Amino terminal domains of human UDP-glucuronosyltransferases (UGT) 2B7 and 2B15 associated with substrate selectivity and autoactivation

A multicentric consortium study demonstrates that dimethylarginine dimethylaminohydrolase 2 is not a dimethylarginine dimethylaminohydrolase

Dimethylarginine dimethylaminohydrolase 1 (DDAH1) protects against cardiovascular disease by metabolising the risk factor asymmetric dimethylarginine (ADMA). However, the question whether the second DDAH isoform, DDAH2, directly metabolises ADMA has remained unanswered. Consequently, it is still unclear if DDAH2 may be a potential target for ADMA-lowering therapies or if drug development efforts should focus on DDAH2's known physiological functions in mitochondrial fission, angiogenesis, vascular remodelling, insulin secretion, and immune responses. Here, an international consortium of research groups set out to address this question using in silico, in vitro, cell culture, and murine models. The findings uniformly demonstrate that DDAH2 is incapable of metabolising ADMA, thus resolving a 20-year controversy and providing a starting point for the investigation of alternative, ADMA-independent functions of DDAH2.

The structural basis of conserved residue variant effect on enzyme activity of UGT2B15

UDP-glucuronosyltransferase 2B15 (UGT2B15) is a crucial phase II drug-metabolizing enzyme, which glucuronidates various compounds, including clinical drugs and hormones. Mutants might affect glucuronidation, leading to a disruption of drug metabolism in vivo and decrease of therapeutic effect. Here, we mainly analyzed two representative mutants, H401P and L446S, on UGT2B15 activity using glucuronidation assays, molecular dynamic (MD) simulation and X-ray diffraction methods. The enzyme activity of L446S obviously increased six-fold than the wild type, although the enzyme activities of P191L, T374A, and H401P were lost apparently. Furthermore, we used MD simulations to calculate the energy change in the catalytic process of H401P and L446S, and the results indicated the free binding energies of H401P mutant to oxazepam and UDPGA were -30.98 ± 1.00 kcal/mol and -36.42 ± 1.04 kcal/mol, respectively, increased obviously compared to wild type, suggesting the mutation on position 401 had a crucial effect on the catalysis. Moreover, the three-dimensional structure of UGT2B15 C-terminal domain L446S was determined through protein crystallography and X-ray diffraction technology and the results suggested that one more hydrogen bonding between S446 and K410 was formed in the S446 crystal structure, compared to the wild type. Isothermal titration calorimetry assay further revealed the K_d values of C-terminal domain of UGT2B15 harbored L446S towards the cofactor UDPGA was similar to the value of wild type. Above all, our results pointed out that H401P and L446S affected the enzyme activity by different mechanism. Our work provided a helpful mechanism for variance explained in the UGTs catalyzation process.

The Variations of Metabolic Detoxification Enzymes Lead to Recurrent Miscarriage and Their Diagnosis Strategy

Spontaneous abortion has been a common obstetrical and gynecological disease, which occurs in 10-15% of all pregnancies. Recurrent miscarriage (RM) refers to the occurrence of three or more times abortions with the same partner. It is generally believed that environmental pollution associated with economic development may cause infertility and RM. When xenobiotics from the environment enter the body, they must be cleared from the body by various metabolic enzymes in the body. The absence or variation of these enzymes may be the genetic basis of RM caused by environmental pollution. The variation of metabolic detoxification enzyme can directly affect the removal of harmful substances from internal and external sources. Therefore, the determination of metabolic enzyme activity may become an important factor in the diagnosis of RM etiology and seeking methods to improve the detoxification ability has a great significance for the treatment of RM.

Arginine-259 of UGT2B7 Confers UDP-Sugar Selectivity

Enzymes of the human UDP-glycosyltransferase (UGT) superfamily typically catalyze the covalent addition of the sugar moiety from a UDP-sugar cofactor to relatively low-molecular weight lipophilic compounds. Although UDP-glucuronic acid (UDP-GlcUA) is most commonly employed as the cofactor by UGT1 and UGT2 family enzymes, UGT2B7 and several other enzymes can use both UDP-GlcUA and UDP-glucose (UDP-Glc), leading to the formation of glucuronide and glucoside conjugates. An investigation of UGT2B7-catalyzed morphine glycosidation indicated that glucuronidation is the principal route of metabolism because the binding affinity of UDP-GlcUA is higher than that of UDP-Glc. Currently, it is unclear which residues in the UGT2B7 cofactor binding domain are responsible for the preferential binding of UDP-GlcUA. Here, molecular dynamics (MD) simulations were performed together with site-directed mutagenesis and enzyme kinetic studies to identify residues within the UGT2B7 binding site responsible for the selective cofactor binding. MD simulations demonstrated that Arg259, which is located within the N-terminal domain, specifically interacts with UDP-GlcUA, whereby the side chain of Arg259 H-bonds and forms a salt bridge with the carboxylate group of glucuronic acid. Consistent with the MD simulations, substitution of Arg259 with Leu resulted in the loss of morphine, 4-methylumbelliferone, and zidovudine glucuronidation activity, but morphine glucosidation was preserved. SIGNIFICANCE STATEMENT: Despite the importance of uridine diphosphate glycosyltransferase (UGT) enzymes in drug and chemical metabolism, cofactor binding interactions are incompletely understood, as is the molecular basis for preferential glucuronidation by UGT1 and UGT2 family enzymes. The study demonstrated that long timescale molecular dynamics (MD) simulations with a UGT2B7 homology model can be used to identify critical binding interactions of a UGT protein with UDP-sugar cofactors. Further, the data provide a basis for the application of MD simulations to the elucidation of UGT-aglycone interactions.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

