PT  - JOURNAL ARTICLE
AU  - Miki Nakajima
AU  - Nozomi Sakata
AU  - Noriko Ohashi
AU  - Toshiyuki Kume
AU  - Tsuyoshi Yokoi
TI  - Involvement of Multiple UDP-glucuronosyltransferase 1A Isoforms in Glucuronidation of 5-(4′-hydroxyphenyl)-5-phenylhydantoin in Human Liver Microsomes
AID  - 10.1124/dmd.30.11.1250
DP  - 2002 Nov 01
TA  - Drug Metabolism and Disposition
PG  - 1250--1256
VI  - 30
IP  - 11
4099  - http://dmd.aspetjournals.org/content/30/11/1250.short
4100  - http://dmd.aspetjournals.org/content/30/11/1250.full
SO  - Drug Metab Dispos2002 Nov 01; 30
AB  - 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′-HPPHO-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′-HPPHO-glucuronide by liquid chromatography-tandem mass spectrometry analysis. The 4′-HPPHO-glucuronosyltransferase activity in human liver microsomes was not saturated at concentrations up to 500 μM 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′-HPPHO-glucuronide. The 4′-HPPHO-glucuronidation in pooled human liver microsomes was inhibited by β-estradiol as a typical substrate for UGT1A1 (IC50 = 21.1 μM) and imipramine as a typical substrate for UGT1A4 (IC50 = 57.7 μM). The inhibitory effects of propofol as a specific substrate for UGT1A9 (IC50 = 167.1 μM) and emodin as a substrate for UGT1A8 and UGT1A10 (IC50 = 287.6 μM) were not prominent. The interindividual difference in the 4′-HPPHO-glucuronidation in 14 human liver microsomes was 28.5-fold (0.023–0.656 nmol/min/mg of protein). The 4′-HPPHO-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′-HPPHO-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. The American Society for Pharmacology and Experimental Therapeutics