Forums ( posts)
GS Survey ( entries)
Survey Results

Gilbert's Syndrome | The Detoxification System | Understanding the Mutations | GS Symptoms | Things That Help & Things To Avoid
Connections To: Allergies | Chronic Fatigue Syndrome | Candida | Soy | Hypothyroidism            Also: Forums | Survey | Contact

Gilbert's Syndrome Mutations

Contents: GS Mutations | The Primary Mutation - UGT1A1*28 | The Second Mutation - UGT1A6*2 | The Third Mutation - UGT1A7*3

This is a very technical section. It would be good to read up on the Detoxification System first to get some background. There are three linked mutations found in Gilbert's Syndrome, each related to an enzyme deficiency.


Gilbert's Syndrome Mutations & Linked Polymorphisms

The UGT1A1*28 mutation is the cause of Gilbert's Syndrome. There are two other linked mutations which 94% of people with Gilbert's Syndrome also have. The first is called UGT1A6*2, in which UGT1A6 is operating at only 50% capacity. The second is called UGT1A7*3, which is operating at only 17% capacity. Both have unique toxins that they act on and contribute to the enzymatic disfunction in Gilbert's Syndrome.

SCIENCE: National Library of Medicine

Two linked polymorphic mutations (A(TA)7TAA and T-3279G) of UGT1A1 as the principal cause of Gilbert syndrome.

Gilbert syndrome is a mild hereditary unconjugated hyperbilirubinemia caused by mutations in the bilirubin UDP-glucuronosyltransferase gene (UGT1A1). The mutation, A(TA)7TAA, is thought to be the sole cause of the syndrome in Caucasians, but an enhancer polymorphism (T-3279G) that lowers transcriptional activity has recently been reported. We have tested the linkage of the two mutations in 11 Caucasians and 12 Japanese patients who were homozygous for A(TA)7TAA. All 23 patients were also homozygous for T-3279G, indicating that T-3279G and A(TA)7TAA were linked. The decrease in transcription caused by both mutations together may be essential to the syndrome.

In Short: Gilbert's Syndrome also contains a linked mutation (T-3279G) that also lowers transcriptional activity. 23/23 tested patients showed this linked mutation.

SCIENCE: National Library of Medicine

Frequent co-occurrence of the TATA box mutation associated with Gilbert's syndrome (UGT1A1*28) with other polymorphisms of the UDP-glucuronosyltransferase-1 locus (UGT1A6*2 and UGT1A7*3) in Caucasians and Egyptians.
Polymorphisms of drug metabolizing enzymes are frequently associated with diseases and side effects of drugs. Recently, a TATA box mutation of UGT1A1 (UGT1A1*28), a common genotype leading to Gilbert's syndrome, and several missense mutations of other UDP-glucuronosyltransferase 1 (UGT1) family members have been described. Furthermore, co-occurrence of UGT1A1*28 and UGT1A6*2 has been observed. In order to elucidate the basis for co-occurrence of UGT1 mutations, fluorescence resonance energy transfer techniques were developed for rapid determination of polymorphisms of three UGT isoforms (UGT1A1*28, 1A6*2, and 1A7*2/*3). Hundred healthy Caucasians and 50 Egyptians were genotyped. All genotypes followed the Hardy-Weinberg equilibrium. Only three major haplotypes were found, including a haplotype consisting of allelic variants of all three isoforms (29% in Caucasians and 22% in Egyptians), all leading to reduced UGT activity. Frequent haplotypes containing several UGT1 allelic variants should be taken into account in studies on the association between diseases, abnormal drug reactions, and UGT1 family polymorphisms.

UGT polymorphisms
Polymorphism Allelic variation UGT activity
UGT1A1*1 (TA)6TAA  
UGT1A1*28 (TA)7TAA Reduced (ca. 3-fold) [2–4]
UGT1A6*1 T181R184  
UGT1A6*2 A181S184 Reduced (ca. 2-fold) [7]
UGT1A7*1 N129R131W208  
UGT1A7*2 K129K131W208 Reduced (2.6-fold) [9]
UGT1A7*3 K129K131R208 Reduced (5.8-fold) [9]
UGT1A7*4 N129R131R208 Reduced (2.8-fold) [9]

Haplotypes and their estimated frequencies in each population
Number Haplotype Frequency (%)
  UGT1A1 UGT1A6 UGT1A7 Caucasians Egyptians
I *1 *1 *1 35.0 +- 3.2 40.9 +- 4.6
II *28 *2 *3 28.5 +- 3.1 21.8 +- 4.1
III *1 *1 *2 26.3 +- 2.4 14.7 +- 3.5
IV *1 *2 *3 3.5 +- 1.3 11.1 +- 3.5
V *1 *1 *3 4.2 +- 1.4 5.1 +- 2.2
VI *1 R184S *2 1.2 +- 0.8 1.1 +- 1.1
VII *28 *1 *2 - 2.3 +- 1.8
VIII *28 R184S *2 - 1.9 +- 1.5
IX *1 *2 *1 - 1.1 +- 1.0
X *28 *1 *1 0.5 +- 0.5 -
XI *1 *2 *2 0.5 +- 0.5 -
XII *1 R184S *3 0.3 +- 0.4 -

In Short: 94% of those with Gilbert's Syndrome also have the UGT1A6*2 and UGT1A7*3 mutations. 5% of the remaining also have the less severe UGT1A7*2 mutation. Only 1% have just the basic mutation for Gilbert's Syndrome. In other words, it is a rare case that those people with Gilbert's Syndrome do not also have mutations in UGT1A6*2 and UGT1A7*3. 8/8 caucasians and 3/4 egyptians with GS who were tested had the other two mutations as well. The 1A6*2 mutation acts at 50% capacity and the 1A7*3 mutation acts at 17% capacity.

SCIENCE: National Library of Medicine

Combined polymorphisms in UDP-glucuronosyltransferases 1A1 and 1A6: implications for patients with Gilbert's syndrome.
BACKGROUND/AIMS: UDP-glucuronosyltransferases (UGTs) are important enzymes involved in glucuronidation of various exogenous and endogenous compounds. Studies were undertaken on the variability of three UGT enzyme activities in human livers. Enzyme activities were associated with genetic polymorphisms in UGT1A1 (UGT1A1*28) and UGT1A6 (UGT1A6*2). UGT1A1*28 is associated with Gilbert's syndrome, a deficiency in glucuronidation of bilirubin leading to mild hyperbilirubinemia, whereas UGT1A6*2 may result in low glucuronidation rates of several drugs. METHODS: Enzyme activities and genetic polymorphisms were assessed in 39 human liver samples, and polymorphisms were also assessed in blood of 253 healthy controls. RESULTS: Associations were found between UGT enzyme activities of bilirubin (B) and 4-nitrophenol (NP; r=0.47, P=0.0024), B and 4-methylumbelliferone (MUB; r=0.54, P=0.0003), and NP and MUB (r=0.89, P<0.0001). In addition to the association between B-UGT enzyme activity and UGT1A1*28 (r=0.45, P=0.0034) as reported earlier, an association between B-UGT and UGT1A6*2 (r=0.43, P=0.007) was found. In 253 Dutch Caucasian controls, co-occurrence of UGT1A1*28 and UGT1A6*2 was found (r=0.9, P<0.0001). CONCLUSIONS: Most patients with Gilbert's syndrome, in addition to their reduced B-UGT enzyme activity, may have abnormalities in the glucuronidation of aspirin or coumarin- and dopamine-derivatives, due to this combination of UGT1A1*28 and UGT1A6*2 genotypes.

In Short: Most patients with Gilbert's Syndrome also have abnormalities in the glucuronidation of UGT1A6 substrates, including aspirin, coumarin, and dopamine-derivatives.

Genetic polymorphisms in UDP-glucuronosyltransferases and glutathione S-transferases and colorectal cancer risk

So far, nine functional UGT1A isoenzymes (UGT1A1, UGT1A3–UGT1A10) have been characterized, all derived from a single gene locus on chromosome 2 (8,20). UGT1A enzymes are involved mainly in the metabolism of exogenous compounds; this is not strictly the case however as bilirubin and steroid hormones are important endogenous substrates... Most UGT1A family members are expressed at low levels in the colon. UGT1A7, UGT1A8 and UGT1A10 are expressed only extra-hepatically and may be highly relevant for colonic detoxification... (describes Gilbert's)...Two missense mutations in exon 1 of UGT1A6 have been described, which results in T181A and R184S amino acid changes (28). These polymorphisms are usually linked on one allele (UGT1A6*2), although alleles carrying only the R184S polymorphism (UGT1A6*3) are found occasionally. Metabolism rates of phenols by recombinant UGT1A6*2 were lower than those of the most common enzyme. For UGT1A7, eight allelic variants of the most common UGT1A7*1 allele have been described (29,30); however, only UGT1A7*1 to *4 have been identified in Caucasians. Complete loss, or a very strong reduction, of activity was reported for UGT1A7*3 (29), whereas substantial reduction of activity was demonstrated for UGT1A7*2 and UGT1A7*4. In UGT1A8, two missense mutations in exon 1 were identified, resulting in an A173G substitution with little impact on catalytic activity, in contrast to the substitution of C277Y yielding an inactive enzyme (22).

In Short: This describes first the UGT1 isoenzyme and then the effects of known polymorphisms in the UGT1A6, UGT1A7, and UGT1A8 enzymes.

