Paracetamol glucuronidation by recombinant rat and human phenol UDP-glucuronosyltransferases

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. Serotonin-UGT activity kinetics of representative human liver microsomes (n = 7) and pooled human kidney, intestinal and lung microsomes and recombinant human UGT1A6 typically followed one enzyme Michaelis-Menten kinetics. Serotonin glucuronidation activity in these human liver microsomes had widely varying V(max) values ranging from 0.62 to 51.3 nmol/min/mg of protein but very similar apparent K(m) values ranging from 5.2 to 8.8 mM. Pooled human kidney, intestine, and lung microsomes had V(max) values (mean +/- standard error of the estimates) of 8.8 +/- 0.4, 0.22 +/- 0.00, and 0.03 +/- 0.00 nmol/min/mg of protein (respectively) and apparent K(m) values of 6.5 +/- 0.9, 12.4 +/- 2.0, and 4.9 +/- 3.3 mM (respectively). In comparison, recombinant UGT1A6 had a V(max) of 4.5 +/- 0.1 nmol/min/mg of protein and an apparent K(m) of 5.0 +/- 0.4 mM. A highly significant correlation was found between immunoreactive UGT1A6 protein content and serotonin-UGT activity measured at 4 mM serotonin concentration in human liver microsomes (R(s) = 0.769; P < 0.001) (n = 52). In conclusion, these results indicate that serotonin is a highly selective in vitro probe substrate for human UGT1A6.

Glucuronidation of dihydroartemisinin in vivo and by human liver microsomes and expressed UDP-glucuronosyltransferases

The aim of this study was to elucidate the metabolic pathways for dihydroartemisinin (DHA), the active metabolite of the artemisinin derivative artesunate (ARTS). Urine was collected from 17 Vietnamese adults with falciparum malaria who had received 120 mg of ARTS i.v., and metabolites were analyzed by high-performance liquid chromatography-mass spectrometry (HPLC-MS). Human liver microsomes were incubated with [12-(3)H]DHA and cofactors for either glucuronidation or cytochrome P450-catalyzed oxidation. Human liver cytosol was incubated with cofactor for sulfation. Metabolites were detected by HPLC-MS and/or HPLC with radiochemical detection. Metabolism of DHA by recombinant human UDP-glucuronosyltransferases (UGTs) was studied. HPLC-MS analysis of urine identified alpha-DHA-beta-glucuronide (alpha-DHA-G) and a product characterized as the tetrahydrofuran isomer of alpha-DHA-G. DHA was present only in very small amounts. The ratio of the tetrahydrofuran isomer, alpha-DHA-G, was highly variable (median 0.75; range 0.09-64). Nevertheless, alpha-DHA-G was generally the major urinary product of DHA glucuronidation in patients. The tetrahydrofuran isomer appeared to be at least partly a product of nonenzymic reactions occurring in urine and was readily formed from alpha-DHA-G by iron-mediated isomerization. In human liver microsomal incubations, DHA-G (diastereomer unspecified) was the only metabolite found (V(max) 177 +/- 47 pmol min(-1) mg(-1), K(m) 90 +/- 16 microM). Alpha-DHA-G was formed in incubations of DHA with expressed UGT1A9 (K(m) 32 microM, V(max) 8.9 pmol min(-1) mg(-1)) or UGT2B7 (K(m) 438 microM, V(max) 10.9 pmol mg(-1) min(-1)) but not with UGT1A1 or UGT1A6. There was no significant metabolism of DHA by cytochrome-P450 oxidation or by cytosolic sulfotransferases. We conclude that alpha-DHA-G is an important metabolite of DHA in humans and that its formation is catalyzed by UGT1A9 and UGT2B7.

The importance of cysteine 126 in the human liver UDP-glucuronosyltransferase UGT1A6

The human UDP-glucuronosyltransferase 1A6 (UGT1A6) isoform is actively involved in the detoxication of phenolic compounds. In an effort to gain insight on active-site amino acids, we investigated the functional relevance of cysteinyl residues in the glucuronidation process. The enzyme was irreversibly inactivated upon exposure to thiol-specific reagents, especially N-phenylmaleimide. Site-directed mutagenesis of the conserved Cys126 into valine led to a fully inactive mutant, whereas conservative substitution with serine significantly restored the glucuronidation activity toward 4-methylumbelliferone used as a reference substrate. This mutant exhibited a reduced affinity toward the acceptor substrate, as evidenced by a 10-times increase in K(m) value, compared to the wild-type enzyme. The two mutations did not alter the stability of UGT1A6 nor change the subcellular localization of the protein in the endoplasmic reticulum of recombinant cells. These results support the conclusion that Cys126 is an essential residue for the integrity of the substrate binding site of UGT1A6.

Glucuronidation of acetaminophen catalyzed by multiple rat phenol UDP-glucuronosyltransferases

Gunn rats glucuronidate acetaminophen (APAP) at reduced rates and show increased susceptibility to APAP-induced hepatotoxicity. This defect is presumed to involve UDP-glucuronosyltransferase (UGT) 1A6, which is nonfunctional in Gunn rats, but it is currently unclear whether other 1A family members are also involved. In humans, two 1A isoforms are known to be active (1A6 and 1A9) but 1A6 form has a 25-fold lower apparent K(m) (2 mM). Rat liver microsomal APAP UGT activity is induced by in vivo treatment with beta-naphthoflavone or oltipraz, an effect correlating with induction of 1A6 and 1A7. To address a possible role of 1A7 in APAP glucuronidation relative to other 1A forms, cDNAs encoding UGTs 1A1, 1A5, 1A6, 1A7, and 1A8 were expressed in human embryonic kidney cells and the contents of expressed enzyme in prepared membrane fractions determined by quantitative immunoblotting. At 2.5 mM APAP, 1A7 showed the highest specific activity (2.8 nmol/min/nmol 1A7 protein), followed by 1A6 (1.1 nmol/min/nmol), and 1A8 (0.27 nmol/min/nmol). 1A1 and 1A5 were essentially inactive. Kinetic comparisons indicated 1A7 had a similar apparent K(m) as 1A6 (4.7 versus 3.9 mM, respectively) but a 2.4-fold higher catalytic activity. These data suggest that in rats, 1A7 plays a major role in APAP glucuronidation and contributes to protection against APAP-induced hepatotoxicity. The involvement of other UGTs besides 1A6 is further underscored by the presence of significant residual APAP-glucuronidating activity by Gunn rat hepatocytes, indicating the activity of an unknown UGT2 family member.

Human variability in glucuronidation in relation to uncertainty factors for risk assessment

The appropriateness of the default uncertainty factor for human variability in kinetics has been investigated for glucuronidation using an extensive database of substrates metabolised primarily by this pathway. Inter-individual variability was quantified for 15 compounds from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve (AUC)) and acute exposure (C(max)). Low inter-individual variability (about 30-35%) was found for all parameters (clearance corrected or not corrected for body weight, metabolic clearance, oral AUC and C(max)) after either iv or oral administration to healthy adults. The overall variability of 31% for glucuronidation in healthy adults supported the validity of the default kinetic uncertainty factor of 3.16 for this group, because it would cover more than 99% of individuals. Comparisons between potentially sensitive subgroups and healthy adults using differences in means and variability indicated that neonates showed the greatest impairment of glucuronidation, and that the 3.16 kinetic default factor applied to the mean data for adults would be inadequate for this subpopulation. The in vivo data have been used to derive pathway-related default factors for compounds eliminated largely via glucuronidation.

