title: Involvement of multiple UDP-glucuronosyltransferase 1A isoforms in glucuronidation of 5-(4'-hydroxyphenyl)-5-phenylhydantoin in human liver microsomes.
abstract: In humans, orally administered phenytoin, 5,5-diphenylhydantoin, is mainly excreted as 5-(4'-hydroxyphenyl)-5-phenylhydantoin (4'-HPPH) O-glucuronide. Phenytoin is oxidized to 4'-HPPH by CYP2C9 and to a minor extent by CYP2C19, and then 4'-HPPH is metabolized to 4'-HPPH O-glucuronide by UDP-glucuronosyltransferase (UGT). In the present study, 4'-HPPH O-glucuronidation in human liver microsomes was investigated. The metabolite formed by incubation with human liver microsomes, 4'-HPPH, and UDP-glucuronic acid was identified as 4'-HPPH O-glucuronide by liquid chromatography-tandem mass spectrometry analysis. The 4'-HPPH O-glucuronosyltransferase activity in human liver microsomes was not saturated at concentrations up to 500 microM of 4'-HPPH. Any commercially available recombinant human UGTs (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B7, and UGT2B15) expressed in baculovirus-infected insect cells did not show detectable 4'-HPPH O-glucuronide. The 4'-HPPH O-glucuronidation in pooled human liver microsomes was inhibited by beta-estradiol as a typical substrate for UGT1A1 (IC(50) = 21.1 microM) and imipramine as a typical substrate for UGT1A4 (IC(50) = 57.7 microM). The inhibitory effects of propofol as a specific substrate for UGT1A9 (IC(50) = 167.1 microM) and emodin as a substrate for UGT1A8 and UGT1A10 (IC(50) = 287.6 microM) were not prominent. The interindividual difference in the 4'-HPPH O-glucuronidation in 14 human liver microsomes was 28.5-fold (0.023-0.656 nmol/min/mg of protein). The 4'-HPPH O-glucuronosyltransferase activity in 11 human liver microsomes was significantly (r = 0.609, P < 0.05) correlated with the 4-nitrophenol glucuronosyltransferase activity, which is catalyzed by UGT1A6 and UGT1A9. These results suggest that multiple UGT1As such as UGT1A1, UGT1A4, UGT1A6, and UGT1A9 are involved in 4'-HPPH O-glucuronidation in human liver microsomes, although the percentage contribution of each UGT1A could not be estimated. Large interindividual differences in the glucuronidation of 4'-HPPH might be responsible for the nonlinearity of the phenytoin plasma concentration or adverse reactions in humans.
authors.affiliation: Division of Drug Metabolism, Faculty of Pharmaceutical Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Miki
authors.initials: M
authors.lastname: Nakajima
authors.forename: Nozomi
authors.initials: N
authors.lastname: Sakata
authors.forename: Noriko
authors.initials: N
authors.lastname: Ohashi
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Kume
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Yokoi
mesh_terms: D001711:Biotransformation; D020719:Glucuronides / Q000378:metabolism; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D013058:Mass Spectrometry; D008862:Microsomes, Liver / Q000201:enzymology*; D010672:Phenytoin / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000652:urine; D013379:Substrate Specificity
chemical_list: D020719:Glucuronides; D007527:Isoenzymes; C001419:hydroxyphenytoin; D010672:Phenytoin; D014453:Glucuronosyltransferase