Glucuronidation of anabolic androgenic steroids by recombinant human UDP-glucuronosyltransferases

Non-steroidal anti-inflammatory drugs interact with testosterone glucuronidation

Testosterone and epitestosterone are secreted mainly as glucuronide metabolites and the urinary ratio of testosterone glucuronide to epitestosterone glucuronide, often called T/E, serves as a marker for possible anabolic steroids abuse by athletes. UDP-glucuronosyltransferase (UGT) 2B17 is the most important catalyst of testosterone glucuronidation. The T/E might be affected by drugs that interact with UGT2B17, or other enzymes that contribute to testosterone glucuronidation. Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used by sportsmen and we have examined the effect of two NSAIDs, diclofenac and ibuprofen, on testosterone and epitestosterone glucuronidation in human liver microsomes. In parallel, we have studied the inhibitory effect of these NSAIDs on recombinant UGT2B17 and UGT2B15, as well as other human hepatic UGTs that revealed low but detectable testosterone glucuronidation activity, namely UGT1A3, UGT1A4, UGT1A9 and UGT2B7. Both diclofenac and ibuprofen inhibited testosterone glucuronidation in microsomes, as well as UGT2B15 and UGT2B17. Interestingly, UGT2B15 was more sensitive than UGT2B17 to the two drugs, particularly to ibuprofen. Human liver microsomes lacking functional UGT2B17 exhibited significantly higher sensitivity to ibuprofen, suggesting that UGT2B15 plays a major role in the residual testosterone glucuronidation activity in UGT2B17-deficient individuals. Nonetheless, a minor contribution of other UGTs, particularly UGT1A9, to testosterone glucuronidation in such individuals cannot be ruled out at this stage. The epitestosterone glucuronidation activity of human liver microsomes was largely insensitive to ibuprofen and diclofenac. Taken together, the results highlight potential interactions between NSAIDs and androgen glucuronidation with possible implications for the validity of doping tests.

The red wine polyphenol resveratrol reduces polycyclic aromatic hydrocarbon-induced DNA damage in MCF-10A cells

Polycyclic aromatic hydrocarbons (PAH) are procarcinogens that can be commonly found in our food and environment. Upon biotransformation in our body system, they can cause DNA damage through the generation of genotoxic species and oxidative stress. Phase I and II enzymes are pivotal in the process of proximate carcinogen formation and elimination. Some dietary phytochemicals are strong inhibitors to the phase I enzymes. In the present study, we investigated the effect of the red wine compound resveratrol on DNA damage induced by PAH in a non-tumorigenic breast cell line MCF-10A. Resveratrol ranging from 1 to 5 μm could significantly suppress the expressions of cytochrome P450 (CYP) 1A1, CYP1B1 and UDP-glucuronosyltransferase (UGT) 1A1 induced by 7,12-dimethylbenz[a]anthracene (DMBA). The comet assay indicated that DMBA introduced DNA damage to these cells, and co-treatment of resveratrol at 5 or 10 μm could alleviate the damage. Further investigation illustrated that resveratrol reduced the binding of DMBA metabolites to DNA with no effect on DMBA-induced oxidative DNA damage. Since the phase II enzyme UGT1A1 was suppressed, the elimination of DMBA metabolites would not have contributed to the reduction in the DMBA metabolite–DNA binding. In summary, resveratrol might protect breast cells against PAH-induced DNA damage. The underlying mechanism was mediated by phase I enzyme suppression rather than phase II enzyme induction or oxidative DNA repair.

Characterization of human hepatic and extrahepatic UDP-glucuronosyltransferase enzymes involved in the metabolism of classic cannabinoids

Tetrahydrocannabinol (Delta(9)-THC), the primary psychoactive ingredient in marijuana, is subject to cytochrome P450 oxidation and subsequent UDP-glucuronosyltransferase (UGT)-dependent glucuronidation. Many studies have shown that CYP2C9 and CYP3A4 are the primary enzymes responsible for these cytochrome P450-dependent oxidations, but little work has been done to characterize phase II metabolic pathways. In this study, we test the hypothesis that there are specific human UGTs responsible for classic cannabinoid metabolism. The activities of 12 human recombinant UGTs toward classic cannabinoids [cannabinol (CBN), cannabidiol (CBD), (-)-Delta(8)-THC, (-)-Delta(9)-THC, (+/-)-11-hydroxy-Delta(9)-THC (THC-OH), and (-)-11-nor-9-carboxy-Delta(9)-THC (THC-COOH)] were evaluated using high-performance liquid chromatography-tandem mass spectrometry and labeling assays. Despite activity by UGT1A1, 1A3, 1A8, 1A9, 1A10, and 2B7 toward CBN, CBD, THC-OH, and THC-COOH, only selected UGTs demonstrate sufficient activity for further characterization of steady-state kinetics. CBN was the most recognized substrate as evidenced by activities from hepatic UGT1A9 and extrahepatic UGT1A7, UGT1A8, and UGT1A10. These results may reflect the introduction of an aromatic ring to Delta(9)-THC, leading to favorable pi stacking with phenylalanines in the UGT active site. Likewise, oxidation of Delta(9)-THC to THC-OH results in UGT1A9 and UGT1A10 activity toward the cannabinoid. Further oxidation to THC-COOH surprisingly leads to a loss in metabolism by UGT1A9 and UGT1A10, while creating a substrate recognized by UGT1A1 and UGT1A3. The resulting glucuronide of THC-COOH is the main metabolite found in urine, and thus these hepatic enzymes play a critical role in the metabolic clearance of cannabinoids. Taken together, glucuronidation of cannabinoids depends on upstream processing including enzymes such as CYP2C9 and CYP3A4.