Evaluation of the marmoset as a model species for drug glucuronidation

1. The in vitro glucuronidation of a wide range of compounds has been studied in microsomes prepared from marmoset liver and kidney. These studies have been undertaken to evaluate the marmoset as a model species for drug glucuronidation and for comparison with conjugation by other species. 2. The compounds studied were glucuronidated by marmoset liver microsomes to varying extents (e.g. naproxen CLint 0.4 microl min(-1) mg(-1), 1-naphthol CLint 43 microl min(-1) mg(-1)) Both marmoset and rat liver microsomes glucuronidated morphine at the 3-position (marmoset CLint 19 microl min(-1) mg(-1), rat CLint 6.3 microl min(-1) mg(-1)) but glucuronidation at the 6-position was below, the level of radiodetection in both species. 3. Interestingly, marmoset liver microsomes were able to catalyse the glucuronidation of the tertiary amine imipramine to a significant extent (0.05 nmol min(-1) mg(-1)). However, no glucuronidation was detected by rat liver microsomes. 4. Conjugation of a range of substrates was detectable by marmoset kidney microsomes in contrast to rat kidney microsomes, which only catalysed the glucurondation of bilirubin and 1-naphthol (CLint 17 microl min(-1) mg(-1) and 18 microl min(-1) mg(-1), respectively). 5. This report and previous work in dog and human tissue microsomes suggest that the marmoset may be an alternative animal model for human drug glucuronidation, especially when the pathway of drug glucuronidation is known to differ between lower laboratory species and man.

Role of ATP and glycogen reserves in both paracetamol sulfation and glucuronidation by cultured precision-cut rat liver slices

Precision-cut rat liver slices (PCLS) were used to investigate the formation of paracetamol conjugates. The time course of biochemical markers such as ATP and GSH content, glycogen levels and protein synthesis rates was recorded over a period of time of 26 h and taken as index of slices viability. Low values of ATP (3.6 nmol/mg prot), GSH (7.1 nmol/mg prot) and protein synthesis rates (94.1 pmol leu/mg prot x min(-1)) were initially observed. Thereafter, they gradually recovered up to 6 h but decreased values were seen after 20 h. Glycogen, however, dropped rapidly during the first 6 h, being no longer detected after 20 h of incubation. The reincubation of PCLS in a fresh medium for 6 h allowed a strong recovery of GSH, ATP and protein synthesis rates, but no gluconeogenesis was observed. Meanwhile, paracetamol sulfate formation was fairly constant (about 3 microg/mg protein) while glucuronide gradually disappeared. The amount of both UGT1A1 and ST1A1 did not correlate with their respective enzymatic activities. We suggest that loss of glycogen impair glucuronide conjugation by decreasing the availability of UDPGA, and that low values of ATP are largely enough to support sulfotransferase activity.

Acetaminophen UDP-glucuronosyltransferase in ferrets: species and gender differences, and sequence analysis of ferret UGT1A6

The principal objective of this study was to determine whether ferrets glucuronidate acetaminophen more slowly compared with other species, and if so investigate the molecular basis for the difference. Acetaminophen-UDP-glucuronosyltransferase (UGT) activities were measured using hepatic microsomes from eight ferrets, four humans, four cats, four dogs, rat, mouse, cow, horse, monkey, pig and rabbit. Gender differences between male and female ferret livers were explored using enzyme kinetic analysis. Immunoblotting of microsomal proteins was also performed using UGT-specific antibodies. Finally, the exon 1 region of UGT1A6, a major acetaminophen-UGT, was sequenced. Glucuronidation of acetaminophen was relatively slow in ferret livers compared with livers from all other species except cat. Gender differences were also apparent, with intrinsic clearance (Vmax/Km) values significantly higher in male compared with female ferret livers. Furthermore, Vmax values correlated with densitometric measurements of two protein bands identified with a UGT1A subfamily-specific antibody. No deleterious mutations were identified in the exon 1 or flanking regions of the ferret UGT1A6 gene. In conclusion, like cats, ferret livers glucuronidate acetaminophen relatively slowly. However, unlike cats, in which UGT1A6 is encoded by a pseudogene and dysfunctional, there are no defects in the ferret UGT1A6 gene which could account for the low activity.

Drug glucuronidation in clinical psychopharmacology

Glucuronidation is a phase II metabolic process and one of the most common pathways in the formation of hydrophilic drug metabolites. At least 33 families of uridine diphosphate-glucuronosyltransferases have been identified in vitro, and specific nomenclature similar to that used to classify the cytochrome (CYP) P450 system has been established. The UGT1 and UGT2 subfamilies represent the most important of these enzymes in human drug metabolism. Factors affecting glucuronidation include the following: cigarette smoking, obesity, age, and gender. In addition, several drugs have been found in vitro to be substrates, inhibitors, or inducers of UGT enzymes. Induction or inhibition of both UGT and CYP isoforms may occur simultaneously. Some important drug interactions involving glucuronidation have been documented and others can be postulated. This review summarizes the relevant literature pertaining to drug glucuronidation and its implications for clinical psychopharmacology.

An alternative promoter contributes to tissue- and inducer-specific expression of the rat UDP-glucuronosyltransferase 1A6 gene

UDP-glucuronosyltransferase 1A6 (UGT1A6), a key enzyme catalyzing the glucuronidation of small planar phenols and amines, is expressed in a tissue- and inducer-dependent manner. Expression is high in kidney, gastrointestinal tract, and induced liver, with low expression in spleen, lung, and ovary. Exposure to certain chemicals, such as 3-methylcholanthrene, benzo[a]pyrene, beta-naphthoflavone, and oltipraz elevates UGT1A6 mRNA in liver and to a lesser extent gastrointestinal tract and kidney, but not in other tissues. The mechanisms underlying this complex pattern of expression have been elusive. We have identified a new type of UGT1A6 mRNA (class 2) that differs in its 5' untranslated sequence. The class 2 transcript is the more abundant type expressed in liver, gastrointestinal tract, and kidney. Transcription of the class 2 mRNA is initiated 107 bases 5' of the UGT1A6 coding exon. The promoter region flanking the transcription start site contains an HNF1-like binding site identical to that in the human UGT1A6 gene. Both class 1 and class 2 mRNAs were elevated in liver by 3-methylcholanthrene, benzo[a]pyrene, beta-naphthoflavone, and oltipraz, with preferential elevation of class 1 occurring after 3-methylcholanthrene and benzo[a]pyrene treatment. These data suggest that transcription from a second promoter contributes to tissue- and inducer-specific expression of rat UGT1A6.

Retinol potentiates acetaminophen-induced hepatotoxicity in the mouse: mechanistic studies

This study was designed to elucidate the mechanism of retinol's potentiation of acetaminophen-induced hepatotoxicity. To accomplish this, the major bioactivation and detoxification pathways for acetaminophen were investigated following retinol (75 mg/kg/day, 4 days), acetaminophen (400 mg/kg), and retinol + acetaminophen treatment. Hepatic microsomes were used to determine the catalytic activity and polypeptide levels of cytochrome P450 enzymes involved in the murine metabolism of acetaminophen. Results showed that the catalytic activity and polypeptide levels of CYP1A2, CYP2E1, and CYP3A were unchanged in the treatment groups compared to vehicle and untreated controls. In combination, retinol + acetaminophen caused a significantly greater depletion of GSH compared to corn oil + acetaminophen (0.36 +/- 0.11 vs 0.89 +/- 0.19 micromol/g, respectively, p < 0.05). This greater GSH depletion correlated with a higher degree of hepatic injury in the retinol + acetaminophen-treated animals but is probably not the cause of the potentiated injury since the results showed that retinol treatment itself did not alter hepatic glutathione (3.34 +/- 0.43 vs 3.44 +/- 0.46 micromol/g for retinol vs vehicle, respectively). However, hepatic UDPGA stores were decreased in the retinol-treated group compared to untreated and corn oil controls (54.6 +/- 10.6 vs 200.6 +/- 17.6 nmol/g for retinol and untreated control, respectively, p < 0.001). This demonstrates that there is significantly less hepatic UDPGA available for conjugation following retinol administration. The results suggest that decreased hepatic UDPGA is likely the cause of retinol's potentiation of acetaminophen-induced hepatic injury.