The Inhibition of Hepatic and Renal Glucuronidation of p-Nitrophenol and 4-Methylumbelliferone by Oil Palm Empty Fruit Bunch Lignin and Its Main Oxidation Compounds

Background:

In order to develop oil palm empty fruit bunch (EFB) lignin as a nutraceutical and health supplement, the investigation of its potential in interacting with other drugs via inhibition of drug-metabolizing enzymes (DMEs) would ensure product safety.

Objective:

The study was aimed to investigate the in vitro effect of oil palm EFB lignin and its main oxidation compounds on phase II DME UDP-glucuronosyltransferases (UGTs) in rat liver and kidney microsomes.

Materials and Methods:

The p-nitrophenol (p-NP) and 4-methylumbelliferone (4-MU) were employed as probe substrates in glucuronidation assays. The effect of soda oil palm EFB lignin on V_max, K_m, CL_int, K_i, and mode of inhibition of 4-MU glucuronidation in RLM was also determined.

Results:

The inhibitory potency of oil palm EFB lignin for both p-NP and 4-MU glucuronidation in rat liver microsome (RLM) and rat kidneys microsomes (RKM) was found to be in the rank order of soda > kraft > organosolv. However, the inhibitory potency of its main oxidation compounds were in the rank order of vanillin > syringaldehyde > p-hydroxybenzaldehyde. Soda oil palm EFB lignin exhibited mixed-type inhibition against 4-MU glucuronidation in RLM, showing the change in apparent V_max and with only a minor effect on K_m compared with control.

Conclusions:

The findings showed that effect of oil palm EFB lignin on both p-NP and 4-MU glucuronidation in RLM and RKM was enhanced by the presence of vanillin as well as flavonoids. Kinetic study showed that soda oil palm EFB lignin exhibited strong inhibition on UGT activity in RLM with mixed-type inhibition mode.

SUMMARY

The inhibitory potential of oil palm EFB lignin extracts for p-NP and 4-MU glucuronidation in RLM and RKM can be listed in the following rank order: soda > kraft > organosolv

The inhibitory potential of oil palm EFB lignin main oxidation compounds for p-NP and 4-MU glucuronidation in RLM and RKM can be listed in the following rank order: vanillin > syringaldehyde > p-hydroxybenzaldehyde

Results suggested that the effect of oil palm EFB lignin on p-NP and 4-MU glucuronidation activity in both RLM and RKM was enhanced by the presence of vanillin as well as total flavonoid content

Results also suggested that oil palm EFB lignin may inhibit glucuronidation of substrate by UGT enzymes, especially UGT1A6, particularly in rat liver

Abbreviations used:p-NP: p-Nitrophenol, 4-MU: 4-Methylumbelliferone, EFB: Empty fruit bunch, DME: Drug-metabolizing enzymes, UGT: UDPglucuronosyltransferase, V_max: Maximal reaction velocity, K_m: Michaelis-Menten constant, CLint: Intrinsic clearance, K_i: Dissociation constant of an inhibitor enzyme complex, 4-MUG: 4-Methylumbelliferone glucuronide, DMSO: Dimethyl sulfoxide, IC50: Half maximal inhibitory concentration, p-NPG: p-Nitrophenol glucuronide, RKM: Rat kidneys microsomes, RLM: Rat liver microsome, UDPGA: UDPglucuronic acid, TCA: trichloroacetic acid, MPA: mycophenolic acid

Effects of mitragynine and 7-hydroxymitragynine (the alkaloids of Mitragyna speciosa Korth) on 4-methylumbelliferone glucuronidation in rat and human liver microsomes and recombinant human uridine 5'-diphospho-glucuronosyltransferase isoforms

Background:

Glucuronidation catalyzed by uridine 5’- diphospho-glucuronosyltransferase (UGT) is a major phase II drug metabolism reaction which facilitates drug elimination. Inhibition of UGT activity can cause drug-drug interaction. Therefore, it is important to determine the inhibitory potentials of drugs on glucuronidation.