This link lists polymorphisms and their effects:

This page has refence links to the discovery of each.

Gilbert's Syndrome
amino acid promoter change - reduced activity
amino acid G71R reduced
amino acid P229Q reduced
amino acid R367G reduced
amino acid F83L (found in a Thai family)

amino acids T181A and R184S
reduced effect

amino acids N129K, R131K, and W208R
reduced effect

no information

SCIENCE: National Library of Medicine

UDP glucuronosyltransferase (UGT) 1A6 pharmacogenetics: II. Functional impact of the three most common nonsynonymous UGT1A6 polymorphisms (S7A, T181A, and R184S).
The objective of this study was to use recombinant enzymes and human liver microsomes (HLMs) to comprehensively evaluate the functional impact of the three most common nonsynonymous polymorphisms (S7A, T181A, and R184S) identified in the human UDP glucuronosyltransferase (UGT) 1A6 gene. Initial studies using different substrates (serotonin, 5-hydroxytryptophol, 4-nitrophenol, acetaminophen, and valproic acid) showed similar results with 2-fold higher glucuronidation by UGT1A6(*)2 (S7A/T181A/R184S) compared with UGT1A6(*)1 (reference), and intermediate activities for other variants. Enzyme kinetic analyses with the UGT1A6-specific substrate (serotonin) showed 50% lower K(m) values for all R184S variants and 2-fold higher V(max) values for both S7A/T181A variants compared with UGT1A6(*)1. Furthermore, intrinsic clearance (V(max)/K(m)) values were highest for the UGT1A6(*)2 allozyme (2.3-fold over UGT1A6(*)1), resulting from additive effects of higher enzyme affinity and activity. As expected, K(m) values of (*)1/(*)1 genotyped HLMs (5.4 +/- 0.2 mM) were similar to recombinant UGT1A6(*)1 (5.8 +/- 0.6 mM). Conversely, (*)2/(*)2 HLMs showed higher K(m) values (7.0 +/- 0.3 mM) rather than the lower K(m) values displayed by recombinant UGT1A6(*)2 (3.6 +/- 0.3 mM), suggesting that this allozyme may display different enzyme kinetic behavior in HLMs compared with HEK293 cells. At best, these polymorphisms were predicted to account for 15 to 20% of the observed 13-fold variability in glucuronidation of UGT1A6 substrates by HLMs, indicating that there are likely other genetic or environmental factors responsible for the majority of this variation.

In Short: This appears to be saying that the UGT1A6(*2) polymorphism associated with Gilbert's Syndrome causes glucuronidation twice as fast as normal. Substrates of UGT1A6 include serotonin, 5-hydroxytryptophol, 4-nitrophenol, acetaminophen, and valproic acid.


The Primary Mutation - UGT1A1*28 - Operating At 30%

UGT1A1 Substrates



UGT1A1 is the most extensively studied enzyme of the UGT superfamily, and is known to contain over 30 genetic variants, many of which influence its expression and functional properties [1,10]. UGT1A1 is responsible for glucuronidating various drugs and endogenous substrates including estrogens and bilirubin. It is the primary UGT responsible for bilirubin glucuronidation.

SCIENCE: National Library of Medicine

Pharmacophore and quantitative structure activity relationship modelling of UDP-glucuronosyltransferase 1A1 (UGT1A1) substrates.

UDP-glucuronosyltransferase 1A1 (UGT1A1) is a polymorphic enzyme responsible for the glucuronidation of structurally diverse drugs, non-drug xenobiotics and endogenous compounds (e.g. bilirubin). Thus, definition of UGT1A1 substrate and inhibitor selectivities and binding affinities assumes importance for the identification of compounds whose elimination may be impaired in subjects with variant genotypes, and for the prediction of potentially inhibitory interactions involving xenobiotics and endogenous compounds metabolized by UGT1A1.
The common features pharmacophore demonstrated the importance of two hydrophobic domains separated from the glucuronidation site by 4 A and 7 A, respectively. These models, which represent the first generalized predictive models for a UGT isoform, complement each other and are an important first step towards computer based (in silico) models of UGT1A1 for high throughput prediction of metabolism.

In Short : UGTA1A detoxifies diverse drugs, non-drug xenobiotics and endogenous compounds such as bilirubin.

Differential Modulation of UDP-Glucuronosyltransferase 1A1 (UGT1A1)-Catalyzed Estradiol-3-glucuronidation by the Addition of UGT1A1 Substrates and Other Compounds to Human Liver Microsomes

Modulatory effects of the following compounds were investigated: bilirubin, eight flavonoids, 17alpha-ethynylestradiol (17alpha-EE), estriol, 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP), anthraflavic acid, retinoic acid, morphine, and ibuprofen. Although the classic UGT1A1 substrate bilirubin was a weak competitive inhibitor of estradiol-3-glucuronidation, the estrogens and anthraflavic acid activated or inhibited estradiol-3-glucuronidation dependent on substrate and effector concentrations. For example, at substrate concentrations of 5 and 10 microM, estradiol-3-glucuronidation activity was stimulated by as much as 80% by low concentrations of 17alpha-EE but was unaltered by flavanone. However, at higher substrate concentrations (25-100 microM) estradiol-3-glucuronidation was inhibited by about 55% by both compounds. Anthraflavic acid and PhIP were also stimulators of estradiol 3-glucuronidation at low substrate concentrations. The most potent inhibitor of estradiol 3-glucuronidation was the flavonoid tangeretin. The UGT2B7 substrates morphine and ibuprofen had no effect on estradiol 3-glucuronidation, whereas retinoic acid was slightly inhibitory. Estradiol-17-glucuronidation was inhibited by 17alpha-EE, estriol, and naringenin but was not activated by any compound. This study demonstrates that the interactions of substrates and inhibitors at the active site of UGT1A1 are complex, yielding both activation and competitive inhibition kinetics.

In Short: The following things have varying affects on UGT1A1 detoxification and are definitely or likely substrates of UGT1A1: bilirubin, 17alpha-EE, flavanone, anthraflavic acid, PhIP, the flavonoid tangeretin (the strongest inhibitor), estriol, naringenin. Retinoic acid is a UGT2B7 substrate but was slightly inhibitory.

Based on evolutionary divergence, two families of UGT enzymes have been identified, UGT1 and UGT2. The UGT1A gene is located on chromosome 2 and encodes for all members of the UGT1A subfamily by differential splicing of the gene product (Ritter et al., 1992). Within the UGT1A subfamily, the catalytic activity of UGT1A1 has been relatively well studied. Important physiological roles of UGT1A1 include glucuronidation of the toxic heme breakdown product bilirubin, as well as the glucuronidation of catechol estrogens, and flavonoids (Senafi et al., 1994). UGT1A1 also glucuronidates anthraqinones (Senafi et al., 1994), the oral contraceptive 17-ethynylestradiol (17-EE) (Ebner et al., 1993), and oripavine opioids such as buprenorphine (Senafi et al., 1994).

In Short: UGT1A1 substrates: bilirubin, catechol estrogens, flavonoids, anthraqinones, the birth control pill 17-ethynylestradiol, and oripavine opioids such as buprenorphine.

(Me: Non-UGT1A1 substrates:) The UGT2B7 substrates retinoic acid (Carrier et al., 2000), morphine (Coffman et al., 1997), and ibuprofen (Jin et al., 1993) were examined as examples of non-UGT1A1 substrates.
The rates of estradiol 3-glucuronidation at all estradiol concentrations examined were basically unaffected by the addition of increasing concentrations of the UGT2B7 substrates ibuprofen and morphine (Fig. 2). The pattern expected for a competitive inhibitor of estradiol 3-glucuronidation is a decrease in activity as the modulator concentration increases. In addition for competitive inhibition, as substrate concentration increases the observed loss of glucuronide formation in the presence of inhibitor should decrease. Such patterns were observed for the effects of tangeretin (Fig. 1) and bilirubin (Fig. 2).
Several compounds in the study were observed to be inhibitors of estradiol 3-glucuronidation by UGT1A1. Flavonoids were the most potent inhibitors with tangeretin being the most potent inhibitor of estradiol 3-glucuronidation. Unlike naringenin and the other hydroxylated flavonoids, which provide hydroxy groups for glucuronidation and are therefore substrates for UGT1A1, tangeretin has methoxy groups at various positions. It therefore appears that the most potent inhibitor determined in this study of UGT1A1-catalyzed estradiol-3-glucuronidation, tangeretin, is not a substrate of the enzyme.

Note: Interesting! While other substrates of UGT1A1 inhibit glucuronidation of the test substrate, other unrelated things can also inhibit this enzyme’s glucuronidation. Tangeretin is not a UGT1A1 substrate but was the most potent inhibitor of it.

Nontraditional kinetics were also observed when other UGT1A1 substrates such as ethinylestradiol, buprenorphine, and anthraflavic acid were studied with both human liver microsomes and recombinant UGT1A1

In Short: UGT1A1 substrates: ethinylestradiol, buprenorphine, and anthraflavic acid.

Farnesol is an isoprenoid found in many aromatic plants and is also produced in humans, where it acts on numerous nuclear receptors and has received considerable attention due to its apparent anticancer properties.
Farnesol is glucuronidated in human liver, kidney and intestine in vitro, and is a novel substrate for UGT2B7 and UGT1A1
We also show the first direct evidence that farnesol can be metabolized to hydroxyfarnesol by human liver microsomes and that hydroxyfarnesol is metabolized further to hydroxyfarnesyl glucuronide.