High-performance liquid chromatographic assay for acetaminophen glucuronide in human liver microsomes

A rapid and specific high-performance liquid chromatographic assay was developed for the determination of acetaminophen glucuronide formed by human liver microsomes. In addition, incubation conditions were systematically evaluated. Conditions that yielded the optimal rate of acetaminophen glucuronide formation over various concentrations of acetaminophen (0.15-30 mM) consisted of the following: 0.1 M potassium phosphate buffer, 1 mM magnesium chloride, 30 microg/mg alamethicin, 4 mM uridine 5'-diphosphoglucuronic acid at a pH of 7.1. Alamethicin produced higher and more consistent APAPG formation rates compared to Brij-58. Adding saccharolactone to the incubation medium reduced the velocity of the reaction. Acetaminophen glucuronide, acetaminophen, and the internal standard (paraxanthine), were analyzed on a C18 column with UV detection at 250 nm. The mean correlation coefficient (r2) of the standard curves for acetaminophen glucuronide was >0.99 over the range of 0.1-25 nmol. The intra- and inter-day coefficients of variation were <4%. This method is suitable for in vitro studies using acetaminophen glucuronide formation as an index reaction for UGT activity.

Importance of histidine residues for the function of the human liver UDP-glucuronosyltransferase UGT1A6: evidence for the catalytic role of histidine 370

The human UDP-glucuronosyltransferase isoform UGT1A6 catalyzes the nucleophilic attack of phenolic xenobiotics on glucuronic acid, leading to the formation of water-soluble glucuronides. Based on the irreversible inhibition of the enzyme activity by the histidyl-selective reagent diethyl pyrocarbonate (DEPC), histidine was suggested to play a key role in the glucuronidation reaction. Therefore, the role of four strictly conserved histidine residues (His38, His361, His370, and His485) in the glucuronidation of 4-methylumbelliferone, as reporter substrate, was examined using site-directed mutagenesis. For this purpose, stable heterologous expression of wild-type and mutant UGT1A6 was achieved in the yeast Pichia pastoris. Replacement of histidine residues by alanine or glutamine led to fully inactive H38A, H38Q, and H485A mutants. Substitution of His361 by alanine affected the interaction of the enzyme with the cosubstrate, as indicated by a 4-fold increase in the K(m) value toward UDP-glucuronic acid. Interestingly, H370A mutant presented a severely impaired catalytic efficiency (with a V(max) value approximately 5% that of the wild-type), whereas conservative substitution of His370 by glutamine (H370Q) led to a significant restoration of activity. Whereas H361A was inactivated by DEPC as the wild-type enzyme, this chemical reagent only produced a minor effect on either H370Q or H370A mutant, providing evidence that His370 is probably the reactive histidine residue targeted by DEPC. The dramatic changes in catalytic efficiency on substitution of His370 by alanine and the ability of glutamine to function in place of histidine along with a weak sensitivity of these mutants to DEPC strongly suggest that His370 plays a catalytic role in the glucuronidation reaction.

Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Glucuronide conjugates represent one of the major types of naturally occurring phase 2 metabolites of xenobiotics and endobiotics. The process underlying their formation, glucuronidation, is normally considered detoxifying, because glucuronides usually possess less intrinsic biological or chemical activity than their parent aglycones and they are rapid excreted. However, a number of glucuronide conjugates are known that are active and may contribute to pharmacological activities or toxicities associated with their parent compounds. These include two classes of glucuronides with electrophilic chemical reactivity (N-O-glucuronides of hydroxamic acids and acyl glucuronides of carboxylic acids) and several types of glucuronides that impart biological effects through non-covalent interactions (morphine 6-O-glucuronide, retinoid glucuronides, and D-ring glucuronides of estrogens). Glucuronides may thus contribute to clinically significant effects, including environmental arylamine-induced carcinogenesis, drug hypersensitivity and other toxicities associated with carboxylic acid drugs, morphine analgesia, and cholestasis from estrogens. This review summarizes the rat and human UDP-glucuronosyltransferases that may be involved in the formation of bioactive glucuronides, including their substrate- and tissue-specificity and genetic and environmental influences on their activity. This knowledge may be useful for enhancing the therapeutic efficacy and minimizing the risk of adverse effects associated with xenobiotics that undergo bioactivating glucuronidation reactions.

Tissue distribution and interindividual variation in human UDP-glucuronosyltransferase activity: relationship between UGT1A1 promoter genotype and variability in a liver bank

The variability in a liver bank and tissue distribution of three probe UDP-glucuronosyltransferase (UGT) activities were determined as a means to predict interindividual differences in expression and the contribution of extrahepatic metabolism to presystemic and systemic clearance. Formation rates of acetaminophen-O-glucuronide (APAPG), morphine-3-glucuronide (M3G), and oestradiol-3-glucuronide (E3G) as probes for UGT1A6, 2B7, and 1A1, respectively, were determined in human kidney, liver, and lung microsomes, and in microsomes from intestinal mucosa corresponding to duodenum, jejunum and ileum. While formation of E3G and APAPG were detectable in human kidney microsomes, M3G formation rates from kidney microsomes approached the levels seen in liver, indicating significant expression of UGT2B7. Interestingly, rates of E3G formation in human intestine exceeded the hepatic rates by several fold, while APAPG and M3G formation rates were low. The intestinal apparent Km value for E3G formation was essentially identical to that seen in liver, consistent with intestinal UGT1A1 expression. No UGT activities were observed in lung. Variability in APAPG and M3G activity across a bank of 20 human livers was modest (< or = 7-fold), compared to E3G formation, which varied approximately 30-fold. The E3G formation rates were found to segregate by UGT1A1 promoter genotype, with wild-type (TA)6 rates significantly greater than homozygous mutant (TA)7 individuals. Kinetic analyses were performed to demonstrate that the promoter mutation altered apparent Vmax without significantly affecting apparent Km. These results suggest that glucuronidation, and specifically UGT1A1 activity, can profoundly contribute to intestinal first pass metabolism and interindividual variability due to the expression of common allelic variants.

Human UDP-glucuronosyltransferases: metabolism, expression, and disease

In vertebrates, the glucuronidation of small lipophilic agents is catalyzed by the endoplasmic reticulum UDP-glucuronosyltransferases (UGTs). This metabolic pathway leads to the formation of water-soluble metabolites originating from normal dietary processes, cellular catabolism, or exposure to drugs and xenobiotics. This classic detoxification process, which led to the discovery nearly 50 years ago of the cosubstrate UDP-glucuronic acid (19), is now known to be carried out by 15 human UGTs. Characterization of the individual gene products using cDNA expression experiments has led to the identification of over 350 individual compounds that serve as substrates for this superfamily of proteins. This data, coupled with the introduction of sophisticated RNA detection techniques designed to elucidate patterns of gene expression of the UGT superfamily in human liver and extrahepatic tissues of the gastrointestinal tract, has aided in understanding the contribution of glucuronidation toward epithelial first-pass metabolism. In addition, characterization of the UGT1A locus and genetic studies directed at understanding the role of bilirubin glucuronidation and the biochemical basis of the clinical symptoms found in unconjugated hyperbilirubinemia have uncovered the structural gene polymorphisms associated with Crigler-Najjar's and Gilbert's syndrome. The role of the UGTs in metabolism and different disease states in humans is the topic of this review.