Objective:

The objective was to evaluate the inhibitory potentials of mitragynine, 7-hydroxymitragynine, ketamine and buprenorphine, respectively on 4-methylumbelliferone (4-MU) glucuronidation in rat liver microsomes, human liver microsomes and recombinant human UGT1A1 and UGT2B7 isoforms.

Materials and Methods:

The effects of the above four compounds on the formation of 4-MU glucuronide from 4-MU by rat liver microsomes, human liver microsomes, recombinant human UGT1A1 and UGT2B7 isoforms were determined using high-performance liquid chromatography with ultraviolet detection.

Results:

For rat liver microsomes, ketamine strongly inhibited 4-MU glucuronidation with an IC₅₀ value of 6.21 ± 1.51 μM followed by buprenorphine with an IC₅₀ value of 73.22 ± 1.63 μM. For human liver microsomes, buprenorphine strongly inhibited 4-MU glucuronidation with an IC₅₀ value of 6.32 ± 1.39 μM. For human UGT1A1 isoform, 7-hydroxymitragynine strongly inhibited 4-MU glucuronidation with an IC₅₀ value of 7.13 ± 1.16 μM. For human UGT2B7 isoform, buprenorphine strongly inhibited 4-MU glucuronidation followed by 7-hydroxymitragynine and ketamine with respective IC₅₀ values of 5.14 ± 1.30, 26.44 ± 1.31, and 27.28 ± 1.18 μM.

Conclusions:

These data indicate the possibility of drug-drug interaction if 7-hydroxymitragynine, ketamine, and buprenorphine are co-administered with drugs that are UGT2B7 substrates since these three compounds showed significant inhibition on UGT2B7 activity. In addition, if 7-hydroxymitragynine is to be taken with other drugs that are highly metabolized by UGT1A1, there is a possibility of drug-drug interaction to occur.

Human UDP-Glucuronosyltransferase (UGT) 2B10: Validation of Cotinine as a Selective Probe Substrate, Inhibition by UGT Enzyme-Selective Inhibitors and Antidepressant and Antipsychotic Drugs, and Structural Determinants of Enzyme Inhibition

Although there is evidence for an important role of UGT2B10 in the N-glucuronidation of drugs and other xenobiotics, the inhibitor selectivity of this enzyme is poorly understood. This study sought primarily to characterize the inhibition selectivity of UGT2B10 by UDP-glucuronosyltransferase (UGT) enzyme-selective inhibitors used for reaction phenotyping, and 34 antidepressant and antipsychotic drugs that contain an amine functional group. Initial studies demonstrated that cotinine is a highly selective substrate of human liver microsomal UGT2B10. The kinetics of cotinine N-glucuronidation by recombinant UGT and human liver microsomes (± bovine serum albumin) were consistent with the involvement of a single enzyme. Of the UGT enzyme-selective inhibitors employed for reaction phenotyping, only the UGT2B4/7 inhibitor fluconazole reduced recombinant UGT2B10 activity to an appreciable extent. The majority of antidepressant and antipsychotic drugs screened for effects on UGT2B10 inhibited enzyme activity with IC50 values <100 µM. The most potent inhibition was observed with the tricyclic antidepressants amitriptyline and doxepin and the tetracyclic antidepressant mianserin, and the structurally related compounds desloratadine and loratadine. Molecular modeling using a ligand-based approach indicated that hydrophobic and charge interactions are involved in inhibitor binding, whereas spatial features influence the potency of UGT2B10 inhibition. Respective mean Ki,u (± S.D.) values for amitriptyline, doxepin, and mianserin inhibition of human liver microsomal UGT2B10 were 0.61 ± 0.05, 0.95 ± 0.18, and 0.43 ± 0.01 µM. In vitro-in vivo extrapolation indicates that these drugs may perpetrate inhibitory drug-drug interactions when coadministered with compounds that are cleared predominantly by UGT2B10.

Hormonal regulation of hepatic drug biotransformation and transport systems

The human body is constantly exposed to many xenobiotics including environmental pollutants, food additives, therapeutic drugs, etc. The liver is considered the primary site for drug metabolism and elimination pathways, consisting in uptake, phase I and II reactions, and efflux processes, usually acting in this same order. Modulation of biotransformation and disposition of drugs of clinical application has important therapeutic and toxicological implications. We here provide a compilation and analysis of relevant, more recent literature reporting hormonal regulation of hepatic drug biotransformation and transport systems. We provide additional information on the effect of hormones that tentatively explain differences between sexes. A brief discussion on discrepancies between experimental models and species, as well as a link between gender-related differences and the hormonal mechanism explaining such differences, is also presented. Finally, we include a comment on the pathophysiological, toxicological, and pharmacological relevance of these regulations.