In Short: Farnesol is a substrate of both UGT1A1 and UGT2B7

Regulation of UGT1A1 gene expression:  UGT1A1 has an unusually long TATAA element [A (TA)6 TAA] 29 nt, that in combination with binding sites for liver-enriched factors serves as the proximal promoter. We showed that an insertion of two additional nucleotides in this promoter reduces the gene expression by 70%, causing Gilbert syndrome, a common cause of mild inherited jaundice. We have also identified two enhancers within the upstream regulatory region, which regulate induction of the gene expression by drugs and its repression by thyroid hormone, via binding of nuclear receptors. Further delineation of the cell-type preference of UGT1A1 expression and its hormonal and xenobiotic regulation is continuing.
Posttranslational regulation of UGT1A1 activity: 

We have recently shown that UGT1A1 forms dimers, which are needed for activation of the import of its donor substrate, UDP-glucuronic acid, into the ER lumen by UDP-N-acetylglucosamine. Since the active sites of all UGTs are located within the ER lumen, this import is critical for the conjugative defense mechanism of the body. Recently, we have shown protein kinase C-mediated phosphorylation of UGT1A1, which enhances the enzyme activity.  The role of these post-translational changes in the regulation of hepatic glucuronidation is being studied actively.  

In Short: I'm not sure, but is this saying that all UGT activity may be affected by those with Gilbert's Syndrome?

Isoflavones modulate the glucuronidation of estradiol in human liver microsomes

Soy food has been associated with a reduced incidence of hormonal cancer in Asian countries, and the soy isoflavones daidzein and genistein are believed to protect against tumors induced by the endogenous hormone 17ß-estradiol (E2). In the present study, we have examined if daidzein and genistein as well as several structurally related isoflavones are able to modulate the in vitro glucuronidation of E2 in human hepatic microsomes. It is known that different isoforms of UDP-glucuronosyltransferase (UGT) are involved in E2 glucuronidation: UGT1A1 leads exclusively to the 3-glucuronide and is stimulated by E2 via homotropic kinetics, whereas UGT2B7 gives rise to the 17-glucuronide of E2 following Michaelis–Menten kinetics. In our study, daidzein markedly stimulated the 3-glucuronidation, thereby enhancing the metabolic clearance of E2. In contrast, genistein inhibited the 3-glucuronidation. The 17-glucuronidation of E2 was not affected by either compound. Formononetin and the daidzein metabolites equol, 3'-hydroxy-daidzein, 6-hydroxy-daidzein and glycitein behaved similar to daidzein, whereas biochanin A resembled genistein. The effect of daidzein on the 3-glucuronidation of E2 in human hepatic microsomes was also obtained with human recombinant UGT1A1. Since the only other compound known to stimulate E2 glucuronidation via allosteric kinetics is 17-ethynylestradiol, our study is the first report of the heterotropic stimulation of a UGT by a non-steroidal and naturally occurring compound. An enhanced rate of glucuronidation of E2 by daidzein and its metabolites may contribute to the putative protection of soy against hormonal cancer.

Note: In short, soy isoflavone daidzein and its metabolites contribute to UGT1A1 glucuronidation, and soy isoflavone genistein inhibits it.

N-Glucuronidation of 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) and N-hydroxy-PhIP by specific human UDP-glucuronosyltransferases
Glucuronidation is a major metabolic pathway in the biotransformation of many xenobiotics. Recent studies have shown that in humans, UDP-glucuronosyltransferase (UGT)-mediated glucuronidation plays a critical role in the detoxification of food-borne carcinogenic heterocyclic amines. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), the most abundant carcinogenic heterocyclic amine found in well-cooked meats, has been shown to be extensively glucuronidated in humans… Incubations containing N-hydroxy-PhIP and UGT1A1 expressing microsomes, with an apparent Km of 4.58 µM and a Vmax of 4.18 pmol/min/mg protein, had the highest capacity to convert N-hydroxy-PhIP to N-hydroxy-PhIP-N2-glucuronide.

In Short: PhIP, the carcinogenic found in well-cooked meats, is mainly processed by UGT1A1.

Glucuronidation of etoposide in human liver microsomes is specifically catalyzed by UDP-glucuronosyltransferase 1A1
Among nine recombinant human UGT isoforms (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A8, UGT1A9. UGT1A10, UGT2B7, and UGT2B15), only UGT1A1 exhibited the catalytic activity of etoposide glucuronidation.

In Short: UGT1A1 substrate: etoposide

UGT1A1 is a member of a superfamily of membrane-bound enzymes. Estrogenic compounds inactivated by UGT1A1 include E2, 2-hydroxyestrone, 2-OHE2, 2-MeE2 and ethinylestradiol [13,33]. UGT1A1 is expressed extensively in the liver [34] and to a lesser extent in other organs. To our knowledge UGT1A1 expression was not investigated in breast epithelium, but was detected in human breast cancer cell lines [35].

In Short: UGT1A1 substrates: Estrogenic compounds, including E2, 2-hydroxyestrone, 2-OHE2, 2-MeE2 and ethinylestradiol.

Cruciferae Interact with the UGT1A1*28 Polymorphism to Determine Serum Bilirubin Levels in Humans1,2

Studies indicate that foods from the botanical families Cruciferae (e.g., broccoli), Rutaceae (citrus), Liliaceae (e.g., onions), and Leguminosae (legumes) may increase UGT activity.
For total, direct, and indirect bilirubin measures, there was no significant association with any botanical group independently. There was a significant inverse association between all 3 bilirubin measures and interaction of UGT1A1*28 genotype with Cruciferae intake (P < 0.02 for each measure); individuals with the 7/7 genotype had reduced bilirubin concentrations with increased intake of cruciferous vegetables, whereas individuals with the 6/6 or 6/7 genotype did not. With regard to UGT1A1-conjugated carcinogens (e.g., heterocyclic amines, polycyclic aromatic hydrocarbons), individuals with decreased UGT1A1 activity due to the 7/7 genotype may be at greater risk for carcinogenesis, but our results imply that they also may have greater opportunity to decrease that risk through dietary intervention.

In Short: UGT1A1 substrates: carcinogens such as heterocyclic amines and polycyclic aromatic hydrocarbon (although it might be refering to those carcinogens processed by UGT1A1 such as PhIP). Cruiferous vegetables (e.g. broccoli) help those with Gilbert's Syndrome by increasing activity of UGT1A1

Uridine Diphosphate Glucuronyltransferase 1A1 (UGT1A1) is an enzyme critical in the detoxification of bilirubin, irinotecan and other substrates. Decreased UGT1A1 enzyme activity is associated with increased risk for neonatal jaundice and irinotecan induced leukopaenia and diarrhoea.

In Short: UGT1A1 substrate: Irinotecan

Significant variability in the pharmacokinetics of drugs such as cyclosporine, tacrolimus, sirolimus and mycophenolic acid, is seen in liver transplant patients. These agents are primarily metabolized by CYP3A4 or UGT1A1, and are also substrates for drug transporters such as P-glycoprotein, multidrug resistance protein 2 (MRP2) and bile salt export pump (BSEP).

In Short: UGT1A1 substrates: cyclosporine, tacrolimus, sirolimus and mycophenolic acid.

SCIENCE: National Library of Medicine

Glucuronidation of acetaminophen is independent of UGT1A1 promotor genotype. The metabolism of acetaminophen (paracetamol) is thought to be altered in patients with Gilbert's syndrome (GS), a chronic unconjugated hyperbilirubinemia. The underlying cause of GS is a polymorphism in the promotor region of the uridine diphosphate glucuronosyltransferase isoform 1A1 gene (UGT1A1*28), its encoded enzyme being responsible for the glucuronidation of bilirubin and presumably acetaminophen. DISPROVEN!... Acetaminophen is likely to be substrate of a UGT isoform other than the UGT1A1.

In Short : Acetaminophen is not a substrate of UGT1A1

Extensive analysis of specific catalytic activities attributed to recombinant UGT proteins has also demonstrated a broad overlap of substrates targeted for glucuronidation. UGT1A proteins have been found to glucuronidate a wide variety of phenolic substrates. As an example, 1-naphthol serves as a substrate for hepatic UGT1A1, UGT1A6 and UGT1A9.

In Short: 1-napthol is a substrate for UGT1A1, UGT1A6, and UGT1A9

Correlation between the UDP-Glucuronosyltransferase (UGT1A1) TATAA Box Polymorphism and Carcinogen Detoxification Phenotype

The UDP-glucuronosyltransferase (UGT) superfamily of enzymes catalyze the glucuronidation of various compounds, including endogenous compounds such as bilirubin and steroid hormones, as well as xenobiotics, including drugs and environmental carcinogens (1, 2, 3, 4) . On the basis of structural as well as sequence homology, UGTs are classified into several families and subfamilies, each containing several highly homologous UGT genes (5) . The entire UGT1A family is derived from a single locus in chromosome 2 coding for nine functional proteins that differ only in their amino terminus as a result of alternate splicing of independent exon 1 regions to a shared carboxy terminus encoded by exons 2–5 (6) . In contrast to the UGT1A family, the UGT2B family is composed of several independent genes, all located on chromosome 4 (7, 8, 9, 10, 11) .