In situ hybridization studies of UDP-glucuronosyltransferase UGT1A6 expression in rat testis and brain

UDP-glucuronosyltransferases (UGTs), in addition to their role in overall pharmacokinetics, play important roles in local protection of cells against toxins and in the control of endogenous receptor ligands. UGT1A6, which conjugates planar phenols, appears to be expressed in many organs, but information on cell-specific expression in these organs is controversial or absent. Therefore, a non-isotopic in situ hybridization method was developed and applied to localize UGT1A6 expression in rat testis and brain. It was found that UGT1A6 is expressed in Sertoli cells and spermatogonia of rat testis and in brain neurons, in particular in hippocampal pyramidal cells and Purkinje cells of the cerebellum.

Molecular genetic basis for deficient acetaminophen glucuronidation by cats: UGT1A6 is a pseudogene, and evidence for reduced diversity of expressed hepatic UGT1A isoforms

The domestic cat has a significantly lower capacity to glucuronidate planar phenolic xenobiotics compared with most other mammalian species. The aim of this study was to determine the mechanistic basis for this anomaly. Current knowledge of the substrate specificity of UDP-glucuronosyltransferase (UGT) isoforms indicates that the cat may either lack or poorly express UGT1A6. Initially, a novel cloning technique was used to identify UGT1A genes expressed in cat liver. Only two unique UGT1A isoforms could be discriminated. The first (28%, of clones) was most homologous to UGT1A1 (the bilirubin-UGT), while the second (72% of clones) showed homology to several isoforms, but could not be unambiguously identified, and was designated cat UGT1A02. Southern blot analysis confirmed the presence of a single UGT1A6-homologous region in the cat genome. Subsequent cloning and sequencing of the entire UGT1A6 exon 1 coding region revealed five deleterious genetic mutations. Identical mutations were found by sequencing of UGT1A6 exon 1 from five other unrelated cats. Four of these five genetic lesions were also identified in the UGT1A6 exon 1 region of a margay (Leopardus wiedii). Finally, RT-PCR of liver mRNA from four different cats confirmed the presence of UGT1A1 and UGT1A02, but not UGT1A6. In conclusion, UGT1A6 is a pseudogene in the domestic cat and in at least one other phylogenetically related species. Furthermore, cats appear to have a less diverse pattern of UGT1A isoform expression compared with other species. Such differences most likely reflect the highly carnivorous diet of Feliform species and resultant minimal exposure to phytoalexins.

Establishment of Salmonella strain expressing catalytically active human UDP-glucuronosyltransferase 1A1 (UGT1A1)

Human uridinediphosphate-glucuronosyltransferase 1A1 (UGT1A1) was expressed in Salmonella typhimurium TA1535 cells by transfection of the cells with plasmids carrying the UGT1A1 cDNA. UGT1A1 cDNA was isolated by a polymerase chain reaction from human liver total RNA and was inserted into the pSE420 plasmid, linked to the trc promoter and terminator. The plasmid thus constructed was introduced into Salmonella TA1535 cells. The expression of human UGT1A1 protein was confirmed by Western blot analysis. The maximal expression was observed at 24 h after the addition of isopropyl-beta-D-thiogalactopyranoside, an inducer. However, the bilirubin conjugation activity of the membrane fraction from the Salmonella cells was not detectable. When a beta-glucuronidase inhibitor such as saccharic acid 1,4-lactone, glycyrrhizin or 1-naphtyl-beta-D-glucuronide was added to the reaction mixture, the bilirubin conjugation activity of the human UGT1A1 was detected. When geniposide was added to the reaction mixture, the bilirubin conjugation activity of UGT1A1 was not seen. Taking these results into account, the established Salmonella strain possesses the beta-glucuronidase activity. Since the beta-glucuronidase activity of the Salmonella was lower than that of E. coli, it was concluded that Salmonella seemed to be a good host to express UGT protein. This is the first study to demonstrate the establishment of a bacterial strain expressing native human UGT protein showing catalytic activity.

Drug-mediated toxicity caused by genetic deficiency of UDP-glucuronosyltransferases

Human gene families encoding UDP-Glucuronosyltransferases (UGTs) have been identified and partially characterised. This family of enzymes catalysed the glucuronidation of drugs, xenobiotics and endobiotics. Genetic mutations and polymorphisms have been identified in several UGT genes and examples should be anticipated in all UGT genes. A common genetic defect in the TATA box promoter of the UGT1A1 gene is associated with Gilbert's Syndrome (GS) causing mild hyperbilirubinaemia. Recently, adverse effects of anticancer agents have been observed in Gilbert's patients due to reduced drug or bilirubin glucuronidation.

Coordinate induction by antioxidants of UDP-glucuronosyltransferase UGT1A6 and the apical conjugate export pump MRP2 (multidrug resistance protein 2) in Caco-2 cells

Treatment of Caco-2 cells with the antioxidants quercetin or t-butylhydroquinone led to induced protein levels of UDP-glucuronosyltransferase UGT1A6 (ca. 3-fold over controls) and of the apical conjugate export pump multidrug resistance protein 2 (MRP2; 1.9-fold over controls). In contrast to UGT1A6, MRP2 (symbol ABCC2) was not inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Immunocytochemistry demonstrated that MRP2 was only expressed at the brush border domain of Caco-2 cell monolayers. The results indicate that UGT1A6 and MRP2 are coordinately induced by antioxidants, facilitating chemoprotection against phenolic toxins and excretion of conjugates into the intestinal lumen.

An internal signal sequence mediates the targeting and retention of the human UDP-glucuronosyltransferase 1A6 to the endoplasmic reticulum

The human UDP-glucuronosyltransferase isoform UGT1A6 is predicted to be a type I transmembrane protein anchored in the endoplasmic reticulum by a single C-terminal transmembrane domain, followed by a short cytoplasmic tail. This topology is thought to be established through the sequential action of a cleavable N-terminal signal peptide and of a C-terminal stop transfer/anchor sequence. We found that the deletion of the signal peptide did not prevent membrane targeting and insertion of this protein expressed in an in vitro transcription/translation system or in yeast Pichia pastoris. Interestingly, the same results were obtained when the protein was depleted of both the signal peptide and the C-terminal transmembrane domain/cytoplasmic tail sequences, suggesting the presence of an internal topogenic element able to translocate and retain UGT1A6 in the endoplasmic reticulum membrane in vitro and in yeast cells. To identify such a sequence, the insertion of several N-terminal deletion mutants of UGT1A6 into microsomal membranes was investigated in vitro. The data clearly showed that the deletion of the N-terminal end did not affect endoplasmic reticulum targeting and retention until residues 140-240 were deleted. The signal-like activity of the 140-240 region was demonstrated by the ability of this segment to confer endoplasmic reticulum residency to the cytosolic green fluorescent protein expressed in mammalian cells. Finally, we show that this novel topogenic sequence can posttranslationally mediate the translocation of UGT1A6. This study provides the first evidence that the membrane assembly of the human UGT1A6 involves an internal signal retention sequence.