Substrate selectivity of human intestinal UDP-glucuronosyltransferases (UGTs): in silico and in vitro insights

The current drug development process aims to produce safe, effective drugs within a reasonable time and at a reasonable cost. Phase II metabolism (glucuronidation) can affect drug action and pharmacokinetics to a considerable extent and so its studies and prediction at initial stages of drug development are very imperative. Extensive glucuronidation is an obstacle to oral bioavailability because the first-pass glucuronidation [or premature clearance by UDP-glucuronosyltransferases (UGTs)] of orally administered agents frequently results in poor oral bioavailability and lack of efficacy. Modeling of new chemical entities/drugs for UGTs and their kinetic data can be useful in understanding the binding patterns to be used in the design of better molecules. This review concentrates on first-pass glucuronidation by intestinal UGTs, including their topology, expression profile, and pharmacogenomics. In addition, recent advances are discussed with respect to substrate selectivity at the binding pocket, structural requirements, and mechanism of enzyme actions.

Substrate inhibition kinetics in drug metabolism reactions

Inhibition of enzyme activity at high substrate concentrations, so-called "substrate inhibition," is commonly observed and has been recognized in drug metabolism reactions since the last decade. Although the importance of such "atypical" kinetics in vivo remains poorly understood, a substrate with substrate inhibition kinetics has been shown to unconventionally alter the metabolism of other substrates. In recent years, it is becoming increasingly evident that the mechanisms for substrate inhibition are highly complex, which are possibly contributed by multiple (at least two) binding sites within the enzyme protein, the formation of a ternary dead-end enzyme complex, and/or the ligand-induced changes in enzyme conformation. This review primarily discusses the mechanisms for substrate inhibition displayed by the important drug-metabolizing enzymes, such as cytochrome p450s, UDP-glucuronyltransferases, and sulfotransferases. Kinetic modeling of substrate inhibition in the absence or presence of a modifier is another central issue in this review because of its importance in the determination of kinetic parameters and in vitro/in vivo predictions.

Uridine diphosphate glucuronosyltransferase isoform-dependent regiospecificity of glucuronidation of flavonoids

The objective of this study was to determine the regiospecificity of the important uridine diphosphate glucuronosyltransferase (UGT) isoforms responsible for the glucuronidation of flavones and flavonols. We systematically studied the glucuronidation of 13 flavonoids (7 flavones and 6 flavonols, with hydroxyl groups at C-3, C-4', C-5, and/or C-7 positions in flavonoid structure) at a substrate concentration of 10 μM by 8 recombinant human UGT isoforms mainly responsible for the metabolism of flavonoids, UGTs 1A1, 1A3, 1A6, 1A7, 1A8, 1A9, 1A10, and 2B7. At 10 μM substrate concentration, different UGT isoforms gave different regiospecific glucuronidation patterns. UGT 1A1 equally glucuronidated 3-O (glucuronic acid substituted at C-3 hydroxyl group), 7-O, and 4'-O, whereas UGTs 1A8 and 1A9 preferably glucuronidated only 3-O and 7-O positions. UGT 1A1 usually showed no regiospecificity for glucuronidating any position, whereas UGT 1A8 and UGT 1A9 showed dominant, moderate, or weak regiospecificity for 3-O or 7-O position, depending on the structure of the compound. UGT 1A3 showed dominant regiospecificity for the 7-O position, whereas UGT 1A7 showed dominant regiospecificity for the 3-O position. We also showed that the glucuronidation rates of 3-O and 7-O positions in flavones and flavonols were affected by the addition of multiple hydroxyl groups at different positions as well as by the substrate concentrations (2.5, 10, and 35 μM). In conclusion, regiospecific glucuronidation of flavonols was isoform- and concentration-dependent, whereas flavones were dominantly glucuronidated at the 7-O position by most UGT isoforms. We also concluded that UGTs 1A3 and 1A7 showed dominant regiospecificity for only the 7-O and 3-O positions, respectively. UGTs 1A8 and 1A9 showed moderate or weak preference on glucuronidating position 3-O over the 7-O position, whereas other UGT isoforms did not prefer glucuronidating any particular positions.

Conjugation of synthetic cannabinoids JWH-018 and JWH-073, metabolites by human UDP-glucuronosyltransferases