Urinary metabolic profile of 19-norsteroids in humans: glucuronide and sulphate conjugates after oral administration of 19-nor-4-androstenediol

19-Nor-4-androstenediol (NOL) is a prohormone of nandrolone (ND). Both substances are included in the WADA List of Prohibited Classes of Substances and their administration is determined by the presence of 19-norandrosterone (NA) with the urinary threshold concentration of 2 ng mL(-1). Routine analytical procedures allow the determination of NA excreted free and conjugated with glucuronic acid, but amounts of ND and NOL metabolites are also excreted in the sulphate fraction. The aim of this study is to determine the urinary metabolic profile after oral administration of a nutritional supplement containing NOL. Urine samples were collected up to 96 h following supplement administration and were extracted to obtain separately three metabolic fractions: free, glucuronide and sulphate. Extraction with tert-butyl methyl ether was performed after the hydrolysis steps and trimethylsilyl derivatives were analyzed by gas chromatography/mass spectrometry (GC/MS). After oral administration of NOL, the main metabolites detected were NA and noretiocholanolone (NE) in the glucuronide and sulphate fractions. The relative abundances of each metabolite in each fraction fluctuate with time; a few hours after administration the main metabolite was NA glucuronide whereas in the last sample (4 days after administration) the main metabolite was the NA sulphate and the second was the NE glucuronide. During the studied period almost half of the dose was excreted and the main metabolites were still found in urine after 96 h. Norepiandrosterone and norepietiocholanolone were also detected only in the sulphate fraction. Our results suggest that sulphate metabolites should be taken into consideration in order to increase the retrospectivity in the detection of 19-norsteroids after oral administration.

Identification of hydroxywarfarin binding site in human UDP glucuronosyltransferase 1a10: phenylalanine90 is crucial for the glucuronidation of 6- and 7-hydroxywarfarin but not 8-hydroxywarfarin

Recent studies show that the extrahepatic human UDP-glucuronosyltransferase (UGT)1A10 is capable of phase II glucuronidation of several major cytochrome P450 metabolites of warfarin (i.e., 6-, 7-, and 8-hydroxywarfarin). This study expands on this finding by testing the hypothesis that the UGT1A10 F(90)-M(91)-V(92)-F(93) amino acid motif is important for proper recognition and conjugation of hydroxywarfarin derivatives. Site-directed mutagenesis studies demonstrate that F(90) is critical for 6- and 7-hydroxywarfarin glucuronidation based on the complete loss of enzymatic activity toward these substrates. In contrast, V92A and F93A mutants lead to higher rates of substrate turnover, have minimum changes in K(m) values, and demonstrate substrate inhibition kinetics. A completely different activity profile is observed in the presence of 8-hydroxywarfarin. No change in either activity or affinity is observed with F90A when compared with wild type, whereas F93A and V92A mutants show increases in V(max) (3- and 10-fold, respectively) and minimum changes in K(m). Liquid chromatographytandem mass spectrometry studies show that enzymatic products produced by mutants are identical to wild-type products produced in the presence of 6-, 7-, and 8-hydroxywarfarin. Because F(90) is not critical for the glucuronidation of 8-hydroxywarfarin, there is likely another, different amino acid responsible for binding this compound. In addition, an inhibitory binding site may be formed in the presence of 6- and 7-hydroxywarfarin. This new knowledge and continued characterization of the hydroxywarfarin binding site(s) for UGT1A10 will help elucidate the molecular mechanism of hydroxywarfarin glucuronidation and potentially result in more effective anticoagulant therapies.

Liquid chromatographic-mass spectrometric analysis of glucuronide-conjugated anabolic steroid metabolites: method validation and interlaboratory comparison

Liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) method for simultaneous and direct detection of 12 glucuronide-conjugated anabolic androgenic steroid (AAS) metabolites in human urine is described. The compounds selected were the main metabolites detected in human urine after dosing of the most widely abused AAS in sports, e.g. methandienone, methenolone, methyltestosterone, nandrolone and testosterone, and certain deuterium-labeled analogs of these metabolites. Sample preparation and the LC-ESI-MS/MS method were optimized, validated, and the overall process was implemented and the results between seven laboratories were compared. All the metabolites were extracted simultaneously by solid-phase extraction (SPE) and analyzed by LC-ESI-MS/MS with positive ionization mode and multiple reaction monitoring (MRM). Recovery of the SPE for the AAS glucuronides was 89-100% and ten out of twelve compounds had detection limits in the range of 1-10 ng/ml in urine. The results for inter/intraday repeatability were satisfactory and the interlaboratory comparison with authentic urine samples demonstrated the ease of method transfer from one instrument setup to another. When equivalent triple quadrupole analyzers were employed the overall performance was independent from instrument manufacturer, electrospray ionisation (ESI) or atmospheric pressure chemical ionization (APCI) and liquid chromatohraphic (LC) column, whereas major differences were encountered when changing from one analyzer type to another, especially in the analysis of those AAS glucuronides ionized mainly as adducts.