In addition to being the major enzyme involved in the metabolism and detoxification of bilirubin (12) , UGT1A1 is one of several UGTs that glucuronidate metabolites of tobacco carcinogens, including benzo(a)pyrene (BaP). Of the hepatic UGTs, only UGT1A1 and UGT1A9 exhibit significant activity against benzo(a)pyrene-trans-7R,8R-dihydrodiol [BPD(-)] (13) , precursor to the highly mutagenic anti-(+)-BaP-7R,8S-dihydrodiol-9S,10R-epoxide. UGTs 1A7, 1A8, and 1A10 also exhibit glucuronidating activity against BPD(-) (13) , but these are extrahepatic enzymes located primarily in the alimentary tract (14, 15, 16, 17, 18) .

Significant correlations between UGT1A1 genotype and both UGT1A1 expression and BPD(-) glucuronidating activity were demonstrated, implicating this polymorphism as a potentially important risk factor for BaP-induced carcinogenesis.
We screened a cohort of 95 subjects…As shown in Table 1 , 12% of these subjects had the homozygous polymorphic UGT1A1(*28/*28) genotype, which was similar to the genotype prevalence observed previously for Caucasians (26) .
After screening for BPD(-) glucuronidation phenotype in all 60 liver microsomes, similar levels of BPD(-) glucuronide formation were observed in subjects with different UGT1A1 TATAA box genotypes (Fig. 4B) . Unlike that observed for bilirubin glucuronidation, no significant differences in BPD(-) glucuronide formation were observed when we compared subjects with the UGT1A1(*28/*28) genotype with either the UGT1A1(*1/*1) or UGT1A1(*1/*28) genotypes. Because UGT1A1 and UGT1A9 are the only two hepatic UGT enzymes that exhibit significant activity against BPD(-) (13) , attempts were made to eliminate UGT1A9-induced BPD(-) glucuronidating activity as a potential confounder in these assays by identifying a substrate that could inhibit UGT1A9 but not UGT1A1.
Decreased BPD-glucuronide formation (3–6-fold) was observed when 0.1 mM -naphthylamine was added to BPD glucuronidation assays of human liver microsomes (Figs. 4B and 7 ), a decrease that was significant regardless of UGT1A1 genotype (P < 0.001 for all genotypes). When 0.1 mM -naphthylamine was added to glucuronidation assays of liver microsomes, differences in the levels of BPD(-) glucuronide formation were observed for subjects with different UGT1A1 TATAA box genotypes (Fig. 7) . We observed a significantly lower rate of BPD(-) glucuronide formation in subjects with the homozygous polymorphic UGT1A1(*28/*28) genotype compared with subjects who were either homozygous (P < 0.02) or heterozygous (P < 0.02) for the UGT1A1*1 allele.
When we used -naphthylamine, an inhibitor of UGT1A9-induced glucuronidation of BPD(-), results from the present study suggest that 30% of BPD(-) glucuronidation is catalyzed by UGT1A1, whereas 70% is catalyzed by UGT1A9 in subjects homozygous for the wild-type UGT1A1*1 allele, a fact that is consistent with both enzymes playing major roles in the hepatic glucuronidation of BPD(-).
Together, these data suggest that the UGT1A1 TATAA box polymorphism is associated with a decreased overall ability to glucuronidate an important metabolite [BPD(-)] within the BaP carcinogenic pathway. This suggests that individuals with the variant UGT1A1(*28/*28) genotype may be less able to detoxify BaP (and potentially other carcinogens metabolized by UGT1A1) than those who are wild-type for UGT1A1, further suggesting that individuals with the variant UGT1A1(*28/*28) genotype are at increased risk for certain cancers.
Although the association between BPD(-) glucuronide formation and UGT1A1 genotype was not observed in assays where hepatic UGT1A9 activity was not inhibited, this was likely because hepatic UGT1A9-induced activity comprised a majority (70%) of the total BPD(-) glucuronidation observed in human liver microsomes. The fact that significant differences in genotype-associated UGT1A1-induced BPD(-) glucuronide formation were not detectable when UGT1A9-associated activities were not inhibited is consistent with the possibility that small but significant long-term decreases in the overall ability to detoxify a procarcinogen such BPD(-) may play an important role in cancer susceptibility.

In Short: UGT1A1 is responsible for glucuronidating 30% of tobacco carcinogens, and UGT1A9 does the rest. In subjects with GS, UGT1A9 compensates entirely for the lower UGT1A1, the result being no change in glucuronidation of tobacco carcinogens. (This apparently conflicts with the findings that UGT1A7 deals with tobacco and other carbon-based carcinogens)

Olanzapine toxicity in unconjugated hyperbilirubinaemia (Gilbert's syndrome)

A 19-year-old male with paranoid features and schizophrenic symptoms was treated with 2.5 mg olanzapine for 2 days, which was increased to 5 mg on the third day. On the fourth day, because of a suicide attempt and extreme agitation, the patient was admitted to a psychiatric centre. He was given oral doses of 10 mg olanzapine and 5 mg lorazepam. The patient was conscious on the sixth day but did not respond to verbal stimuli and his symptoms of mutism persisted over the next few days. Communication was possible by monosyllables on day eight. On day ten he was bradypsychic, oriented and capable of articulating short sentences with great effort. Speech returned to normal on day twelve. The patient described his experience as a sensation of not being able to find the words in his head. He had not previously displayed speech alterations, nor did they appear later.

Over the past few years a number of different mutations affecting this gene have been characterised, in which a greater frequency of schizophrenia has been described (Miyaoka et al, 2000). Olanzapine is metabolised in the liver through direct glucuronidation reactions. Polymorphisms in glucuronosyltransferases, which often result in a decreased capacity for bilirubin glucuronidation, may have a significant impact on our capacity to detoxify and eliminate drugs and toxins (Mackenzie et al, 2000). Drug-mediated toxicity caused by genetic deficiency of UPD-glucuronosyl-transferases is known (Burchell et al, 2000), as in the case of the administration of phenothiazine antipsychotics or tricyclic antidepressants. Mutism with olanzapine use has been reported in cases of overdose (Hanel et al, 1998; Cohen, 1999).

The use of therapeutic dosages of olanzapine can cause toxic symptoms if a lack of bilirubin UDP-glucuronosyltransferase is present. We should keep in mind idiopathic unconjugated hyperbilirubinaemia when prescribing olanzapine.

In Short: Yet another substrate. But interestingly, the failure to remove this from the subject's system led to an extreme inability to find the words he was looking for.

Glucose Transferase (the enzyme we are low on) is one of the enzymes that clear the blood stream of the following: carcinogens, some drugs (especially: bromosulfophthalein), something called indocyanine green, free fatty acids, tolbutamide, hormones (PMS/PMT MORE LIKELY IN GS!), and neurotransmitters (maybe why we get anxious?).

Note: compare this list against what I have and look up differences

Polychlorinated biphenyls (PCBs) are a family of 209 chemicals with varying numbers of chlorine atoms attached in varying positions to two connected benzene rings... Because of their insulating and nonflammable properties, PCBs have been used as heat exchange and dielectric fluids in transformers and capacitors, hydraulic and lubricating fluids, diffusion pump oils, plasticizers, extenders for pesticides, and as ingredients of caulking compounds, paints, adhesives, and flame retardants. PCBs have also been used in inks and carbonless paper.

Other populations potentially more sensitive to PCBs are persons with compromised hepatic functioning, including those with incompletely developed glucuronide conjugation mechanisms due to congenital disorders such as Gilbert's syndrome, and persons with hepatic infections.

Note: Not sure if this is UGT1A1 or not.


Investigation Into Specific UGT1A1 Substrates

UGTA1A Substrates: Estrogenic compounds - including catechol estrogens, E2, 2-hydroxyestrone, 2-OHE2, 2-MeE2 and ethinylestradiol

REFERENCE: Wikipedia - Estrogen

The three major naturally occurring estrogens in women are estradiol, estriol and estrone. In the body these are all produced from androgens through enzyme action. Estradiol is produced from testosterone and estrone from androstenedione. Estrone is weaker than estradiol, and in post-menopausal women more estrone is present than estradiol.

Estrogen is produced primarily by developing follicles in the ovaries, the corpus luteum and the placenta. Some estrogens are also produced in smaller amounts by other tissues such as the liver, adrenal glands and the breasts.

UGTA1A Substrates: flavanone, flavonoids (but not the flavonoid tangeretin, although it is the strongest inhibitor in one test), anthraflavic acid, soy isoflavone daidzein and its metabolites (though soy isoflavone genistein inhibits it).

(see section on soy)


Bilirubin & The UGT1A1*28 Mutation

Things That Affect Bilirubin Levels

Drugs that can increase bilirubin measurements include allopurinol, anabolic steroids, some antibiotics, antimalarials, azathioprine, chlorpropamide, cholinergics, codeine, diuretics, epinephrine, meperidine, methotrexate, methyldopa, MAO inhibitors, morphine, nicotinic acid, oral contraceptives, phenothiazines, quinidine, rifampin, salicylates, steroids, sulfonamides, and theophylline.

Drugs that can decrease bilirubin measurements include barbiturates, caffeine, penicillin, and high-dose salicylates.

Hemolysis of blood will falsely increase bilirubin levels
Lipids in the blood will falsely decrease bilirubin levels
Bilirubin is light-sensitive; it decomposes in light

Note: Aha! That's why I itch after I take percaset! Perhaps that's why I like swimming and sunshine, too. I wonder if this contributes to SAD as well.