K2, a synthetic cannabinoid (SC), is an emerging drug of abuse touted as "legal marijuana" and marketed to young teens and first-time drug users. Symptoms associated with K2 use include extreme agitation, syncope, tachycardia, and visual and auditory hallucinations. One major challenge to clinicians is the lack of clinical, pharmacological, and metabolic information for the detection and characterization of K2 and its metabolites in human samples. Information on the metabolic pathway of SCs is very limited. However, previous reports have shown the metabolites of these compounds are excreted primarily as glucuronic acid conjugates. Based on this information, this study evaluates nine human recombinant uridine diphosphate-glucuronosyltransferase (UGT) isoforms and human liver and intestinal microsomes for their ability to glucuronidate hydroxylated metabolites of 1-naphthalenyl-1(1-pentyl-1H-indol-3-yl)-methanone (JWH-018) and (1-butyl-1H-indol-3-yl)-1-naphthalenyl-methanone (JWH-073), the two most common SCs found in K2 products. Conjugates were identified and characterized using liquid chromatography/tandem mass spectrometry, whereas kinetic parameters were quantified using high-performance liquid chromatography-UV-visible methods. UGT1A1, UGT1A3, UGT1A9, UGT1A10, and UGT2B7 were shown to be the major enzymes involved, showing relatively high affinity with K(m) ranging from 12 to 18 μM for some hydroxylated K2s. These UGTs also exhibited a high metabolic capacity for these compounds, which indicates that K2 metabolites may be rapidly glucuronidated and eliminated from the body. Studies of K2 metabolites will help future development and validation of a specific assay for K2 and its metabolites and will allow researchers to fully explore their pharmacological actions.

Effect of UDP-glucuronosyltransferase 2B15 polymorphism on bisphenol A glucuronidation

Bisphenol A (BPA) is one of a number of potential endocrine-disrupting chemicals, which are metabolized mainly by UDP-glucuronosyltransferase 2B15 (UGT2B15) in humans. Six UGT2B15 allelic variants (UGT2B15*2, UGT2B15*3, UGT2B15*4, UGT2B15*5, UGT2B15*6, and UGT2B15*7; wild-type, UGT2B15*1) with amino acid substitutions have been found in Caucasian, African-American, Hispanic, and Oriental populations to date. In this study, the effects of amino acid substitutions in UGT2B15 on BPA glucuronidation were studied using recombinant UGT2B15 enzymes of wild-type (UGT2B15.1) and all identified variants (UGT2B15.2, UGT2B15.3, UGT2B15.4, UGT2B15.5, UGT2B15.6, and UGT2B15.7) expressed in insect (Sf9) cells. The K (m), V (max), and CL (int) values of UGT2B15.1 for BPA glucuronidation were 3.9 μM, 650 pmol/min/mg protein, and 170 μL/min/mg protein, respectively. Although there is no significant difference in the K (m) value between wild-type and any variant UGT2B15, the V (max) and CL (int) values of UGT2B15 variants having D85Y substitution were markedly reduced to 14 and 10% for UGT2B15.2, and 4.3 and 3.9% for UGT2B15.5 compared with those of UGT2B15.1, respectively. However, the K (m), V (max), and CL (int) values of UGT2B15.3, UGT2B15.4, UGT2B15.6, and UGT2B15.7 having L86S, T352I, and/or K523T substitution(s) for BPA glucuronidation were comparable to those of UGT2B15.1. These findings suggest that D85Y substitution in UGT2B15 decreases enzymatic function and that the polymorphic alleles of UGT2B15 are closely associated with variations in the metabolism and toxicity of BPA. The information gained in this study should help with in vivo extrapolation to assess the toxicity of endocrine-disrupting chemicals.

The prediction of drug-glucuronidation parameters in humans: UDP-glucuronosyltransferase enzyme-selective substrate and inhibitor probes for reaction phenotyping and in vitro-in vivo extrapolation of drug clearance and drug-drug interaction potential

Major advances in the characterization of uridine diphosphate (UDP)-glucuronosyltransferase (UGT) enzyme substrate and inhibitor selectivities and the development of experimental paradigms to investigate xenobiotic glucuronidation in vitro now permit the prediction of a range of drug-glucuronidation parameters in humans. In particular, the availability of substrate and inhibitor "probes" for the major hepatic drug metabolizing UGTs together with batteries of recombinant enzymes allow the reaction phenotyping of drug glucuronidation reactions. Additionally, in vitro experimental approaches and scaling strategies have been successfully applied to the quantitative prediction of in vivo clearance via glucuronidation and drug-drug interaction potential.

Evidence for oxazepam as an in vivo probe of UGT2B15: oxazepam clearance is reduced by UGT2B15 D85Y polymorphism but unaffected by UGT2B17 deletion

AIMS

Although in vitro studies indicate that oxazepam is an isoform-selective substrate probe for UDP-glucuronosyltransferase 2B15, the utility of this drug as an in vivo probe is uncertain. The main aim of this study was to determine whether common missense polymorphisms in the UGT2B15 gene (D85Y and K523T) are associated with altered oxazepam pharmacokinetics and pharmacodynamics. We also determined the possible influence of a common deletion polymorphism in the gene encoding UGT2B17, which shows substantial substrate specificity overlap with UGT2B15.

METHODS

Thirty healthy male subjects were administered 15 mg of oxazepam by mouth followed by plasma oxazepam concentration monitoring for 36 h, and pharmacodynamic testing for 8 h. Genotypes were determined by genomic polymerase chain reaction and commercial 5'-nuclease assays.

