RT Journal Article SR Electronic T1 IDENTIFICATION OF CYP1A2 AS THE MAIN ISOFORM FOR THE PHASE I HYDROXYLATED METABOLISM OF GENISTEIN AND A PRODRUG CONVERTING ENZYME OF METHYLATED ISOFLAVONES JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 924 OP 931 DO 10.1124/dmd.31.7.924 VO 31 IS 7 A1 Ming Hu A1 Kristopher Krausz A1 Jun Chen A1 Xia Ge A1 Jianqi Li A1 Harry L. Gelboin A1 Frank. J. Gonzalez YR 2003 UL http://dmd.aspetjournals.org/content/31/7/924.abstract AB This study determined the cytochrome P450 (P450) isoforms responsible for metabolism of isoflavones using human liver microsomes (HLM) and expressed P450s. The primary metabolite of genistein is 3′-OH-genistein, as identified with an authentic chemically synthesized standard. CYP1A2 was predominantly responsible for 3′-OH-genistein formation since its formation was inhibited (>50%, p < 0.05) by a monoclonal antibody specific for CYP1A2, was correlated with CYP1A2 activities of HLM, and was catalyzed by expressed CYP1A2. In addition to CYP1A2, CYP2E1 also catalyzed, although to a lesser extent, its formation. The contribution of these P450s to the formation of 3′-OH-genistein was also confirmed with a panel of expressed enzymes. Methylated isoflavones biochanin A, prunetin, and formononetin (10–100 μM) were rapidly converted by HLM and expressed CYP1A2 to more active genistein and daidzein. The conversion of biochanin A to genistein appears to be mainly mediated by CYP1A2 because of the strong correlation between the conversion rates and CYP1A2 activities in HLM. Thus, CYP1A2 is an effective prodrug-converting enzyme for less active methylated isoflavones. CYP1A2-catalyzed conversion of biochanin A to genistein (Km, 7.80 μM; Vmax, 903 pmol/min/mg of protein; Vmax/Km, 116 μl/min/mg of protein) was much faster than 3′-hydroxylation of genistein (Km, 12.7 μM and Vmax, 109 pmol/min/mg of protein; Vmax/Km, 8.6 μl/min/mg of protein). The interaction studies showed that genistein inhibited formation of acetaminophen from phenacetin with an IC50 value of 16 μM. Additional studies showed that phenacetin and genistein were mutually inhibitory. In conclusion, CYP1A2 and CYP2E1 metabolized genistein and CYP1A2 acted as prodrug-converting enzymes for other less active methylated isoflavones. The American Society for Pharmacology and Experimental Therapeutics