REFERENCE: Wikipedia - Bilirubin

Drugs (especially anti-psychotic, some sex hormones, and a wide range of other drugs).

REFERENCE: Wikipedia - Sex hormone

In many contexts, the two main classes of sex steroids are androgens and estrogens, of which the most important human examples are testosterone and estradiol respectively. Other contexts will include progestagen as a third class of sex steroids, distinct from androgens and estrogens. Progesterone is the most important and only naturally occurring human progestagen.

There are also many synthetic sex steroids. Synthetic androgens are often referred to as anabolic steroids. Synthetic estrogens and progestins are used in oral contraceptive pills. Diethylstilbestrol (DES) is a synthetic estrogen.

Sex steroids include:

• testosterone
• androstenedione
• dihydrotestosterone
• dehydroepiandrosterone
• anabolic steroid

• estradiol
• diethylstilbestrol

• progesterone
• progestins

Bilirubin Metabolism

SCIENCE: The Institute of Biomedical Science

Normal bilirubin production and excretion

The bilirubin in blood plasma is derived almost entirely from the breakdown of haemoglobin. At the end of their 120 day life span, red cells are removed from the circulation by the reticuloendothelial system and the haemoglobin contained within them is broken down to its constituent parts: haem and globin. The haem part is converted to bilirubin.

As a waste product of haemoglobin breakdown, with no physiological functions, bilirubin must be removed from the body. It is in fact excreted by the liver, in bile. The first phase in the process of bilirubin excretion is transport in blood plasma (bound to albumin) from the reticuloendothelial system, where it is produced, to the liver. Bilirubin arriving at the liver is lipid soluble; for excretion in bile it must first be made water soluble. This is accomplished in the cells of the liver (hepatocytes) by joining (conjugation) with glucuronide acid or conjugated bilirubin. The process of conjugation within hepatocytes is dependent on a key enzyme, uridine diphosphate-glucuronosyltransferase (UPD-GT).
NOTE: Hmm, I wonder if the excess bilirubin is taking up room in albumin which could be used for oxygen instead. Another possible reason for fatigue?

Conjugated bilirubin is secreted from hepatocytes to the bile canaliculi of the liver and outwards in bile from the liver via the gall bladder and common bile duct to the gastrointestinal tract. Bacterial action within the ileum and colon converts bilirubin to stercobilinogen. Finally stercobilin, the product of stercobilinogen oxidation, is excreted in faeces.

Although most bilirubin is excreted as stercobilin in faeces, a small amount of stercobilinogen is reabsorbed from the gastrointestinal tract back into the blood system and subsequently excreted in urine as urobilinogen.

Normally, heme is released from old red blood cells or those broken open by vigorous exercise. The heme in the hemoglobin is the carried in the bloodstream to the liver, where a few enzymes (proteins) break it down into something to be excreted in the urine. This is necessary because bilirubin (BR) is mildly neurotoxic and generally unwanted as a metabolite. It is, in chemical terms, "hydrophobic", which means it cannot dissolve well in water. The way the body takes care of hydrophobic compunds like this is to chemically add a sugar molecule to the structure of it. This makes it dissolve well in water, and allows it to be metabolized further into urobilinogen and other products that can be eliminated by urination. Normally, the unsugared "unconjugated" bilirubin attaches to a blood protein called albumin, which also carries all kinds of fatty substances. When the unconjugated BR reaches the liver, and enzyme in a particular place in the cell (called the endoplasmic reticulum) puts the sugar molecule on, the bilirubin becomes "conjugated" and water soluble.

Why are plasma Bilrubin levels variable?
a) Water, Sugar, and Bilirubin, the three substrates.
For one, because the UGT enzyme requires glucose (sugar) molecules to operate. In biochemistry terms, you must have a molecule of UDP-Glucoronic acid PLUS a bilirubin, otherwise, there is no productive turnover. This means that even if you have enough UGT in your liver, you aren't giving it the fuel it needs for getting rid of bilirubin, when you fast and your blood sugar is low. So, keep your blood sugar off the bottom, and you'll be better off because the enzyme will have its second "substrate", its food. Enzymes run faster when you give them the things that they will work on. We see this in retrospect, when, knowing that Gilbert's patients have lesser amounts of UGT, and higher BR levels, the lesser amount of enzyme is working harder to metabolize the BR, than in normal patients. This is why the BR levels are asymptotic, and level off, rather than escalating over time until jaundice.

b) Changeable amounts of enzyme produced.
The other thing that makes this biology interesting is that you can temporarily raise the amount of UGT in your liver, by consuming compounds that stimulate UGT production. In effect, you can trick the liver into making the enzyme, but really not have anything for it to work on. Generally, the production of the enzyme is stimulated by things that UGT1 should metabolize. One report shows that green tea flavonoids can stimulate production of the enzyme. Also, many food substances modulate the levels of the UGT enzyme, as we saw above. The liver senses everything in the bloodstream and reacts accordingly. It may start up more production of the UGT1 enzyme, which would lead to less bilirubin. Or, it may slow down.


The bulk of bilirubin (approximately 80%) is produced as a result of the breakdown of senescent red cells. The remainder comes from catabolism of other pigments (e.g., cytochromes and catalases, particularly cytochrome p450). Reduction of heme occurs primarily in the liver and spleen where hemoglobin released from senescent red cells is phagocytosed by cells of the mononuclear phagocytic system. Globin is enzymatically degraded to amino acids, iron is bound to transferrin and transported to the bone marrow, and the heme is converted to biliverdin by heme oxygenase and subsequently to bilirubin by biliverdin reductase. Bilirubin is the only major breakdown product that requires excretion since the globin and heme molecules are reused.

Bilirubin is released from the mononuclear phagocyte system into the circulation where it circulates bound to albumin. Bilirubin is insoluble in water and the binding to albumin allows it to be transported and inhibits its diffusion into the tissues. This type of albumin-bound bilirubin is referred to as unconjugated, indirect reading, or lipid soluble bilirubin. Binding of bilirubin to albumin is disrupted by a low serum albumin concentration and competition from a variety of drugs such as sulfonamides, thyroxine and salicylates. Transport of bilirubin by albumin ends with its disassociation at the hepatic sinusoidal membrane.

Note: Bilirubin competes with thyroxine for albumin binding!! Salicylates include aspirin.

Hepatocyte Uptake

Hepatocyte uptake of unconjugated bilirubin is a carrier mediated process. The process may exhibit saturation as the carrier is shared by conjugated bilirubin and a variety of organic anions including bile acids and bacterial endotoxin. Within the hepatocytes bilirubin is bound by two proteins called y and z binding proteins which allow the process of conjugation to take place.


Conjugation renders bilirubin more water soluble, thereby facilitating the secretion of bilirubin into the bile. Bilirubin is conjugated to glucuronic acid, taurine (and to a lesser extent glucose) primarily via glucuronosyltransferase to form bilirubin monoglyceride and diglucuronide. This form of bilirubin is called conjugated, direct reacting or water soluble bilirubin.

Biliary Secretion

Secretion of bilirubin across the hepatocyte canaliculus membrane is the rate limiting step in bilirubin metabolism. This active transport process occurs against a large concentration gradient and is ordinarily not disrupted in hepatic disease.

Post Hepatic Bilirubin Metabolism

Bilirubin flows in the bile from the biliary canaliculi down the bile ducts into the gallbladder where it is stored until feeding initiates gallbladder contraction. Conjugated bilirubin in bile then flows into the duodenum via the common bile duct. In the distal small intestine and colon bacteria reduce most of the conjugated bilirubin to a group of colorless compounds known as urobilinogens that are readily absorbed by the intestinal and colonic mucosa. Most reabsorbed urobilinogen is re-secreted into the bile, but a small amount is excreted in the urine. Some urobilinogen is not reabsorbed but is instead passed in the feces either unchanged or further degraded into stercobilins, which are believed to impart color to the feces.

SCIENCE: National Library of Medicine

Under normal conditions conjugation of bilirubin takes place in the hepatic cell, and two or one molecules of gluguronic acid leads to form bilirubin diglucuronides (BDG) and bilirubin monoglucuronides (BMG) respectively. In humans, BDG constitutes the major conjugate in bile (about 80%). But in subjects with Gilbert's syndrome the proportion of BDG/BMG is inverted.

SCIENCE: National Library of Medicine

Genetic lesions of bilirubin uridine-diphosphoglucuronate glucuronosyltransferase (UGT1A1) causing Crigler-Najjar and Gilbert syndromes: correlation of genotype to phenotype.

Uridine-diphosphoglucuronate glucuronosyltransferases (UGTs) are a family of enzymes that conjugate various endogenous and exogenous compounds with glucuronic acid and facilitate their excretion in the bile. Bilirubin-UGT(1) (UGT1A1) is the only isoform that significantly contributes to the conjugation of bilirubin.

Unconjugated bilirubin has to be conjugated with glucuronic acid in the hepatocyte to form water-soluble bilirubin glucuronides to be excreted from the body. A specific hepatic enzyme isoform (1A1) belonging to the uridinediphosphoglucuronate glucuronosyltransferase (UGT) family of enzymes catalyzes this process. UGT is a group of enzymes that mediate the conjugation of many substances to glucuronic acid. This group of enzymes is normally concentrated in the lipid bilayer of the endoplasmic reticulum of hepatocytes, intestinal cells, kidneys, and other tissues.