Screening of in vitro synthesised metabolites of 4,9,11-trien-3-one steroids by liquid chromatography mass spectrometry

The aim of the work was to develop a flexible in vitro synthesis procedure, which can be applied in order to study and predict the metabolic patterns of new derivatives of anabolic androgenic steroids (AAS) with respect to most prominent target compounds for doping control purposes. Microsomal and S9 fraction of human liver preparations were used as a source of metabolising enzymes and the co-substrates of the synthesis mixture were selected to favour phase-I metabolic reactions and glucuronidation as phase-II conjugation reactions. Model compounds within the study were 4,9,11-trien-3-one steroids, structural derivatives of gestrinone and trenbolone, which both are included in the list of prohibited compounds in sports by the World Anti-Doping Agency (WADA). The correlation between in vitro metabolism of human microsomes and in vivo excretion studies in human was compared with gestrinone and subsequently, the applicability of the in vitro model for prediction of AAS metabolic pathways for new doping agents was evaluated. All the AAS examined within this study were successfully metabolised using the developed in vitro model, hydroxylation, reduction and glucuronide conjugation being the most prominent reaction pathways. Hydroxylated and glucuronide-conjugated metabolites of in vivo experiment with gestrinone were the same metabolites formed in the enzyme-driven process, thus showing good in vitro-in vivo correlation. Liquid chromatographic-mass spectrometric and tandem mass spectrometric methods were developed, relying on the positive polarity of electrospray ionisation, which also allowed the direct detection of intact glucuronide-conjugated AAS metabolites. Due to charge delocalisation and high proton affinity, the developed method was proven effective in the analysis of AAS metabolites bearing extensive conjugated double bond systems in their structures.

The first aspartic acid of the DQxD motif for human UDP-glucuronosyltransferase 1A10 interacts with UDP-glucuronic acid during catalysis

All UDP-glucuronosyltransferase enzymes (UGTs) share a common cofactor, UDP-glucuronic acid (UDP-GlcUA). The binding site for UDP-GlcUA is localized to the C-terminal domain of UGTs on the basis of amino acid sequence homology analysis and crystal structures of glycosyltransferases, including the C-terminal domain of human UGT2B7. We hypothesized that the (393)DQMDNAK(399) region of human UGT1A10 interacts with the glucuronic acid moiety of UDP-GlcUA. Using site-directed mutagenesis and enzymatic analysis, we demonstrated that the D393A mutation abolished the glucuronidation activity of UGT1A10 toward all substrates. The effects of the alanine mutation at Q(394),D(396), and K(399) on glucuronidation activities were substrate-dependent. Previously, we examined the importance of these residues in UGT2B7. Although D(393) (D(398) in UGT2B7) is similarly critical for UDP-GlcUA binding in both enzymes, the effects of Q(394) (Q(399) in UGT2B7) to Ala mutation on activity were significant but different between UGT1A10 and UGT2B7. A model of the UDP-GlcUA binding site suggests that the contribution of other residues to cosubstrate binding may explain these differences between UGT1A10 and UGT2B7. We thus postulate that D(393) is critical for the binding of glucuronic acid and that proximal residues, e.g., Q(394) (Q(399) in UGT2B7), play a subtle role in cosubstrate binding in UGT1A10 and UGT2B7. Hence, this study provides important new information needed for the identification and understanding of the binding sites of UGTs, a major step forward in elucidating their molecular mechanism.

Glucuronidation of monohydroxylated warfarin metabolites by human liver microsomes and human recombinant UDP-glucuronosyltransferases

Our understanding of human phase II metabolic pathways which facilitate detoxification and excretion of warfarin (Coumadin) is limited. The goal of this study was to test the hypothesis that there are specific human hepatic and extrahepatic UDP-glucuronosyltransferase (UGT) isozymes, which are responsible for conjugating warfarin and hydroxylated metabolites of warfarin. Glucuronidation activity of human liver microsomes (HLMs) and eight human recombinant UGTs toward (R)- and (S)-warfarin, racemic warfarin, and major cytochrome P450 metabolites of warfarin (4'-, 6-, 7-, 8-, and 10-hydroxywarfarin) has been assessed. HLMs, UGT1A1, 1A8, 1A9, and 1A10 showed glucuronidation activity toward 4'-, 6-, 7-, and/or 8-hydroxywarfarin with K(m) values ranging from 59 to 480 microM and V(max) values ranging from 0.03 to 0.78 microM/min/mg protein. Tandem mass spectrometry studies and structure comparisons suggested glucuronidation was occurring at the C4'-, C6-, C7-, and C8-positions. Of the hepatic UGT isozymes tested, UGT1A9 exclusively metabolized 8-hydroxywarfarin, whereas UGT1A1 metabolized 6-, 7-, and 8-hydroxywarfarin. Studies with extrahepatic UGT isoforms showed that UGT1A8 metabolized 7- and 8-hydroxywarfarin and that UGT1A10 glucuronidated 4'-, 6-, 7-, and 8-hydroxywarfarin. UGT1A4, 1A6, 1A7, and 2B7 did not have activity with any substrate, and none of the UGT isozymes evaluated catalyzed reactions with (R)- and (S)-warfarin, racemic warfarin, or 10-hydroxywarfarin. This is the first study identifying and characterizing specific human UGT isozymes, which glucuronidate major cytochrome P450 metabolites of warfarin with similar metabolic rates known to be associated with warfarin metabolism. Continued characterization of these pathways may enhance our ability to reduce life-threatening and costly complications associated with warfarin therapy.

