TY - JOUR T1 - Glucuronidation of Etoposide in Human Liver Microsomes Is Specifically Catalyzed by UDP-Glucuronosyltransferase 1A1 JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 589 LP - 595 DO - 10.1124/dmd.31.5.589 VL - 31 IS - 5 AU - Yuichiro Watanabe AU - Miki Nakajima AU - Noriko Ohashi AU - Toshiyuki Kume AU - Tsuyoshi Yokoi Y1 - 2003/05/01 UR - http://dmd.aspetjournals.org/content/31/5/589.abstract N2 - A metabolite formed by incubation of human liver microsomes, etoposide, and UDP-glucuronic acid was identified as etoposide glucuronide by liquid chromatography-tandem mass spectrometry analysis. According to the derivatization with trimethylsilylimidazole (Tri-Sil-Z), it was confirmed that the glucuronic acid is linked to an alcoholic hydroxyl group of etoposide and not to a phenolic group. Among nine recombinant human UGT isoforms (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A8, UGT1A9. UGT1A10, UGT2B7, and UGT2B15), only UGT1A1 exhibited the catalytic activity of etoposide glucuronidation. The enzyme kinetics in pooled human liver microsomes and recombinant UGT1A1 microsomes showed a typical Michaelis-Menten plot. The kinetic parameters of etoposide glucuronidation wereKm = 439.6 ± 70.7 μM andVmax = 255.6 ± 19.2 pmol/min/mg of protein in human liver microsomes andKm = 503.2 ± 110.2 μM andVmax = was 266.5 ± 28.6 pmol/min/mg of protein in recombinant UGT1A1. The etoposide glucuronidation in pooled human liver microsomes was inhibited by bilirubin (IC50 = 31.7 μM) and estradiol (IC50 = 34 μM) as typical substrates for UGT1A1. The inhibitory effects of 4-nitrophenol (IC50 = 121.0 μM) as a typical substrate for UGT1A6 and UGT1A9, imipramine (IC50 = 393.8 μM) as a typical substrate for UGT1A3 and UGT1A4, and morphine (IC50 = 109.3 μM) as a typical substrate for UGT2B7 were relatively weak. The interindividual difference in etoposide glucuronidation in 13 human liver microsomes was 78.5-fold (1.4–109.9 pmol/min/mg of protein). The etoposide glucuronidation in 10 to 13 human liver microsomes was significantly correlated with β-estradiol-3-glucuronidation (r= 0.841, p < 0.01), bilirubin glucuronidation (r = 0.935, p < 0.01), and the immunoquantified UGT1A1 protein content (r = 0.800,p < 0.01). These results demonstrate that etoposide glucuronidation in human liver microsomes is specifically catalyzed by UGT1A1. The American Society for Pharmacology and Experimental Therapeutics ER -