The Second Mutation - UGT1A6*2 - Operating At 50%

The Mutation

SCIENCE: National Library of Medicine

Genetic polymorphism in the human UGT1A6 (planar phenol) UDP-glucuronosyltransferase: pharmacological implications.

Two missense mutations were uncovered in the UGT1A6 (HLUG P1) cDNA which codes for a human phenol-metabolizing UDP-glucuronosyltransferase. The mutant and a wild-type UGT1A6 cDNAs were isolated from a custom synthesized human liver lambda Zap cDNA library. Both an A to G transition at nucleotide 541 (T181 A) and an A to C transversion at nucleotide 552 (R184S) occurred in exon 1 of the UGT1A6 (UGT1F) gene at the UGT1 locus. The two mutations on a single allele created a heterozygous genotype. Newly created BsmI and BsoFI sites at the T181 A and R184S locations, respectively, were confirmed by endonuclease treatment of PCR-generated DNA using the donor-liver genomic DNA as template. Screens with endonuclease treatment showed that 33/98 DNA samples were heterozygous with both mutations on one allele. One other individual also carried the R184S mutation on the second allele. Wild-type UGT1A6 generated a broad plateau of activity from pH 5.0 to pH 8.0 with certain experimental phenols, while activity was 1.3-2.5-fold higher at pH 6.4 than at pH 7.2 for others. UGT1A6*2 (181 A+ and 184S+) metabolized 4-nitrophenol, 4-tert-butylphenol, 3-ethylphenol/4-ethylphenol, 4-hydroxycoumarin, butylated hydroxy anisole and butylated hydroxy toluene, with the pH 6.4 preference, at only 27-75% of the rate of the wild-type isozyme whereas 1-naphthol, 3-iodophenol, 7-hydroxycoumarin, and 7-hydroxy-4-methylcoumarin were metabolized at essentially the normal level. Furthermore, UGT1A6*2 metabolized 3-O-methyl-dopa and methyl salicylate at 41-74% of that of the wild-type, and a series of beta-blockers at 28-69% of the normal level. This evidence suggests that the UGT1A6 enzyme activity is affected by different amino acids depending upon the substrate selection.

In Short: This refers to the UGT1A6*2 mutation that most people with Gilbert's Syndrome have, and the effect on processing of many substrates. ***



Two missense mutations leading to T181A (541A.G) and R184S (552A.C) have been identified in the UGT1A6 gene. The mutations have been detected on a single allele (UGT1A6*2) at a frequency of 30% in a Caucasian population, and a second variant allele containing only the R184S change was reported to occur at a frequency of 2% in the same population [28]. UGT1A6*2 encodes an enzyme that has reduced catalytic activity against many, but not all UGT1A6 substrates. UGT1A6 has been demonstrated as an important enzyme in metabolism of aspirin and other non-steroidal anti-inflamitory agents (NSAIDs).. Bigler and colleagues [30] have recently demonstrated that UGT1A6*2 allele positively modified the protective effect of aspirin with the hypothesis being that decreased glucuronidation leads to higher levels of aspirin and its various active metabolites and enhances their protective influence. However, the study failed to demonstrate the same genotype-phenotype association in individuals who were using NSAIDs other than aspirin.

A recent study by Peters et al. [31], found that 87% of individuals homozygous for the UGT1A1*28 allele, were also homozygous for the UGT1A6*2 allele, implying that such individuals might have reduced capacity to glucuronidate bilirubin, as well as several drugs such as aspirin and coumarin due to the combined UGT1A1*28 and UGT1A6*2 genotypes.

UGT1A6 Substrates

Quantitative Structure Activity Relationships for the Glucuronidation of Simple Phenols by Expressed Human UGT1A6 and UGT1A9

UGT1A6 and UGT1A9 have both been demonstrated to rapidly glucuronidate simple phenolic compounds.. UGT1A6 showed a more restricted acceptance of phenolic substrates compared with UGT1A9. However, the affinity of UGT1A6 for these compounds exhibited higher Km values than UGT1A9, although rates of turnover were similar.. The larger UGT1A6 substrates were typified by low activity and lower Km values than their smaller counterparts. Extrapolating from this, it was demonstrated that phenols with large 4-substituents, which were not UGT1A6 substrates, could inhibit 4-ethylphenol glucuronidation.
There are numerous instances where drugs are cleared extensively by direct glucuronidation (3'-azido-2',3'-dideoxythymidine, valproic acid, propofol, and morphine; Bertz and Granneman, 1997), although in many other cases, the involvement of glucuronidation is restricted to the conjugation of glucuronic acid to metabolites of phase I oxidative metabolism. Where drugs are significantly glucuronidated independently of phase I metabolism, the functional group to which the glucuronic acid is transferred can be a hydroxy (phenolic or aliphatic), a carboxylic acid or, in some cases, an amino group (primary, secondary, or tertiary) moiety.
Many UGT family 1 isoforms are capable of glucuronidating phenols to varying degrees including UGT1A1 (Senafi et al., 1994), UGT1A3 (Green et al., 1998), UGT1A4 (Green and Tephly, 1996), UGT1A8, and UGT1A10 (Cheng et al., 1999). UGT family 2 isoforms also display activity toward phenols, although substrate acceptance and rate of glucuronidation seems to be more restricted (UGT2B15, Green et al., 1994; UGT2B7, Coffman et al., 1998). Two of the earliest UGT isoforms to be characterized were done so on the basis of their ability to glucuronidate phenols. UGT1A6 and UGT1A9 were both classified as phenol UGT isoforms due to the high turnover rates of these substrates (Ebner and Burchell, 1993). UGT1A9 demonstrated greater proficiency in glucuronidating bulky and complex phenols than UGT1A6, which was considered to be only capable of glucuronidating simple or planer phenols (Ebner and Burchell, 1993). The high glucuronidation activity toward simple phenolic substrates has been clearly illustrated to be present in human liver microsomes (Temellini et al., 1991), and UGT1A6 comprises a significant proportion of liver UGT 1-naphthol glucuronidation capacity (Ouzzine et al., 1994).
Twenty-four of these phenols were substrates of UGT1A9 and 12 were substrates of UGT1A6. Two of the 12 phenols glucuronidated by UGT1A6 (4-fluorophenol and 4-methoxyphenol) were turned over to such a low extent that it was not possible to accurately measure kinetic parameters.
Kinetic constants for glucuronidation of 4-substituted phenols catalysed by UGT1A6

Standard errors in Vmax were in the range of ±4.0 to ±11.4% of the mean with a mean error of ±7.30%. Standard errors in Km were in the range of ±9.0 to ±22.8% of the mean with a mean error of ±14.8%. The mean of control 1-naphthol activities was 1.51 ± 0.21 nmols/min/mg.

Substrate Control Adjusted Vmax Km Vmax/Km Correlation Coefficient (R)
  nmol/min/mg µM ml/min/mg  
4-Methylphenol 4.61  ± 0.42 1800  ± 230 2.56 0.998
4-Ethylphenol 2.13  ± 0.10 551  ± 54 3.87 0.997
4-n Propylphenol 0.09  ± 0.01 325  ± 58 0.28 0.986
4-iso Propylphenol 0.28  ± 0.02 312  ± 44 0.89 0.993
4-tert Butylphenol 0.20  ± 0.02 233  ± 53 0.59 0.989
4-Hydroxyacetophenone 0.53  ± 0.03 833  ± 75 0.64 0.998
4-Chlorophenol 1.67  ± 0.07 246  ± 28 6.79 0.995
4-Bromophenol 3.00  ± 0.28 419  ± 69 7.16 0.996
4-Iodophenol 2.04  ± 0.11 243  ± 28 8.40 0.997
4-Nitrophenol 1.53  ± 0.2 595  ± 140 2.57 0.987

Kinetic constants for glucuronidation of 4-substituted phenols catalysed by UGT1A9

Standard errors in Vmax were in the range of ±2.5 to ±11.1% of the mean with a mean error of ±6.3%. Standard errors in Km were in the range of ±7.8 to ±31.8% of the mean with a mean error of ±19.4%. The mean of control propofol activities was 0.465 ± 0.12 nmols/min/mg.