Steroid metabolism and profile of steroidogenic gene expression in Episkin: high similarity with human epidermis

The skin is a well-recognized site of steroid formation and metabolism. Episkin is a cultured human epidermis. In this report, we investigate whether Episkin possesses a steroidogenic machinery able to metabolize adrenal steroid precursors into active steroids. Episkin was incubated with [14C]-dehydroepiandrosterone (DHEA) and 4-androstenedione (4-dione) and their metabolites were analyzed by liquid chromatography/mass spectrometry (LC/MS/MS). The results show that the major product of DHEA metabolism in Episkin is DHEA sulfate (DHEAS) (88% of the metabolites) while the other metabolites are 7alpha-OH-DHEA (8.2%), 4-dione (1.3%), 5-androstenediol (1.3%), dihydrotestosterone (DHT) (1.4%) and androsterone (ADT) (2.3%). When 4-dione is used as substrate, much higher levels of C19-steroids are produced with ADT representing 77% of the metabolites. These data indicate that 5alpha-reductase, 17beta-hydroxysteroid dehydrogenase (17beta-HSD) and 3alpha-hydroxysteroid dehdyrogenase (3alpha-HSD) activities are present at moderate levels in Episkin, while 3beta-HSD activity is low and represents a rate-limiting step in the conversion of DHEA into C19-steroids. Using realtime PCR, we have measured the level of mRNAs encoding the steroidogenic enzymes in Episkin. A good agreement is found between the mRNAs expression in Episkin and the metabolic profile. High expression levels of steroid sulfotransferase SULT2B1B and type 3 3alpha-HSD (AKR1C2) correspond to the high levels of DHEA sulfate (DHEAS) and ADT formed from DHEA and 4-dione, respectively. 3beta-HSD is almost undetectable while the other enzymes such as type 1 5alpha-reductase, types 2, 4, 5, 7, 8, and 10 17beta-HSD and 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) (AKR1C1) are highly expressed. Except for UGT-glucuronosyl transferase, similar mRNA expression profiles between Episkin and human epidermis are observed.

Differential effect of over-expressing UGT1A1 and CYP1A1 on xenobiotic assault in MCF-7 cells

Gene mutation has been considered as a major step of carcinogenesis. Some defective genes may induce spontaneous tumorigenesis, while others are required to interact with the environment to induce cancer. CYP1A1 and UGT1A1 are encoded for the respective phase I and II drug-metabolizing enzymes. Their expressions have been associated with breast cancer incidence in women, and some xenobiotics are substrates of these two enzymes. In the current study, cytochrome P450 (CYP) 1A1 and UDP-glucuronosyltransferase (UGT) 1A1 were over-expressed in the breast cancer MCF-7 cells, and potential interactions between these enzymes and estrogen or polycyclic aromatic hydrocarbon were evaluated. Compared with control cells (MCF-7(VEC)), reduced cell proliferation was seen in cells expressing UGT1A1 (MCF-7(UGT1A1)) under estradiol treatment. 7,12-Dimethylbenz[a]anthracene (DMBA) is an established breast cancer initiator in animal model. Over-expressing UGT1A1 reduced the binding of DMBA to DNA, and increased MCF-7(UGT1A1) intact cells under DMBA treatment was verified by comet assay. On the other hand, intensified DMBA binding and damages were observed in MCF-7(CYP1A1) cells. This study supported that UGT1A1 but not CYP1A1 expression could protect against xenobiotic assault.

Interactions with other human UDP-glucuronosyltransferases attenuate the consequences of the Y485D mutation on the activity and substrate affinity of UGT1A6

OBJECTIVES

To explore the possible role of hetero-oligomerization among the human UDP-glucuronosyltransferases in attenuating the consequences of the pathological Y486D mutation (UGT1A1 numbering) that often causes hyperbilirubinaemia. Owing to exon sharing in the human UGT1A gene, the equivalent mutation is present in all other UGT1As of the affected individuals. It is unknown, however, if this mutation results in clinical conditions, other than impaired bilirubin conjugation by UGT1A1.