Molecular genetic basis of Gilbert's syndrome

Gilbert's syndrome, an hereditary, chronic, mild, unconjugated hyperbilirubinaemia resulting from impaired hepatic bilirubin clearance and otherwise normal liver function, is arguably the most common syndrome known in humans. Recent molecular genetic studies have determined that the clinical phenotype can be described by a dinucleotide polymorphism in the TATA box promoter of the bilirubin uridine diphosphate-glucuronosyltransferase (UGT-1A1) gene, most frequently (TA)7TAA, affecting up to 36% of Africans, but only 3% of Asians. However, a second common heterozygous mutation in the coding exon 1 of the UGT-1A1 gene (G71R) can also cause the Gilbert's phenotype in Japanese and Asians. The clinical phenotype may not be apparent as frequently as the determined genotype, due to environmental factors such as alcohol-induced hepatic bilirubin glucuronidation, reducing serum bilirubin levels and causing a latent condition. Gilbert's disease is a contributory factor of prolonged neonatal jaundice in breast-fed infants and may precipitate jaundice when coinherited with other disorders of haem metabolism. The genetic variation described as Gilbert's syndrome may lead to pharmacological variation in drug glucuronidation and unexpected toxicity from therapeutic agents.

Photoaffinity labeling of the aglycon binding site of the recombinant human liver UDP-glucuronosyltransferase UGT1A6 with 7-azido-4-methylcoumarin

7-Azido-4-methylcoumarin (AzMC) is a fluorescent photoactive compound structurally related to 4-methylumbelliferone (4-MU), a marker substrate of the human liver recombinant UDP-glucuronosyltransferase (UGT) 1A6. AzMC was synthesized and utilized to label the substrate binding site of UGT1A6. AzMC exhibits a fluorescence spectrum with maximum excitation and emission wavelengths of 380 and 442 nm, respectively. Upon irradiation, the probe irreversibly inhibited glucuronidation activity measured with para-nitrophenol (pNP) as substrate and interacted with UGT1A6 according to a saturable process indicative of reversible binding before covalent incorporation of the photoaffinity label. This inhibition was both time and concentration dependent and led to the calculation of an inhibition constant, k(2) = 0.113 mM min(-1), and dissociation constant, K(d) = 2.89 mM, for the reaction. Partial photoinactivation of UGT1A6 with AzMC revealed that the probe decreased the apparent V(max) of the pNP glucuronidation reaction, but not the K(m). Moreover, inhibition was partially prevented by 1-naphthol, a surrogate substrate for the enzyme, or by preincubation with an active-site directed inhibitor, 5'-O-[[(2-decanoylamino-3-phenyl-propyloxycarbonyl)amino]-su lfonyl]-2 ',3'-O-isopropylideneuridine. In contrast, UDP-glucuronic acid (UDP-GlcUA) did not have any protective effect against photoinactivation and AzMC did not affect the photoaffinity labeling of UGT1A6 by 5-[beta-(32)P]N(3)UDP-GlcUA, a photoaffinity analog of UDP-GlcUA. Additionally, in the absence of irradiation, AzMC was found to be a competitive inhibitor of 4MU glucuronidation. Collectively, these results strongly indicate that AzMC specifically binds to the UGT1A6 aglycon binding site. Amino acid alignment of phenol-binding proteins revealed a conserved motif, YXXXKXXPXP. It is possible that this motif is involved in phenol binding to UGT1A6 and other phenol-accepting proteins.

Expression of a functionally active human hepatic UDP-glucuronosyltransferase (UGT1A6) lacking the N-terminal signal sequence in the endoplasmic reticulum

UDP-glucuronosyltransferase 1A6 (UGT1A6) is a membrane glycoprotein of the endoplasmic reticulum playing a key role in drug metabolism. It is synthesized as a precursor with an N-terminal cleavable signal peptide. We demonstrate that deletion of the signal peptide sequence does not prevent membrane targeting and integration of this human isoform when expressed in an in vitro transcription-translation system, as shown by N-glycosylation, resistance to alkaline treatment and protease protection. Furthermore, UGT1A6 lacking the signal peptide (UGT1A6delta sp) was targeted to the endoplasmic reticulum in mammalian cells as shown by immunofluorescence microscopy and was catalytically active with kinetic constants for 4-methylumbelliferone glucuronidation similar to that of the wild-type. These results provide evidence that the signal peptide is not essential for the membrane assembly and activity of UGT1A6 suggesting that additional topogenic element(s) mediate(s) this process.

Functions and transcriptional regulation of PAH-inducible human UDP-glucuronosyltransferases

Functions and regulation of selected human UDP-glucuronosyltransferases (UGT1A1, UGT1A4, UGT1A6, UGT1A9, UGT2B7, UGT2B15) are summarized. Evidence for at least two PAH-inducible UGTs (UGT1A6 and UGT1A9) is presented, which, however, are also constitutively expressed in a tissue- and cell-specific manner. These isoforms have recently been characterized to conjugate planar and bulky phenols, respectively. Using a selective RT-PCR method, UGT1A6 expression was detected in a variety of tissues (liver, kidney, lung, intestine, and pharyngeal mucosa). PAH-inducible UGTs may cooperate in the metabolism of phenolic metabolites of benzo(a)pyrene. Studies with stably expressed isoforms suggest that UGT1A9 is responsible for the formation of benzo(a)pyrene-3.6-diphenol diglucuronide, the major biliary metabolite of benzo(a)pyrene.

Expression of UDP-glucuronosyltransferases (UGTs) 2B7 and 1A6 in the human brain and identification of 5-hydroxytryptamine as a substrate

The extrahepatic expression of UDP-glucuronosyltransferases (UGTs) is important in the detoxification of a number of endogenous and exogenous compounds, including 5-hydroxytryptamine and morphine. Studies were designed to investigate the extrahepatic expression of human UGTs using RT-PCR techniques and to determine the UGTs involved in the glucuronidation of 5-hydroxytryptamine. Human UGT2B7 expression was found in the human liver, kidney, pancreas, and brain, while UGT1A6 expression is found in the liver, kidney, and brain. This is the first observation of UGTs present in the human central nervous system. Using glucuronidation assays, a significant amount of 5-hydroxytryptamine glucuronide was found to be catalyzed by UGT1A6. These studies suggest that UGT2B7 may play an important role in the overall contribution of morphine analgesia by serving to generate the potent morphine-6-O-glucuronide in situ. UGT1A6 could play an important role in the glucuronidation of 5-hydroxytryptamine in vivo, therefore terminating the actions of the neurotransmitter.

Mono- and Diglucuronide formation from benzo[a]pyrene and chrysene diphenols by AHH-1 cell-expressed UDP-glucuronosyltransferase UGT1A7

Polycyclic aromatic hydrocarbon (PAH)-type compounds induce at least two rat UDP-glucuronosyltransferase isoforms, UGT1A6 and UGT1A7. Among the glucuronidation reactions of PAH metabolites studied, mono- and diglucuronide formation of benzo[a]pyrene and chrysene-3,6-diphenol showed the highest induction factors in rat liver microsomes. Availability of AHH-1 cells stably expressing UGT1A7 allowed us to study whether this PAH-inducible isoform could catalyze benzo[a]pyrene and chrysene-3,6-diphenol glucuronidation. It was found that UGT1A7 indeed catalyzed mono- and diglucuronide formation of both benzo[a]pyrene and chrysene 3,6-diphenols. V79 cell-expressed rat UGT1A6 also catalyzed these reactions, except for chrysene diphenol diglucronide formation (Bock et al., Mol Pharmacol 42: 613-618, 1992). Enzyme kinetic studies of the glucuronidation of 6-hydroxychrysene (used as a stable PAH phenol) indicated that UGT1A7 conjugated this compound with a lower apparent Km value (0.1 microM) than UGT1A6 (10 microM). The results suggest that the two PAH-inducible UGTs may cooperate in conjugating PAH metabolites, but that UGT1A7 is more efficient.