Substrate Control Adjusted Vmax Km Vmax/Km Correlation Coefficient (R)
  nmol/min/mg µM ml/min/mg  
4-Methylphenol 0.47  ± 0.026 75  ± 13 6.20 0.965
4-Ethylphenol 0.38  ± 0.016 34  ± 6 11.24 0.981
4-n Propylphenol 0.56  ± 0.036 53  ± 13 10.53 0.973
4-iso Propylphenol 0.78  ± 0.087 51  ± 15 15.29 0.973
4-n Butylphenol 0.57  ± 0.036 154  ± 38 3.69 0.991
4-sec Butylphenol 0.91  ± 0.039 99  ± 13 9.20 0.993
4-tert Butylphenol 1.26  ± 0.12 75  ± 16 16.80 0.989
4-Cyclopentylphenol 1.02  ± 0.03 160  ± 13 6.38 0.997
4-Phenylphenol 0.36  ± 0.03 67  ± 21 5.43 0.945
Methyl 4-hydroxybenzoate 1.11  ± 0.05 82  ± 11 13.54 0.993
Ethyl 4-hydroxybenzoate 0.96  ± 0.05 94  ± 16 10.23 0.988
Propyl 4-hydroxybenzoate 0.90  ± 0.04 120  ± 16 7.53 0.993
Butyl 4-hydroxybenzoate 0.91  ± 0.03 113  ± 12 8.03 0.993
4-Hydroxybenzophenone 0.97  ± 0.06 57  ± 12 17.02 0.983
4-Hydroxyacetophenone 1.07  ± 0.05 298  ± 36 3.59 0.994
4-Fluorophenol 0.81  ± 0.03 213  ± 51 3.78 0.993
4-Chlorophenol 1.02  ± 0.07 43  ± 9 23.72 0.986
4-Bromophenol 0.94  ± 0.09 41  ± 11 22.90 0.974
4-Iodophenol 0.78  ± 0.05 21  ± 6 37.10 0.962
4-Phenylazophenol 0.28  ± 0.01 79  ± 15 3.54 0.989
4-Nitrophenol 0.93  ± 0.07 88  ± 19 10.58 0.979
4-Methoxyphenol 0.90  ± 0.06 236  ± 35 3.80 0.994
4-Ethoxyphenol 1.12  ± 0.08 271  ± 38 4.13 0.995
4-Propoxyphenol 0.53  ± 0.05 211  ± 49 2.53 0.984

Good image here: [in a new window] (

SCIENCE: National Library of Medicine

Genetic variants in the UGT1A6 enzyme, aspirin use, and the risk of colorectal adenoma.

Genetic variation in the uridine diphosphate glucuronosyltransferase 1A6 (UGT1A6) enzyme is associated with impaired metabolism of aspirin.

Validation of Serotonin (5-Hydroxtryptamine) as an in Vitro Substrate Probe for Human UDP-Glucuronosyltransferase (UGT) 1A6

Investigation of human UDP-glucuronosyltransferase (UGT) isoforms has been limited by a lack of specific substrate probes. In this study serotonin was evaluated for use as a probe substrate for human UGT1A6 using recombinant human UGTs and tissue microsomes. Of the 10 commercially available recombinant UGT isoforms, only UGT1A6 catalyzed serotonin glucuronidation. Serotonin-UGT activity at 40 mM serotonin concentration varied more than 40-fold among human livers (n = 54), ranging from 0.77 to 32.9 nmol/min/mg of protein with a median activity of 7.1 nmol/min/mg of protein. In conclusion, these results indicate that serotonin is a highly selective in vitro probe substrate for human UGT1A6.

In Short: Serotonin is a highly selective substrate for UGT1A6. Serotonin UGT activity varied by more than 40-fold among human livers.


The Third Mutation - UGT1A7*3 - Operating At 17%

Structural heterogeneity at the UDP-glucuronosyltransferase 1 locus: functional consequences of three novel missense mutations in the human UGT1A7 gene.

One of the most important mechanisms involved in host defense against xenobiotic chemicals and endogenous toxins is the glucuronidation catalysed by UDP-glucuronosyltransferase enzymes (UGT). The role of genetic factors in determining variable rates of glucuronidation is not well understood, but phenotypic evidence in support of such variation has been reported. In the present study, six single nucleotide polymorphisms were discovered in the first exon of the UGT1A7 gene, which codes for the putative substrate-binding domain, revealing a high structural heterogeneity at the UGT1 gene locus. The new UGT1A7 proteins differ in their primary structure at amino acid positions 129, 131 and 208, creating four distinct UGT1A7 allelic variants in the human population: UGT1A7*1 (N129 R131 W208), *2 (K129 K131 W208), *3 (K129 K131 R208), and *4 (N129 R131 R208). In functional studies, HEK cells stably transfected to express the four allelic UGT1A7 variants exhibited significant differences in catalytic activity towards 3-, 7-, and 9-hydroxy-benzo(a)pyrene. UGT1A7*3 exhibited a 5.8-fold lower relative Vmax compared to wild-type *1, whereas *2 and *4 had a 2.6- and 2.8-fold lower relative Vmax than *1, respectively, suggesting that these mutations confer slow glucuronidation phenotype. Kinetic characterization suggested that these differences were primarily attributable to altered Vmax. Additionally, it suggested that each amino acid substitutions can independently affect the UGT1A7 catalytic activity, and that their effects are additive. The expression pattern of UGT1A7 studied herein and its catalytic activity profile suggest a possible role of UGT1A7 in the detoxification and elimination of carcinogenic products in lung. A population study demonstrated that a considerable proportion of the population (15.3%) was found homozygous for the low activity allele containing all three missense mutations, UGT1A7*3. These findings suggest that further studies are needed to investigate the impact of the low UGT1A7 conjugator genotype on individual susceptibility to chemical-induced diseases and responses to therapeutic drugs.

In Short: This refers to the UGT1A7*3 mutation that is closely linked to the mutation in Gilbert's Syndrome. It deals with detoxification of carcinogenic products in the lung.

UGT1A7 Substrates



UGT1A7 has been demonstrated to glucuronidate known carcinogens such as polycyclic aromatic hydrocarbons and dietary heterocyclic amines [1].

In two small case-control studies, UGT1A7*3 allele was reported to be significantly more common in patients with sporadic colorectal carcinomas [34] and hepatocellular carcinomas [35]. Such reports based on small sample size will certainly require confirmation in larger studies, but they perhaps demonstrate the potentially important role UGTs might play in detoxification of the gastrointestinal tract, and generate plausible hypothesis regarding UGT variants' capacity to alter cancer risk.

We compared the activities and tissue expression of UGT1A7 and UGT1A8, which exhibit 77% identity in their amino terminal sequence. UGT1A7 shows broad specificity, catalyzing the glucuronidation of 31 of 40 randomly selected substrates (100 µM) at rates >0.1 nmol/mg/min. UGT1A7 substrates included both planar and nonplanar compounds, mono- and polycyclic aromatics, and compounds with bulky side chain ring substitutions. UGT1A8 exhibited a narrower substrate specificity that completely overlapped with UGT1A7. UGT1A8 was most active toward the 1-OH, 4-OH, 5-OH, 6-OH, 7-OH, 10-OH, 11-OH, and 12-OH derivatives of benzo[a]pyrene. Other effective UGT1A8 substrates (>0.1 nmol/mg/min) included 9-OH-benzo[a]pyrene, 1-naphthol, 4-methylumbelliferone, 7-hydroxycoumarin, chrysin, quercetin, 4-nitrophenol, and estriol. In general, substrates preferred by UGT1A8 were polyaromatic planar structures with nonbulky substituents and a superimposable 1-naphtho ring structure. Studies of the tissue expression of the UGT1A7 and 1A8 mRNAs using RNase protection analysis suggested that each is expressed in liver and kidney of control rats. A major difference is the higher expression of UGT1A7 mRNA in intestine.
Glucuronide conjugates tend to have reduced biological activity, more restricted patterns of distribution, and enhanced rates of excretion from the body.
In both humans and rodents, two families of UGTs1 are known. The UGT1A family is unique in using an exonsharing arrangement to encode a family of UGTs having identical C-terminal sequence (245 amino acids) (Ritter et al., 1992; Emi et al., 1995). Analysis of the various amino terminal coding exons of UGT1A reveals two distinct subclasses or clusters based on sequence similarity. One of these, the "cluster B" subgroup (Gong et al., 2001), includes UGT1A7 and the "UGT1A7-like" forms. Humans have four functional cluster B forms (UGT1A7, UGT1A8, UGT1A9, and UGT1A10)
Rat UGT1A7 has been reported previously by our laboratory to catalyze glucuronidation of various phenol, diphenol, and dihydrodiol metabolites of the environmental carcinogen BaP (Grove et al., 1997; Bock et al., 1999) and the therapeutic analgesic drug acetaminophen (Kessler et al., 2002). In the latter study, recombinant expressed UGT1A8 also was found to catalyze activity toward acetaminophen but at 25-fold lower rate than UGT1A7.
At the regulation level, UGT1A7 mRNA has been reported to be expressed in liver and many extrahepatic tissues, including kidney, intestine, ovary, spleen, and lung (Emi et al., 1995; Grove et al., 1997). In the liver and to a lesser extent the intestine and kidney, UGT1A7 is induced by exposure to polycyclic aromatic hydrocarbon-type inducing agents such as 3-methylcholanthrene (Emi et al., 1995; Metz and Ritter, 1998; Grams et al., 2000) and ß-naphthoflavone (Kobayashi et al., 1998) and also by exposure to the dithiole thione oltipraz (Grove et al., 1997; Metz and Ritter, 1998).
Substrates that were highly glucuronidated (>20% substrate conversion) are indicated by +++.