METHODS

The main experimental approach in this study was to try and form hetero-oligomers of selected UDP-glucuronosyltransferases by coinfecting insect cells with recombinant baculoviruses that encode different human UDP-glucuronosyltransferases and mutants thereof. The infected cells were analysed for both relative expression levels and catalytic activity in each case, the combination of which yielded normalized activity. Kinetic analyses and copurification by affinity chromatography were also performed.

RESULTS

Coinfections with UGT1A4 increased the normalized scopoletin glucuronidation of 6YD (the Y485D mutant of UGT1A6) much more than it affected 1YD (the Y486D mutant of UGT1A1). Serotonin glucuronidation analyses revealed that coexpression of 6YD with most other human UDP-glucuronosyltransferases significantly increased the normalized activity of this mutant. Using 1-naphthol as the aglycone substrate, the Km of 6YD for the cosubstrate UDP-glucuronic acid was about 50 times higher than in UGT1A6. Yet, coexpression of 6YD with UGT1A4 lowered the Km for UDP-glucuronic acid to the level of UGT1A6. Coexpression also influenced wild-type UGT1A6 and UGT2B7, increasing the normalized activity of UGT1A6, but decreasing it for UGT2B7.

CONCLUSION

Hetero-oligomerization may play an important role in UDP-glucuronosyltransferases activity.

Phenylalanine(90) and phenylalanine(93) are crucial amino acids within the estrogen binding site of the human UDP-glucuronosyltransferase 1A10

Human UDP-glucuronosyltransferase 1A10 has been identified as the major isoform involved in the biotransformation of a wide range of phenolic substrates, including native estrogens and their oxidized metabolites. Our recent studies point to the F(90)-M(91)-V(92)-F(93) amino acid motif of UGT1A10, which was identified using photoaffinity labeling followed by LC-MS/MS analysis, as a key determinant of the binding of phenolic substrates. In this report, we have evaluated the role of F(90), V(92), and F(93) in the recognition of estrogens by UGT1A10 using site-directed mutagenesis. Kinetic studies using five mutants revealed that F(90) and F(93) are critical residues for the recognition of all estrogen substrates. The substitution of F(90) with alanine totally abolished the activity of this enzyme toward all the estrogens investigated. Overall, sequential removal for the aromatic ring (F to L) and of the hydrophobic chain (F to A and V to A) from amino acids 90, 92, and 93 effectively alters estrogen recognition. This demonstrates that individual features of the native and hydroxylated estrogens determine the specific binding properties of the compound within the binding site of the human UGT1A10 and the mutants. The resulting activities are completely abolished, unchanged, increased, or decreased depending on the structures of both the mutant and the substrate. The novel identification of UGT1A10 as the major isoform involved in the glucuronidation of all estrogens and the discovery of the importance of the FMVF motif in the binding of steroids will help to elucidate the molecular mechanism of glucuronidation, resulting in the design of more effective estrogen-based therapies.

The Glucuronidation of Δ4-3-Keto C19- and C21-Hydroxysteroids by Human Liver Microsomal and Recombinant UDP-glucuronosyltransferases (UGTs): 6α- and 21-Hydroxyprogesterone Are Selective Substrates for UGT2B7

The stereo- and regioselective glucuronidation of 10 Delta(4)-3-keto monohydroxylated androgens and pregnanes was investigated to identify UDP-glucuronosyltransferase (UGT) enzyme-selective substrates. Kinetic studies were performed using human liver microsomes (HLMs) and a panel of 12 recombinant human UGTs as the enzyme sources. Five of the steroids, which were hydroxylated in the 6beta-, 7alpha-, 11beta- or 17alpha-positions, were not glucuronidated by HLMs. Of the remaining compounds, comparative kinetic and inhibition studies indicated that 6alpha- and 21-hydroxyprogesterone (OHP) were glucuronidated selectively by human liver microsomal UGT2B7. 6alpha-OHP glucuronidation by HLMs and UGT2B7 followed Michaelis-Menten kinetics, whereas 21-OHP glucuronidation by these enzyme sources exhibited positive cooperativity. UGT2B7 was also identified as the enzyme responsible for the high-affinity component of human liver microsomal 11alpha-OHP glucuronidation. In contrast, UGT2B15 and UGT2B17 were the major forms involved in human liver microsomal testosterone 17beta-glucuronidation and the high-affinity component of 16alpha-OHP glucuronidation. Activity of UGT1A subfamily enzymes toward the hepatically glucuronidated substrates was generally low, although UGT1A4 and UGT1A9 contribute to the low-affinity components of microsomal 16alpha- and 11alpha-OHP glucuronidation, respectively. Interestingly, UGT1A10, which is expressed only in the gastrointestinal tract, exhibited activity toward most of the glucuronidated substrates. The results indicate that 6alpha- and 21-OHP may be used as selective "probes" for human liver microsomal UGT2B7 activity and, taken together, provide insights into the regio- and stereoselectivity of hydroxysteroid glucuronidation by human UGTs.