RESULTS

Allele frequencies for D85Y, K523T, UGT2B17del were 47%, 23% and 19%, respectively. Median oxazepam apparent oral clearance was significantly lower in 85YY subjects (1.62 ml min(-1) kg(-1)) compared with 85DD subjects (3.35 ml min(-1) kg(-1); P= 0.003, Student-Newman-Keuls test), whereas 85DY subjects were intermediate (2.34 ml min(-1) kg(-1); P= 0.018 vs. 85DD, P= 0.034 vs. 85YY). Regression analysis indicated that UGT2B15 D85Y genotype accounted for 34% of interindividual variability. However, neither UGT2B15 K523T nor UGT2B17del was associated with altered oxazepam disposition. Furthermore, no differences in pharmacodynamic measures, including quantitative electroencephalography, digit-symbol substitution test, self- or observer-rated visual analogue scales, could be demonstrated for any of the polymorphisms evaluated.

CONCLUSIONS

These results identify UGT2B15 D85Y as a major determinant of oxazepam clearance, and indicate that oxazepam may be useful as an in vivo probe for glucuronidation by UGT2B15.

A molecular model of the human UDP-glucuronosyltransferase 1A1, its membrane orientation, and the interactions between different parts of the enzyme

The vertebrate UDP-glucuronosyltransferases (UGTs) are membrane-bound enzymes of the endoplasmic reticulum that process both endogenous and exogenous substrates. The human UGTs are well known biologically, but biophysical understanding is scarce, largely because of problems in purification. The one resolved crystal structure covers the C-terminal domain of the human UGT2B7. Here, we present a homology model of the complete monomeric human UGT1A1, the enzyme that catalyzes bilirubin glucuronidation. The enzyme can be seen as composed of four different domains: two large ones, the N- and C-terminal domains, and two small ones, the "envelope" helices and the transmembrane segment that includes the cytoplasmic tail. The hydrophobic core of the N-terminal domain and the two envelope helices that connect the large domains are shown to be structurally well conserved even among distant homologs and can thus be modeled with good certainty according to plant and bacterial structures. We consider alternative solutions for the highly variable N-terminal regions that probably contribute to substrate binding. The bilirubin binding site, known pathological mutations in UGT1A1, and other specific residues have been examined in the context of the model with regard to available experimental data. A putative orientation of the protein relative to the membrane has been derived from the location of predicted N-glycosylation sites. The model presents extensive interactions between the N- and C-terminal domains, the two envelope helices, and the membrane. Together, these interactions could allow for a concerted large-scale conformational change during catalysis.

How many and which amino acids are responsible for the large activity differences between the highly homologous UDP-glucuronosyltransferases (UGT) 1A9 and UGT1A10?

The amino acid sequences of the human UDP-glucuronosyltransferases (UGTs) 1A9 and 1A10 are 93% identical, yet there are large differences in their activity and substrate selectivity. For example, the regioselectivity in propranolol glucuronidation, the regioselectivity in dobutamine glucuronidation, and the glucuronidation rate of alpha- and beta-estradiol differ greatly between UGT1A9 and UGT1A10. To identify the residue responsible for the activity differences, we divided the N-terminal half of the two UGTs into five comparable segments by inserting four unique restriction sites at identical positions in both genes and constructing chimeras in which segments of UGT1A9 were individually replaced by the corresponding segments from UGT1A10. Activity analyses of the resulting mutants, 910A [1A10((1-83))/1A9((84-285))], 910B [1A9((1-83))/1A10((84-147))/1A9((148-285))], 910C [1A9((1-147))/1A10((148-181))/1A9((182-285))], 910D [1A9((1-181))/1A10((182-235))/1A9((236-285))], and 910E [1A9((1-235))/1A10((236-285))] indicated that more than one residue is responsible for the differences between UGT1A9 and UGT1A10. We next prepared four double chimeras, in which two of the above UGT1A9 segments were replaced simultaneously by the corresponding UGT1A10 segments. However, none of the double chimeras glucuronidated either estradiol, propranolol, or dobutamine at rates that resembled those of UGT1A10. On the other hand, studying the kinetics of 1-naphthol glucuronidation yielded more focused results, indicating that residues within segment B (84-147) contribute directly to the K(m) value for this substrate. Further mutagenesis and activity assays suggested that Phe117 of UGT1A9 participates in 1-naphthol binding. In addition, it appears that residues within segment C of the N-terminal domain, mainly at positions 152 and 169, contribute to the higher glucuronidation rates of UGT1A10.

In vitro and in vivo glucuronidation of midazolam in humans

AIMS

Midazolam (MDZ) is a benzodiazepine used as a CYP3A4 probe in clinical and in vitro studies. A glucuronide metabolite of MDZ has been identified in vitro in human liver microsome (HLM) incubations. The primary aim of this study was to understand the in vivo relevance of this pathway.