No. Compound Name Control (HEK Cells) Mean UGT1A7 Mean UGT1A8
1 10-OH BaP N.D. +++ +++
2 5-OH BaP N.D. +++ +++
3 12-OH BaP 0.04, 0.01 +++ +++
4 7-OH BaP N.D. +++ 2.08 ± 0.15
5 11-OH BaP N.D. +++ 1.33 ± 0.08
6 1-Naphthol 0.07, 0.004, 0.006 +++ 1.28 ± 0.12
7 1-OH BaP N.D. +++ 1.00 ± 0.04
8 4-OH BaP N.D. +++ 0.90 ± 0.23
9 6-OH BaP N.D. 0.45 ± 0.03 0.79 ± 0.26
10 4-Methylumbelliferone N.D. +++ 0.64 ± 0.01
11 7-OH coumarin N.D. +++ 0.64 ± 0.01
12 Chrysin N.D. +++ 0.40 ± 0.03
13 4-Nitrophenol N.D. +++ 0.33 ± 0.17
14 BaP-4,5 (±) diol N.D. +++ 0.28 ± 0.01
15 Quercetin 0.005, 0.03, 0.02 +++ 0.22 ± 0.05
16 9-OH BaP N.D. +++ 0.17 ± 0.07
17 Estriol 0.010, 0.03 +++ 0.15 ± 0.05
18 4-OH-biphenyl 0.01, 0.002, 0.008 +++ 0.07 ± 0.04
19 3-OH BaP 0.02, 0.02, 0.005 0.62 ± 0.01 0.06 ± 0.03
20 17ß-Estradiol 0.002, 0.005 0.66 ± 0.01 0.05 ± 0.03
21 16-OH pregnenolone 0.001, 0.004 0.02 ± 0.01 0.05 ± 0.03
22 8-OH BaP N.D. 0.22 ± 0.04 0.03 ± 0.02
23 2-OH BaP N.D. +++ 0.03 ± 0.00
24 17ß-Ethynylestradiol 0.009, N.D., 0.006 +++ 0.03 ± 0.01
25 Phenol red 0.001, 0.006 0.07 ± 0.01 N.D.
26 Dinitrocatechol N.D. 0.52 ± 0.03 N.D.
27 Estrone N.D. 0.17 ± 0.01 N.D.
28 4-OH estradiol 0.003, N.D. 0.05 ± 0.01 N.D.
29 Propofol 0.002, 0.001 +++ N.D.
30 BaP-7,8 (+) diol N.D. 0.52 ± 0.01 N.D.
31 BaP-7,8 (-) diol N.D. 0.57 ± 0.08 N.D.
32 Mycophenolic acid N.D. 0.05 ± 0.03 N.D.
33 Serotonin N.D. 0.15 ± 0.03 N.D.
34 Acetaminophen N.D. 0.03 ± 0.01 N.D.
35 Bisphenol A N.D. +++ N.D.
36 Bilirubin N.D. N.D. N.D.
37 Butylated hydroxytoluene N.D. 0.04 ± 0.00 N.D.
38 Butylated Hydroxyquinone N.D. 0.03 ± 0.00 N.D.
39 Buprenorphine N.D. 0.04 ± 0.00 N.D.
40 Epicatechin N.D. 0.65 ± 0.19 N.D.

N.D., not detectable (<0.01 nmol/min/mg protein).

Rat UGT1A7 enzyme has previously been shown to glucuronidate BaP phenols and diols (Grove et al., 1997), quinol metabolites of polycyclic aromatic hydrocarbons (Bock et al., 1999), and the analgesic drug acetaminophen (Kessler et al., 2002).
As documented previously (Grove et al., 1997), UGT1A7 was highly active toward virtually every BaP monophenols and dihydrodiols tested. This finding alone suggests a more open or flexible substrate binding site for the UGT1A7 enzyme, capable of accommodating substrates in orientation(s) necessary for transfer of glucuronic acid to eligible functional groups. This suggestion is further supported by our finding that UGT1A7 is very active toward many bulky, nonplanar type substrates, such as bisphenol A and the estrogenic steroids such as 17ß-ethynylestradiol and 17ß-estradiol.
Note: interesting.. it shares the processing of estrogens, and is whacked out by this GS mutation as well.
UGT1A7 expression is increased after exposure to arylhydrocarbon receptor agonist in both liver and intestine (Kobayashi et al., 1998; Metz and Ritter, 1998).

So what ARE these things?
Polycyclic aromatic hydrocarbons:
REFERENCE: Wikipedia - Polycyclic Aromatic Hydrocarbons
Many of them are known or suspected carcinogens. They are formed by incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, or tobacco. (there is also a list of examples here, none of which I recognize)
Acetominophen: Tylenol
BaP: benzo[a]pyrene
REFERENCE: Wikipedia - Benzopyrene
Benzo[a]pyrene is a five-ring polycyclic aromatic hydrocarbon that is mutagenic and highly carcinogenic.. Benzo[a]pyrene is a product of incomplete combustion at temperatures between 300 and 600°C. Is found in coal tar, in automobile exhaust fumes (especially from diesel engines), tobacco smoke, and in charbroiled food. Recent studies have revealed that levels of benzopyrene in burnt toast are significantly higher than once thought, although it is unproven whether burnt toast is itself carcinogenic.
Napthol: (same as 1-napthol?)
REFERENCE: Wikipedia - Napthol
They are soluble in simple alcohols, ethers, and chloroform.
Coumarin: (same as 7-OH Coumarin?)
REFERENCE: Wikipedia - Courmarin
Coumarin is a chemical compound found in many plants, notably in high concentration in the tonka bean, woodruff, and bison grass. It has a sweet scent, readily recognised as the scent of newly-mown hay.. Coumarin is often found in tobacco products and artificial vanilla substitutes, though it has been banned as a food additive in numerous countries since the mid-20th century because it is moderately toxic to the liver and kidneys. Coumarin was banned as an adulterant by tobacco companies in 1997 but due to the lack of reporting requirements for pipe tobacco to the US Department of Health and Human Services it was still being used as a flavoring additive in pipe tobacco. Coumarin was banned as a food additive in the United States in 1940, however it was still used in tobacco until 1997. Coumarin is on the FDA's Generally Recognized as Safe (GRAS) list as an additive to alcoholic beverages, most notably as an additive to German white wines such as maitrank or May drink. Concerns of coumarin damage to the kidneys and liver may to some extent be associated with coumarin derivatives, many of which are more toxic.
Chrysin is the chemical name for a type of isoflavone molecule that has been demonstrated to be a potent aromatazation blocker. In other words, Chrysin minimizes the conversion of testosterone to either estrogen or DHT (dihyrdotestosterone).

SCIENCE: National Library of Medicine
Chrysin (5,7-dihydroxyflavone) is a natural and biologically active compound extracted from many plants, honey, and propolis. It possesses potent anti-inflammatory, anti-oxidant properties, promotes cell death, and perturbing cell cycle progression.
Suspected: Cardiovascular or blood toxicant, neurotoxicant, skin or sense organ toxicant
Used in pesticide products. This pesticide is used as a fungicide.
REFERENCE: Wikipedia - Quercetin
Quercetin is a flavonoid that forms the "backbone" for many other flavonoids, including the citrus flavonoids rutin, hesperidin, naringin and tangeritin. Quercetin is found to be the most active of the flavonoids in studies, and many medicinal plants owe much of their activity to their high quercetin content. Quercetin has demonstrated significant anti-inflammatory activity because of direct inhibition of several initial processes of inflammation. For example, it inhibits both the manufacture and release of histamine and other allergic/inflammatory mediators. In addition, it exerts potent antioxidant activity and vitamin C-sparing action.. Foods rich in quercetin include apples, black & green tea, onions (higher concentrations of quercetin occur in the outermost rings[1]), raspberries, red wine, red grapes, citrus fruits, broccoli & other leafy green vegetables, and cherries. A study[2] by the University of Queensland, Australia, has also indicated the presence of quercetin in varieties of honey, including honey derived from eucalyptus and tea tree flowers.
Biphenyl: (same as 4-OH-biphenyl?)
REFERENCE: Wikipedia - Biphenyl
Biphenyl (or diphenyl or 1,1'-biphenyl or lemonene) is a solid organic compound… It has a distictive pleasant smell. Biphenyl is an aromatic hydrocarbon… Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be produced from these sources by distillation... Biphenyl prevents the growth of molds and fungus, and is therefore used as a preservative (E230, in combination with E231, E232 and E233), particularly in the preservation of citrus fruits during transportation.
Note: A surprising number of these form yellowish crystals
REFERENCE: Wikipedia - Propofol
Propofol is a short-acting intravenous anesthetic agent used for the induction of general anesthesia in adult patients and pediatric patients older than 3 years of age
REFERENCE: Wikipedia - Bisphenol A
Bisphenol A is a chemical compound with two phenol functional groups in its molecule that belongs to the phenol class of aromatic organic compounds.. BPA has been known to leach from plastics which are cleaned with harsh detergents or used to contain acidic or high temperature liquids. The chemical has been found in nearly every human tested in the United States.. BPA can activate estrogen receptors leading to similar physiological effects as the body's own estrogens.. Some hormone disrupting effects in studies on animals and human cancer cells have been shown to occur at levels as low as 2-5 ppb (parts per billion). It has been claimed that these effects lead to health problems such as, in men, lowered sperm count and infertile sperm. (Note: damn, more estrogen disruption)


Correspondences: obviously, I have long-hated tobacco and fumes from burning stuff. Wood smoke's had bad effects on me, even though I like the smell. And consider- there's the wood-burning house in our neighborhood, and we live in this valley – perhaps another reason the walks bring me down like they often do. I've never much liked Tylenol either.
Wondering.. Some of these are positive – such as the citrus flavones. They help is a hundred ways. I've seen evidence of competition for these enzymes. So, the odd effect might be this – if I was to drink a lot of pineapple juice, my enzyme would strain to process it, and I'd have the positive effects of that pineapple juice running through me for perhaps 6 times as long. But the same goes for smoke. Now if I was to drink pineapple juice while sitting by a fire, that enzyme would be double-loaded, and I may have 12 hour full-dose effects for both smoke and pineapple juice.
No wonder people have such a hard time figuring this out.