Prominent but Reverse Stereoselectivity in Propranolol Glucuronidation by Human UDP-Glucuronosyltransferases 1A9 and 1A10

Propranolol is a nonselective beta-adrenergic blocker used as a racemic mixture in the treatment of hypertension, cardiac arrhythmias, and angina pectoris. For study of the stereoselective glucuronidation of this drug, the two propranolol glucuronide diastereomers were biosynthesized, purified, and characterized. A screen of 15 recombinant human UDP-glucuronosyltransferases (UGTs) indicated that only a few isoforms catalyze propranolol glucuronidation. Analysis of UGT2B4 and UGT2B7 revealed no significant stereoselectivity, but these two enzymes differed in glucuronidation kinetics. The glucuronidation kinetics of R-propranolol by UGT2B4 exhibited a sigmoid curve, whereas the glucuronidation of the same substrate by UGT2B7 was inhibited by substrate concentrations above 1 mM. Among the UGTs of subfamily 1A, UGT1A9 and UGT1A10 displayed high and, surprisingly, opposite stereoselectivity in the glucuronidation of propranolol enantiomers. UGT1A9 glucuronidated S-propranolol much faster than R-propranolol, whereas UGT1A10 exhibited the opposite enantiomer preference. Nonetheless, the Km values for the two enantiomers, both for UGT1A9 and for UGT1A10, were in the same range, suggesting similar affinities for the two enantiomers. Unlike UGT1A9, the expression of UGT1A10 is extrahepatic. Hence, the reverse stereoselectivity of these two UGTs may signify specific differences in the glucuronidation of propranolol enantiomers between intestine and liver microsomes. Subsequent experiments confirmed this hypothesis: human liver microsomes glucuronidated S-propranolol faster than R-propranolol, whereas human intestine microsomes glucuronidated S-propranolol faster. These findings suggest a contribution of intestinal UGTs to drug metabolism, at least for UGT1A10 substrates.

Study of the genetic determinants of UGT1A1 inducibility by phenobarbital in cultured human hepatocytes

UGT1A1 is induced by phenobarbital. We investigated whether three common UGT1A1 variants are associated with the variability in UGT1A1 inducibility. Human hepatocytes were incubated with 2 mM phenobarbital for 2 and 6 days followed by 5 microM SN-38 (1 h), a UGT1A1 probe. SN-38 glucuronidation in the cell media was measured by high-performance liquid chromatography. Three UGT1A1 promoter variants [-53(TA)(6>7), -3156G > A and -3279T > G] were genotyped. Significant induction of UGT1A1 catalytic activity was observed in 82% and 100% of the cultures treated with phenobarbital for 2 days (median fold-induction = 1.6, range 1.3-2.8; n = 28) and 6 days (median fold-induction = 2.8, range 1.6-6.4; n = 16), respectively. After 2 days of treatment, a negative correlation was observed between the UGT1A1 basal activities and the fold-induction (Spearman r = -0.52, P < 0.005). By contrast, the UGT1A1 activities in the basal and induced states were highly correlated (Spearman r = 0.95, P < 0.0001). Similar results were observed after 6 days of treatment. The allele frequencies were not significantly different between induced (n = 22) and non-induced preparations (n = 6) (P > 0.05). The fold-induction was not associated with any variants (P > 0.05). The basal and induced activities were correlated with -53(TA)(6>7) (and with -3156G > A due to almost complete linkage with the -53 indel) (P = 0.001). No association was found with the -3279T > G single nucleotide polymorphism (P > 0.05). The indel at -53 affects the basal phenotype and appears to limit the hepatocyte capability of maximal induction after phenobarbital. However, variants at -53, -3156 and -3279 are not associated with variability in UGT1A1 inducibility.

Rapid simultaneous determination of metabolic clearance of multiple compounds catalyzed in vitro by recombinant human UDP-glucuronosyltransferases

The purpose of this study was to test the applicability of n-in-one (cocktail) incubations in the determination of intrinsic clearance (Cl(int)) as the slope of the linear portion of the Michaelis-Menten curve (velocity V vs. substrate concentration [S]) where substrate concentrations were low. A rapid, sensitive, and selective liquid chromatography tandem mass spectrometry (LC/MS/MS) method was developed for the analysis of samples produced by single-substrate and n-in-one (seven substrates: entacapone, 17beta-estriol, umbelliferone, 4-methylumbelliferone, tolcapone, hydroxyquinoline, and paracetamol) incubations conducted in 96-well plates with different recombinant UDP-glucuronosyltransferases (UGTs). The Cl(int) values obtained with n-in-one incubations were compared with those obtained in single-compound incubations and with V(max)/K(m) values determined by estimating the enzyme kinetic parameters V(max) and K(m) from the Michaelis-Menten curve. When substrate concentrations were well below their K(m) values, Cl(int) values determined as the slope of the linear part of the Michaelis-Menten fitting correlated well with the values determined as V(max)/K(m) ratios from the Michaelis-Menten curve. The correlation between Cl(int) values determined in single-substrate and n-in-one incubations was high as well. Together, the n-in-one incubations, the determination of Cl(int) values as the slope of the linear part of the Michaelis-Menten fitting, and LC/MS/MS as an analytical method proved to be effective approaches for increasing throughput in the first-phase screening of metabolic properties.