METHODS

An authentic standard of N-glucuronide was generated from microsomal incubations and isolated using solid-phase extraction. The structure was confirmed using proton nuclear magnetic resonance (NMR) and (1)H-(13)C long range correlation experiments. The metabolite was quantified in vivo in human urine samples. Enzyme kinetic behaviour of the pathway was investigated in HLM and recombinant UGT (rUGT) enzymes. Additionally, preliminary experiments were performed with 1'-OH midazolam (1'-OH MDZ) and 4-OH-midazolam (4-OH MDZ) to investigate N-glucuronidation.

RESULTS

NMR data confirmed conjugation of midazolam N-glucuronide (MDZG) standard to be on the alpha-nitrogen of the imidazole ring. In vivo, MDZG in the urine accounted for 1-2% of the administered dose. In vitro incubations confirmed UGT1A4 as the enzyme of interest. The pathway exhibited atypical kinetics and a substrate inhibitory cooperative binding model was applied to determine K(m) (46 microM, 64 microM), V(max) (445 pmol min(-1) mg(-1), 427 pmol min(-1) mg(-1)) and K(i) (58 microM, 79 microM) in HLM and rUGT1A4, respectively. From incubations with HLM and rUGT enzymes, N-glucuronidation of 1'-OH MDZ and 4-OH MDZ is also inferred.

CONCLUSIONS

A more complete picture of MDZ metabolism and the enzymes involved has been elucidated. Direct N-glucuronidation of MDZ occurs in vivo. Pharmacokinetic modelling using Simcyp illustrates an increased role for UGT1A4 under CYP3A inhibited conditions.

Influence of N-terminal domain histidine and proline residues on the substrate selectivities of human UDP-glucuronosyltransferase 1A1, 1A6, 1A9, 2B7, and 2B10

An N-terminal domain histidine [corresponding to position 39 of UDP-glucuronosyltransferase (UGT) 1A1] is conserved in all UGT1A and UGT2B subfamily proteins except UGT1A4 (Pro-40) and UGT2B10 (Leu-34). Unlike most UGT1A and UGT2B xenobiotic-metabolizing enzymes, UGT1A4 and UGT2B10 lack the ability to glucuronidate 4-methylumbelliferone (4MU) and 1-naphthol (1NP), both planar phenols, and naproxen (a carboxylic acid). However, only UGT1A4 glucuronidates the tertiary amines lamotrigine (LTG) and trifluoperazine (TFP). In this study, we sought to elucidate the influence of specific N-terminal histidine and proline residues on UGT enzyme substrate selectivity. The conserved N-terminal domain histidine of UGT1A1, UGT1A6, UGT1A9, and UGT2B7 was mutated to proline and leucine 34 of UGT2B10 was substituted with histidine, and the capacity of the wild-type and mutant proteins to glucuronidate 4MU, 1NP, LTG, TFP, and naproxen was characterized. Whereas UGT1A1(H39P), UGT1A6(H38P), and UGT1A9(H37P) lacked the ability to metabolize 4MU, 1NP, and naproxen, all glucuronidated LTG. K(m) values for UGT1A1(H39P) and UGT1A9(H37P) were 774 and 3812 microM, respectively, compared with 1579 microM for UGT1A4. UGT1A1(H39P) also glucuronidated TFP with a V(max)/K(m) value comparable to that of UGT1A4. In contrast to the wild-type enzyme, UGT2B10(L34H) glucuronidated 4MU and 1NP with respective K(m) values of 260 and 118 microM. UGT2B7(H35P) lacked activity toward all substrates. The data confirm a pivotal role for an N-terminal domain proline in the glucuronidation of the tertiary amines LTG and TFP by UGT1A subfamily proteins, whereas glucuronidation reactions involving proton abstraction generally, although not invariably, require a histidine at the equivalent position in both UGT1A and UGT2B enzymes.

Amino acid positions 69-132 of UGT1A9 are involved in the C-glucuronidation of phenylbutazone

Phenylbutazone (PB) is known to be biotransformed to its O- and C-glucuronide. Recently, we reported that PB C-glucuronide formation is catalyzed by UGT1A9. Interestingly, despite UGT1A8 sharing high amino acid sequence identity with UGT1A9, UGT1A8 had no PB C-glucuronidating activity. In the present study, we constructed eight UGT1A9/UGT1A8 chimeras and evaluated which region is important for PB C-glucuronide formation. All of the chimeras and UGT1A8 and UGT1A9 had 7-hydroxy-(4-trifluoromethyl)coumarin (HFC) O-glucuronidating activity. The K(m) values for HFC glucuronidation of UGT1A8, UGT1A9 and their chimeras were divided into two types, UGT1A8 type (high K(m)) and UGT1A9 type (low K(m)), and these types were determined according to whether their amino acids at positions 69-132 were those of UGT1A8 or UGT1A9. Likewise, PB O-glucuronidating activity was also detected by all of the chimeras, and their K(m) values were divided into two types. On the contrary, PB C-glucuronidating activity was detected by UGT1A9((1-132))/1A8((133-286)), UGT1A9((1-212))/1A8((213-286)), UGT1A8((1-68))/1A9((69-286)), and UGT1A8((1-68))/1A9((69-132))/1A8((133-286)) chimeras. The region 1A9((69-132)) was common among chimeras having PB C-glucuronidating activity. Of interest is that UGT1A9((1-68))/1A8((69-132))/1A9((133-286)) had lost PB C-glucuronidation activity, but retained activities of PB and HFC O-glucuronidation. These results strongly suggested that amino acid positions 69-132 of UGT1A9 are responsible for chemoselectivity for PB and affinity to substrates such as PB and HFC.