Heterogeneity of paracetamol metabolism in Gilbert's syndrome

Gilbert's syndrome (GS) is an inherited bilirubin UDP-glucuronosyl transferase deficiency. The object of this study was to investigate the possible effects of this disorder on the metabolism of a drug, such as paracetamol, which is basically eliminated by hepatic glucuronidation. We studied 32 healthy volunteers and 18 people with GS, all of whom were given 1.5 g of paracetamol orally. In the 24 h urine collected, we determined the elimination of free paracetamol, the conjugates (glucuronide, sulphate) and the oxidation products (cysteine, mercapturic acid) by high pressure liquid chromatography (HPLC). The results are given as a percentage of the total quantity of paracetamol eliminated. The patients with GS were divided into 2 subgroups (GS-I and GS-II) according to whether glucuronidation was more or less than 50%. The overall results of the GS group showed no significant difference in the urinary elimination of metabolites as compared to the control group. However, in subgroup GS-I, a reduction in glucuronidation (P = 0.0012) and an increase in oxidation (P = 0.0051) was seen, as compared with the other 2 groups. There was inverse correlation between the glucuronide produced by conjugation and the oxidation products (r = -0.8718; P<0.005). People with GS are a heterogeneous group with respect to the metabolism of paracetamol. In one subgroup this was normal. In the other subgroup there was a marked reduction in glucuronidation and an increase in oxidation. These changes could mean that people in this subgroup are more liable to liver damage after an overdose of paracetamol.

AH receptor-controlled transcriptional regulation and function of rat and human UDP-glucuronosyltransferase isoforms

Transcriptional regulation and function of rat and human PAH-inducible UDP-glucuronosyltransferase (UGT) isoforms have been studied. 1. At least two PAH-inducible UGT isoforms are expressed in a variety of tissues, the rat isoforms UGT1A6 and UGT1A7, and the human isoforms UGT1A6 and UGT1A9. 2. For the rat and human UGT1A6 isoforms two modes of tissue- and cell-specific regulation were found, PAH-inducible and constitutive expression. 3. Transient transfection studies, using human UGT1A6/CAT fusion constructs and colon carcinoma Caco-2 cells, revealed that PAH induction of human UGT1A6 is mediated by the Ah receptor. 4. Cell-expressed UGT isoforms were used to study their function in PAH metabolism. Rat UGT1A7 and human UGT1A9 appear to be more efficient than the corresponding UGT1A6 isoforms in catalyzing glucuronide formation of PAH phenols and diphenols. Several isoforms may act together in the formation of benzo(a)pyrene-3.6-diol diglucuronide, the major glucuronide found in rat bile. The results suggest complex modes of transcriptional regulation of PAH-inducible UGTs. They also suggest a major role of these UGT isoforms in detoxication of PAHs.

Identification of the uridine diphosphate glucuronosyltransferase isoform UGT1A6 in rat brain and in primary cultures of neurons and astrocytes

The expression of a phenol uridine diphosphate glucuronosyltransferase (UGT) was investigated in rat brain homogenate and in primary cultures of astrocytes and neurons, by means of model substrates (1-naphthol and 4-methylumbelliferone) assays, Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR) experiments. Glucuronidation of these substances occurred in cerebral cell or brain homogenates, although to different extents. The specific activity was the highest in astrocytes, with values more than 10- and 100-fold those found in neurons or total brain, respectively. Using antibodies able to recognize several rat liver UGT isoforms, only one protein with an apparent molecular mass of 54 kDa was detected in astrocyte and neuron homogenates and brain microsomes. RT-PCR experiments run with primers specifically designed for the rat liver UGT1A6 revealed amplificons of the expected sizes in accordance with the presence of UGT1A6 mRNA. The nucleotide sequence of the 330-base pair product was 100% homologous to that of exon 1 of rat liver isoform UGT1A6. In conclusion, this work allowed us to identify for the first time a constitutive cerebral UGT isoform identical to rat liver UGT1A6, which glucuronidates planar phenolic substances in cultured astrocytes, neurons, and the entire brain.

Transcriptional activation of the UDP-glucuronosyltransferase 1A7 gene in rat liver by aryl hydrocarbon receptor ligands and oltipraz

UDP-glucuronosyltransferase UGT1A7 catalyzes the glucuronidation of benzo(a)pyrene metabolites and other bulky aromatic compounds. Both UGT1A7 mRNA and an associated enzyme activity (benzo(a)pyrene7, 8-dihydrodioltransferase activity) are markedly increased in livers of rats treated with beta-naphthoflavone or 4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (oltipraz). Nuclear runoff assays show that the effects of both inducers are primarily due to transcriptional activation. A 27-kilobase region that included the UGT1A7/UGT1A6 promoter regions was cloned. Primer extension and RNase protection studies indicated >/=30 transcription start sites in five clusters between bases -85 and -40 respective to the translation start codon. There was no recognizable TATA box, but the promoter region is TA-rich. Sequence analysis revealed potential binding sites for CCAAT enhancer-binding protein, activator protein 1, and hepatic nuclear factors 1, 3, and 4, but no xenobiotic response elements or antioxidant response elements, implicated in the regulation of other genes by beta-naphthoflavone or oltipraz, were found. A UGT1A7 gene reporter plasmid directed strong constitutive expression in transient transfection assays using primary rat hepatocytes. Treatment with 3-methylcholanthrene or oltipraz had no effect compared with similarly treated pGL3-Basic-transfected cells. These results suggest that the regulatory elements controlling xenobiotic inducibility of UGT1A7 transcription are located either 5' or 3' of bases -1600 to +54. One possibility is that the polycyclic aromatic-mediated regulation of UGT1A7 occurs via the xenobiotic response element flanking the UGT1A6 locus 7 kilobase pairs downstream.

Aryl hydrocarbon receptor-inducible or constitutive expression of human UDP glucuronosyltransferase UGT1A6

Transcriptional regulation of human UGT1A6, a UDP glucuronosyltransferase isoform conjugating a wide variety of planar phenols, has been studied using transfection experiments with plasmids containing its 3-kb 5' upstream region and chloramphenicol acetyltransferase as reporter gene. Previously, two modes of expression of the isoform have been described: in colon carcinoma Caco-2 cells UGT1A6 was found to be TCDD-inducible, whereas in lung carcinoma A549 cells it was constitutively expressed. Therefore functional analysis of UGT1A6 regulation was carried out using these two cell lines. In the upstream region of human UGT1A6 one xenobiotic-responsive element (XRE) was found between-1498 and -1502 bp. In Caco-2 cells the reporter gene activity of the entire plasmid and of deletion mutants containing the XRE were TCDD-inducible, in contrast to experiments with a deletion mutant which did not contain the XRE. TCDD induction was marginal in transfection studies with A549 cells. Gel mobility shift analysis indicated that the aryl hydrocarbon receptor and its partner Arnt bind to the XRE. Furthermore, primer extension studies suggest cell-specific use of multiple TATA boxes. Hence, regulation of human UGT1A6 appears to be cell-specific including both constitutive and aryl hydrocarbon receptor-controlled expression.

High induction of phenol UDP-glucuronosyltransferase in the kidney medulla of beta-naphthoflavone-treated rats

Phenol UDP-glucuronosyltransferase activity was highly induced in the microsomes of the kidney medulla of rats by beta-naphthoflavone treatment. In the medulla, phenol UDP-glucuronosyltransferase and its mRNA were greatly increased in both immunoblotting and Northern blot analyses following beta-naphthoflavone treatment of the rats. In untreated rat kidneys, phenol UDP-glucuronosyltransferase was detected by immunohistochemical analysis only in proximal convolution tubular cells located in the cortex. After beta-naphthoflavone treatment of the rats, UDP-glucuronosyltransferase appeared in the epithelial cells in the straight portion of the distal tubules located in the medulla. In conclusion, the medullary distal tubular cells have high latent glucuronidation activity and are thought to play an important role in drug excretion.