KINETIC CHARACTERIZATION OF THE 1A SUBFAMILY OF RECOMBINANT HUMAN UDP-GLUCURONOSYLTRANSFERASES

The initial glucuronidation rates were determined for eight recombinant human UDP-glucuronosyltransferases (UGTs) of the 1A subfamily, and the bisubstrate kinetics and inhibition patterns were analyzed. At low substrate concentrations, the reactions followed general ternary complex kinetics, whereas at higher concentrations of both substrates, the reactions were mostly characterized by ternary complex kinetics with substrate inhibition. The glucuronidation of entacapone by UGT1A9 was inhibited by 1-naphthol in a competitive fashion, with respect to entacapone, and an uncompetitive fashion, with respect to UDP-glucuronic acid (UDPGA). Its inhibition by UDP, on the other hand, was noncompetitive with respect to entacapone and competitive with respect to UDPGA. These inhibition patterns are compatible with a compulsory ordered bi bi mechanism in which UDPGA is the first-binding substrate. Despite the identical primary structure of the C-terminal halves of the UGT1A isoforms, there were marked differences in the respective K(m) values for UDPGA, ranging from 52 microM for UGT1A6 to 1256 microM for UGT1A8. Relative specificity constants were calculated for the eight UGT1A isoforms with 1-hydroxypyrene, 4-nitrophenol, scopoletin, 4-methylumbelliferone, and entacapone as aglycone substrates. The results demonstrated that seven of the UGT1A isoforms are capable of conjugating phenolic substrates with similar highest k(cat) values, and UGT1A4 has a lower relative turnover rate. The highest specificity constants were obtained for 1-hydroxypyrene, even with UGT1A6, which has been regarded as a specific isoform for small planar phenols. A k(cat) value of 1.9 s(-1) was calculated for the glucuronidation of scopoletin by purified UGT1A9.

BIOSYNTHESIS OF DOBUTAMINE MONOGLUCURONIDES AND GLUCURONIDATION OF DOBUTAMINE BY RECOMBINANT HUMAN UDP-GLUCURONOSYLTRANSFERASES

Selected aspects of dobutamine glucuronidation were studied in detail. There are potentially four sites at which dobutamine can be conjugated to glucuronic acid. Three of the four dobutamine monoglucuronides that can be formed were enzymatically synthesized using pig liver microsomes, isolated, and characterized by tandem mass spectrometry, and (1)H and (13)C NMR spectroscopy. Analysis of dobutamine glucuronidation by liver microsomes from various sources revealed large variability in the ratios of the three regioisomers. Interestingly, catecholic dobutamine meta-O-glucuronide, by far the major product synthesized with human liver microsomes, was only a minor product for rat liver microsomes. Rabbit liver microsomes yielded diglucuronides, in addition to monoglucuronides. Activities of individual recombinant human UDP-glucuronosyltransferases (UGTs) were investigated, and the results suggested that dobutamine glucuronidation in the human liver is mainly carried out by UGTs 2B7 and 1A9. Among the extrahepatic UGTs, the formation of monoglucuronides, mainly catecholic meta-O-glucuronide, by UGTs 1A7 and 1A8 was similar to that by 1A9, whereas UGT1A10 also efficiently catalyzed the formation of catecholic dobutamine para-O-glucuronide.

The interactions between the N-terminal and C-terminal domains of the human UDP-glucuronosyltransferases are partly isoform-specific, and may involve both monomers

The pathological mutation Y486D was previously shown to reduce the activities of the UDP-glucuronosyltransferases (UGTs) 1A1 and 1A6 by about 88% and 99%, respectively. Surprisingly, the corresponding mutation in UGT1A9 (Y483D) doubled the Vmax of scopoletin glucuronidation, whereas the entacapone glucuronidation rate was decreased by about 50%. Due to the primary structure identity of the C-terminal half of all the human UGTs of the 1A subfamily, the sharp differences between them in the effect of a mutation deep inside the C-terminal half suggested that there are isoform-specific interactions between the variable N- and the conserved C-terminal halves. In dimeric enzymes, like the UGTs, such interactions might either occur within the same polypeptide, or between opposite monomers. The latter implies functional monomer-monomer interactions, and this was investigated using hetero-dimeric UGTs. Insect cells were co-infected with mixtures containing different combinations of recombinant baculoviruses encoding either UGT1A4 or 1A9Sol. The UGT1A4 was selected because it glucuronidates neither entacapone nor scopoletin at significant rates. The active enzyme in these hetero-dimers was 1A9Sol, a truncation mutant of UGT1A9 that exhibited a very low ratio of entacapone to scopoletin glucuronidation rates. Interestingly, the ratio of entacapone to scopoletin glucuronidation rates in the co-infected cells was dependent on, and markedly increased with, the probability that 1A9Sol forms hetero-dimers with UGT1A4. In addition, the apparent Km for entacapone in the hetero-dimers was much lower than in 1A9Sol, and resembled the corresponding value in full-length UGT1A9. The results, thus, revealed important monomer-monomer interactions within the UGTs.