Stereoselective metabolism of racemic carvedilol by UGT1A1 and UGT2B7, and effects of mutation of these enzymes on glucuronidation activity

Carvedilol, an alpha- and beta-adrenergic blocking drug, is mainly metabolized by CYP2D6, UGT1A1, UGT2B4 and UGT2B7. This drug is administered orally as a racemic mixture of R(+)- and S(-)-enantiomers. It has been reported that CYP2D6 prefers metabolizing S-carvedilol to R-carvedilol stereoselectively. On the other hand, stereoselective metabolism of carvedilol by UGTs is still unclear. Moreover, we have reported that patients with chronic heart failure who had polymorphism in CYP2D6, UGT1A1 and/or UGT2B7 had lower metabolic activity and oral clearance than did patients with no polymorphism. The aim of this study was to clarify stereoselective metabolism of carvedilol by UGT1A1 and UGT2B7 and to determine by using a recombinant enzyme-introduced mutation whether genetic mutation in UGT1A1 and UGT2B7 causes reduction in metabolic activity for carvedilol. A glucuronidation assay using human liver microsomes and recombinant UGT1A1 and UGT2B7 expressed in HeLa cells demonstrated that UGT1A1 prefers metabolizing R-carvedilol to S-carvedilol. On the other hand, UGT2B7 prefers metabolizing S-carvedilol to R-carvedilol. Moreover, G71R mutation of UGT1A1 reduced both affinity and capacity but did not affect stereoselective metabolism. On the other hand, both A71S and H268Y mutations of UGT2B7 reduced capacity but did not affect affinity and, as a result, the efficiency of metabolism was remarkably reduced. However, as in the case of UGT1A1, neither of the mutations affected stereoselective metabolism.

Critical Roles of Residues 36 and 40 in the Phenol and Tertiary Amine Aglycone Substrate Selectivities of UDP-Glucuronosyltransferases 1A3 and 1A4

Despite high sequence identity, UGT1A3 and UGT1A4 differ in terms of substrate selectivity. UGT1A3 glucuronidates the planar phenols 1-naphthol (1-NP) and 4-methylumbelliferone (4-MU), whereas UGT1A4 converts the tertiary amines lamotrigine (LTG) and trifluoperazine (TFP) to quaternary ammonium glucuronides. Residues 45 to 154 (which incorporate 21 of the 35 amino acid differences) and 45 to 535 were exchanged between UGT1A3 and UGT1A4 to generate UGT1A3-4((45-535)), UGT1A3-4((45-154))-3, UGT1A4-3((45-535)), and UGT1A4-3((45-154))-4 hybrid proteins. Although differences in kinetic parameters were observed between the parent enzymes and chimeras, UGT1A4-3((45-535)) and UGT1A4-3((45-154))-4 [but not UGT1A3-4((45-535)) and UGT1A3-4((45-154))-3] retained the capacity to glucuronidate LTG and TFP. Likewise, UGT1A3-4((45-535)) and UGT1A3-4((45-154))-3 retained the capacity to glucuronidate 1-NP and 4-MU, but UGT1A4-3((45-535)) and UGT1A4-3((45-154))-4 exhibited low or absent activity. Within the first 44 residues, UGT1A3 and UGT1A4 differ in sequence at positions 36 and 40. "Reciprocal" mutagenesis was performed to generate the UGT1A3(I36T), UGT1A3(H40P), UGT1A4(T36I), and UGT1A4 (P40H) mutants. The T36I and P40H mutations in UGT1A4 reduced in vitro clearances for LTG and TFP glucuronidation by >90%. Conversely, the I36T and H40P mutations in UGT1A3 reduced the in vitro clearances for 1-NP and 4-MU glucuronidation by >90%. Introduction of the single H40P mutation in UGT1A3 conferred LTG and TFP glucuronidation, whereas the single T36I mutation in UGT1A4 conferred 1-NP and 4-MU glucuronidation. Thus, residues 36 and 40 of UGT1A3 and UGT1A4 are pivotal for the respective selectivities of these enzymes toward planar phenols and tertiary amines, although other regions of the proteins influence binding affinity and/or turnover.