The influence of age and some inducers on UDP-glucuronyltransferase activity

UDP-glucuronyltransferase (UDP-GT) activity was examined in male Wistar rats aged 0.5, 1, 2, 4, 8, 12, 20, and 28 months. The rats were treated with phenobarbital (75 mg/kg, 72 and 48 h before death), beta-naphthoflavone or dexamethasone (40 mg/kg and 20 mg/kg, respectively, for three days before death). Prior to decapitation the rats were fasted for 12 h. Hepatic microsomes were prepared according to the method of Dallner. UDP-GT activity was determined by the method of Burchell and Weatherill. p-Nitrophenol was used as an aglucone. UDP-GT activity decreased rapidly in the control rats aged from two weeks to four months. In the older control rats the activity tended to increase. Two-week-old rats treated with phenobarbital showed a slightly increased UDP-GT activity. In the older animals (up to one year) UDP-GT activity increased to 150% of the control value and stayed at this level in the remaining age groups. beta-Naphthoflavone was a more potent inducer of UDP-GT than phenobarbital. The activity of beta-naphthoflavone-induced UDP-GT was low in the youngest rats. It was about 180% in two-month-old rats and reached 260% of the control value in eight-month-old rats. Although the activity decreased in the older rats, it still exceeded 200%. Dexamethasone did not affect UDP-GT activity. Only in two-week-old and two-month-old rats did we observe a slight increase in the activity of UDP-GT.

Molecular basis for deficient acetaminophen glucuronidation in cats. An interspecies comparison of enzyme kinetics in liver microsomes

Cats are highly susceptible to acetaminophen toxicity because of deficient glucuronidation of this drug in vivo. The enzyme kinetic basis for this defect is unknown. Therefore, the kinetic properties of acetaminophen UDP-glucuronosyltransferase (acetaminophen-UGT) were investigated, using hepatic microsomes from cats (N = 4) compared with those of species that are less sensitive to acetaminophen intoxication including dogs (N = 4), humans (N = 4), and six other mammalian species (one liver from each). Gunn rats were also studied, since they express defective UGT family 1 isoenzymes and are also prone to acetaminophen toxicity. Acetaminophen kinetics were biphasic in all instances with distinct high and low affinity components. Km values for the high affinity activity in cat microsomes (0.31 +/- 0.1 mM; mean +/- SEM) were intermediate between those of dogs (0.11 +/- 0.02 mM) and humans (0.60 +/- 0.06 mM) and other species (0.22 to 6.7 mM; range). On the other hand, high affinity Vmax values were over 10-fold less in cat microsomes (0.025 +/- 0.006 nmol/min/mg) than in dogs (0.92 +/- 0.09 nmol/min/mg) and humans (0.27 +/- 0.09 nmol/min/mg); and over 5-fold less compared with microsomes from other species (range 0.13 to 7.63 nmol/min/mg). Gunn rat microsomes showed a similar 10-fold difference in high affinity Vmax values between the homozygous mutant (0.67 nmol/min/mg) and homozygous normal (6.75 nmol/min/mg) animals. These results demonstrate that, relative to a number of other species, cats have remarkably low hepatic levels of a high affinity acetaminophen-UGT. This difference is sufficient enough to explain poor glucuronidation of acetaminophen in vivo and susceptibility to acetaminophen intoxication.

Biochemical basis for deficient paracetamol glucuronidation in cats: an interspecies comparison of enzyme constraint in liver microsomes

Unlike most other mammalian species, domestic cats glucuronidate phenolic compounds poorly and are therefore highly susceptible to the toxic side effects of many drugs, including paracetamol. In this study, we evaluated the role of enzyme constraint, a characteristic that limits the activity of all uridine 5'-diphosphoglucuronosyltransferase (UGT) enzymes, in the aetiology of this species-dependent defect of drug metabolism. Detergent activation experiments were performed using hepatic microsomes from cats (4), dogs (4), man (4), and 6 other mammalian species (1 liver each). In addition, we used microsomes from Gunn rats which are sensitive to paracetamol toxicity because of a genetic defect affecting all family 1 UGTs. Increase in paracetamol-UGT activity at optimum concentrations of detergent was used as an index of enzyme constraint. Native activity (measured in the absence of detergent) was less than one-sixth in cats compared with other species. Optimum detergent treatment tended to enhance rather than abolish this difference, however, indicating relatively lower levels of constraint of paracetamol-UGT in cats compared with other species. Similarly, detergent treatment failed to reduce the native activity difference between homozygous mutant and normal Gunn rats. Initially CHAPS (3-(3-cholamidopropyl)-dimethylammonio-1-propanesulphonic acid) was used as the detergent activator; in 3 of 4 microsomal preparations from man, however, inhibition rather than activation was observed at all detergent concentrations used. Studies were repeated using the non-ionic detergent, Brij 58 (polyoxyethylene 20-cetyl ether), which resulted in similar although more profound activation and no inhibition. We conclude that deficient paracetamol glucuronidation in cats does not result from increased paracetamol-UGT constraint in this species compared with other mammalian species. Other causes, such as differences in enzyme protein concentration or substrate affinity might be responsible.

Monometoxytrityl derivatives of uridine as inhibitors of a human recombinant UDP-glucuronosyltransferase: UGT1*6

A series of inhibitors of the human liver recombinant UDP-glucuronosyltransferase 1*6 derived from uridine were synthetized as probes of the binding site of the cosubstrate, UDP-glucuronic acid. If triphenylmethanol or uridine alone failed to inhibit the glucuronidation of 4-methylumbelliferone, the trityl derivatives of uridine were found to be very effective inhibitors of the enzyme (Ki 4.4 to 73 microM). The type of inhibition (competitive or mixed) varied with the substitutions on the uracile or on the triphenylmethyl moiety by halogen atoms or methyl groups. Structural features for the binding of the cofactor are postulated.

Expression and inducibility of UDP-glucuronosyltransferases 1-naphthol in human cultured hepatocytes and hepatocarcinoma cell lines

The UDP-glucuronosyltransferase (UGTs) isoforms involved in the conjugation of 1-naphthol were characterized in human cultured hepatocytes and in two human hepatocarcinoma cell lines, KYN-2 and Mz-Hep-1 in terms of expression, kinetics and induction by drugs. Their properties were compared to those of UGT1*6 stably expressed in the V79 cell line (V79UGT1*6), which glucuronidates 1-naphthol preferentially. The determination of kinetic constants for glucuronidation of 1-naphthol revealed a two-site model in human hepatocytes, but a one-site model in the two hepatocarcinoma cell lines. Southern blot analysis of RT-PCR products, showed that the UGT1*6 mRNA was expressed in KYN-2, but not in Mz-Hep-1 cells. However, a mRNA encoding a UGT different from UGT1*6 was expressed in Mz-Hep-1 cells. The two inducers, beta-naphthoflavone and rifampicin exerted a differential effect, depending on the cell lines considered. Altogether, the results suggest that, in hepatocytes, two UGT isoforms, which glucuronidate 1-naphthol are expressed and are differentialy regulated by inducers. Both KYN-2 and Mz-Hep-1 cells express one of the two different UGT isoforms found in hepatocytes. The UGT isoform present in KYN-2 cells corresponds to UGT1*6, whereas in Mz-Hep-1 cells the UGT isoform present was different from UGT1*6 and UGT1*7.