Glucuronidation of oxidized fatty acids and prostaglandins B1 and E2 by human hepatic and recombinant UDP-glucuronosyltransferases

Arachidonic acid (AA) can be metabolized to various metabolites, which can act as mediators of cellular processes. The objective of this work was to identify whether AA, prostaglandin (PG) B1 and E2, and 15- and 20-hydroxyeicosatetraenoic acids (15- and 20-HETE) are metabolized via glucuronidation. Assays with human recombinant UDP-glucuronosyltransferase 1A (UGT1A) isoforms revealed that AA and 15-HETE were glucuronidated by UGT1A1, 1A3, 1A4, 1A9, and 1A10, whereas 20-HETE was glucuronidated by UGT1A1 and 1A4 and PGB1 was glucuronidated by UGT1A1, 1A9, and 1A10. All substrates were glucuronidated by recombinant UGT2B7, with AA and 20-HETE being the best substrates. Kinetic analysis of UGT1A1 and 1A9 with AA resulted in Km values of 37.9 and 45.8 microM, respectively. PGB1 was glucuronidated by UGT1A1 with a Km of 26.3 microM. The Km values for all substrates with UGT2B7 were significantly higher than with the UGT1A isoforms. Liquid chromatography-mass spectrometry of glucuronides biosynthesized from PGB1 and 15-HETE showed that hydroxyl groups were the major target of glucuronidation. This work demonstrates a novel metabolic pathway for HETEs and PGs and the role of UGT1A isoforms in this process. These results indicate that glucuronidation may play a significant role in modulation of the availability of these fatty acid derivatives for cellular processes.

UDP-Glucuronosyltransferase (UGT) 2B15 Pharmacogenetics: UGT2B15 D85Y Genotype and Gender Are Major Determinants of Oxazepam Glucuronidation by Human Liver

Oxazepam is a commonly used 1,4-benzodiazepine anxiolytic drug that is polymorphically metabolized in humans. However, the molecular basis for this phenomenon is currently unknown. We have previously shown that S-oxazepam glucuronide, the major oxazepam metabolite, is selectively formed by UDP-glucuronosyltransferase (UGT) 2B15, whereas the minor R-oxazepam glucuronide is produced by multiple UGTs other than UGT2B15. Phenotype-genotype studies were conducted using microsomes and DNA prepared from the same set of 54 human livers. Sequencing of the UGT2B15 gene revealed three nonsynonymous polymorphisms, D85Y, T352I, and K523T, with variant allele frequencies of 0.56, 0.02, and 0.40, respectively. D85Y genotype showed a significant effect (p = 0.012) on S-oxazepam glucuronidation with lower median activities in 85Y/Y livers (49 pmol/min/mg protein) compared with 85D/D livers (131 pmol/min/mg), whereas 85D/Y livers were intermediate in activity (65 pmol/min/mg). There was also a significant trend (p = 0.049) for higher S-oxazepam activities in the two 352T/I livers (135 and 210 pmol/min/mg) compared with the remaining 352T/T livers (median, 64 pmol/min/mg). Conversely, K523T genotype had no apparent effect on oxazepam glucuronidation (p > 0.05). Donor gender also significantly influenced S-oxazepam glucuronidation with higher median activities in male (65 pmol/min/mg) compared with female (39 pmol/min/ mg) livers (p = 0.042). R-Oxazepam glucuronidation was not affected by either genotype or gender (p > 0.05). In conclusion, gender and D85Y genotype are identified as major determinants of S-oxazepam glucuronidation by human liver and may explain in part polymorphic oxazepam glucuronidation by human subjects.

An Active and Water-Soluble Truncation Mutant of the Human UDP-Glucuronosyltransferase 1A9

The UDP-glucuronosyltransferases (UGTs) are integral membrane proteins, and previous attempts to generate a water-soluble UGT by removing the single trans-membrane helix yielded inactive and membrane-bound proteins. We have now replaced the 45 C-terminal amino acids of the human UGT1A9, including its trans-membrane helix, with a fusion peptide ending with six His residues. Detergent-free extraction of insect cells expressing this mutant, UGT1A9Sol, released scopoletin glucuronidation activity into the supernatant, and subsequent ultracentrifugation did not sediment that activity. UGT1A9Sol was purified by immobilized metal affinity chromatography (IMAC) in the absence of detergents throughout the entire process. The IMAC purification increased somewhat the apparent K(m) of UGT1A9 toward scopoletin and rendered the enzyme sensitive to freezing. The activity of UGT1A9Sol in the cell extract was partly inhibited by Triton X-100, irrespective of the presence or absence of phospholipids. UGT1A9Sol exhibited a relatively high rate of scopoletin glucuronidation, whereas its activity toward 1-naphthol, entacapone, umbelliferone, and 4-nitrophenol was much lower. The kinetics and substrate specificity of UGT1A9Sol resembled the detergent-suspended full-length UGT1A9 rather than the membrane-bound UGT1A9. The apparent K(m) value of UGT1A9Sol for scopoletin was similar to that of the full-length UGT1A9 in the presence of detergent, but much higher than the respective value in the membrane-bound enzyme. The results suggest that either the detergent binding to the trans-membrane helix within the full-length UGT1A9, or the removal of this helix by gene manipulation, affect the interaction of the enzyme with its aglycone substrate in a similar manner.