Specificity of human UDP-glucuronosyltransferases and xenobiotic glucuronidation

Several human liver UDP-Glucuronosyltransferases (UGTs) have been cloned and the cDNAs expressed in heterologous cell lines. This technological advance has allowed the assessment of the functional substrate specificity of these UGTs. The problems which may be encountered with the latency and assay of UGTs are briefly described. The data accumulated to date indicate that the Km, and possibly the Vmax/Km, for individual substrates are the best parameters to assess the specificity of the enzymes towards xenobiotic molecules. The substrate specificity of seven UGTs has been summarised from the currently available information. Of these, UGT1*02 and UGT2B8 appear to be key isoforms in the glucuronidation of a wide range of xenobiotic substrates. Additional UGTs have yet to be identified and characterised and their future inclusion may provide further insights. Finally, the functional role of each UGT in vivo has to be determined.

Glucuronidation in the Caco-2 human intestinal cell line: induction of UDP-glucuronosyltransferase 1*6

The ability of the differentiated human intestinal cell line, Caco-2, to glucuronidate various endobiotic and xenobiotic molecules was investigated. Glucuronidation of hydroxylated or carboxylic acid compounds such as 1-naphthol, thymol, androsterone, estriol, hyodeoxycholic acid, lithocholic acid, chloramphenicol, paracetamol and morphine could be determined in microsomal fractions of Caco-2 cells. The activity toward 1-naphthol was the highest glucuronidation activity measured in Caco-2 cells. This activity was specifically increased four-fold upon addition of beta-naphthoflavone into culture medium but not by rifampicine or clofibrate and was related to a biosynthesis of UDP-glucuronosyltransferase 1*6 (UGT1*6). alpha-Naphthoflavone did not affect the inducing property of beta-naphthoflavone. 7-Ethoxyresorufin-O-dealkylation activity, supported by cytochrome P4501A1, was induced more than 1000-times in Caco-2 cells by beta-naphthoflavone treatment, and this effect was partially abolished by alpha-naphthoflavone treatment. The results suggest that several isoforms, including UGT1*6, are expressed in Caco-2 cells.

Formation of mono- and diglucuronides and other glycosides of benzo(a)pyrene-3,6-quinol by V79 cell-expressed human phenol UDP-glucuronosyltransferases of the UGT1 gene complex

Glucuronidation of quinols of polycyclic aromatic hydrocarbons (PAHs) represents an important detoxication pathway preventing toxic quinone/quinol redox cycles. Therefore, mono- and diglucuronide formation of benzo(a)pyrene-3,6-quinol was investigated and compared to that of structurally related 3,6-dihydroxychrysene and simple phenols (1-naphthol and 4-methylumbelliferone) using V79 cell-expressed human UGT1.6 (= P1) and human UGT1.7 (= P4). Properties of human UGT1.6 were compared to those of the rat ortholog. Cofactors related to UDP-glucuronic acid such as UDP-galacturonic acid and UDP-glucose were also studied. It was found that rat and human UGT1.6 and human UGT1.7 catalyse monoglucuronide formation of planar PAH quinols. Diglucuronide formation was only detectable with human UGT1.7. The UGT isozymes studied also formed galacturonides and, although only to a minor extent, glucosides. Rat UGT1.6 (but not the human ortholog) catalysed digalacturonide formation of benzo(a)pyrene-3,6-quinol; the in vivo significance of galacturonide formation remains to be established. The results suggest that planar PAH phenols and quinols are conjugated more efficiently by human UGT1.7 than by UGT1.6, which preferentially conjugates simple planar phenols.

The chemical modification of human liver UDP-glucuronosyltransferase UGT1*6 reveals the involvement of a carboxyl group in catalysis

The treatment of UDP-glucuronosyltransferase UGT1*6 stably expressed in V79 cells with three carboxyl-specific reagents, dicyclohexylcarbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide and N-ethyl-5-phenylisoxazolium-3'-sulfonate (Woodward's reagent K), resulted in a fast, dose-dependent decrease of the 4-methylumbelliferone glucuronidation. The inactivation reactions followed pseudo-first order kinetics. The pKa of the modified residue was close to 5.0. A partial protection against inactivation by Woodward's reagent was observed at pH 7.4 in the presence of UDP-glucuronic acid, UDP, and, to a lesser extent, in the presence of 4-methylumbelliferone. Dicyclohexylcarbodiimide significantly decreased the Vmax, without affecting the apparent Km towards UDP-glucuronic acid and 4-methylumbelliferone. The results support the involvement of a carboxyl group in the catalytic process.

Tissue-specific constitutive and inducible expression of rat phenol UDP-glucuronosyltransferase

To investigate constitutive and inducible expression of rat phenol UDP-glucuronosyltransferase (UGT1A1) in liver and extrahepatic tissues, a selective cDNA probe for its unique exon 1 was utilized. 6-Hydroxychrysene was used as a functional probe of UGT1A1 activity. Constitutive expression of UGT1A1 was low in liver, but high in kidney, testis, epididymis and ovary. After treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10 micrograms/kg for 7 days) the UGT1A1 mRNA level was markedly increased in liver (ca. 10-fold), and only moderately enhanced (up to 2-fold) in extrahepatic tissues where constitutive enzyme expression was high. UGT activity toward 6-hydroxychrysene was strongly inducible in liver (ca. 9-fold) and only moderately inducible in extrahepatic tissues (up to 2-fold). The results suggest complex tissue-specific regulation of UGT1A1 including positive and negative transcriptional factors and marked inducibility by TCDD in liver.

Expression of the human UDP-glucuronosyltransferase UGT1*6 in Escherichia coli. Influence of bacterial signal peptides on the production and localization of the recombinant protein

The membrane-bound human liver UDP-glucuronosyltransferase UGT1*6 was expressed in Escherichia coli. Exchange of the natural signal peptide by the bacterial signal peptides of pclB or OmpT proteins considerably increased the level of expression and, as the natural signal peptide, targeted the protein to the membranes. The extent of maturation of SpelB-UGT1*6 precursor was about 30%. No processing of sOmpT-UGT1*6 occurred but the processing rate of this precursor could be significantly increased by mutagenesis of the first two amino acid residues of the mature sequence. These expression vectors allowed us to produce high levels of recombinant mature UGT1*6 required for further structural studies.

Glucuronidation and isomerization of all-trans- and 13-cis-retinoic acid by liver microsomes of phenobarbital- or 3-methylcholanthrene-treated rats

Glucuronidation and isomerization of all-trans-retinoic acid (tr-RA) and 13-cis-retinoic acid (13-cis-RA) were investigated in an in vitro system using liver microsomes of differently pretreated rats. In agreement with their thermodynamic stability, more retinoic acid was isomerized from the 13-cis form to the all-trans form than vice versa. Also some 9-cis-retinoic acid (9-cis-RA) could be found. Isomerization was reduced, but in contrast to glucuronidation was still important if boiled microsomes were used. This supports the view that isomerization can proceed as a non-enzymatic process. 3-Methylcholanthrene (MC) pretreatment of the rats increased the microsomal glucuronidation of 13-cis-RA and tr-RA and the formation of 13-cis-retinoyl-beta-glucuronide was enhanced up to 7-fold by MC-induced rat microsomes. The rates of glucuronidation by uninduced and phenobarbital-induced rat microsomes differed only slightly. In addition to glucuronides of the applied retinoic acid isomers (13-cis-RA and tr-RA), 9-cis-RA and its glucuronide were found. Induction of retinoid glucuronidation by pretreatment with MC indicates that this metabolic reaction is catalysed by a MC-inducible UGT isozyme. After two recently described pathways (conversions of retinol to retinal and of retinyl methyl ether to retinol) this is a third step of retinoid metabolism, induced by pretreatment with MC. With human microsomes no more than traces of glucuronides were detected; also, incubations with human microsomes resulted in a lower degree of isomerization than with rat microsomal fractions.