BIOTRANSFORMATION OF PARATHION IN HUMAN LIVER - PARTICIPATION OF CYP3A4 AND ITS INACTIVATION DURING MICROSOMAL PARATHION OXIDATION

Studies in rat liver have shown that cytochrome P450 (CYP) enzymes med iate the oxidative biotransformation of the phosphorothioate pesticide parathion to paraoxon and 4-nitrophenol. Transfer of the phosphorothi oate thionosulfur atom to the CYP apoprotein results in amino acid mod ification and enzyme inactivation. Our study investigated the role of human hepatic CYP in parathion oxidation and their relative susceptibi lities to inhibition and inactivation. Rates of parathion oxidation va ried about 10-fold in microsomes from 23 individual livers (1.72-18.33 nmol total metabolites/mg protein/min). Linear regression of rates of parathion oxidation with those of other microsomal CYP reactions impl icated CYP3A4 in the reaction. Thus, parathion oxidation was correlate d strongly with testosterone 6 beta-hydroxylation (r(2) = 0.95, n = 11 ), but not with activities mediated by CYP 1A2, 2C9 or 2E1. CYP 3A4 ex pressed in lymphoblastoid cell lines was an efficient catalyst of para thion oxidation, although CYP 1A2 and 2B6 also catalyzed the activity. The CYP3A4 inhibitors ketoconazole and triacetyloleandomycin decrease d the observed rate of microsomal parathion oxidation, but chemicals k nown to interact preferentially with other human CYP were essentially noninhibitory. P450 was lost during parathion biotransformation in hum an hepatic microsomes. Thus, incubation (10 min) of parathion (25 mu M ) with NADPH-supplemented microsomes led to an apparent 19 +/- 4% decr ease in holo-P450 content. Several CYP-specific oxidation reactions we re inhibited and inactivated by parathion. Testosterone 6 beta-hydroxy lation (mediated by CYP3A4), 7-ethylresorufin O-deethylation (CYP1A2) and tolbutamide methyl hydroxylation (CYP2C9/10), but not aniline 4-hy droxylation (CYP2E1), were inhibited effectively by parathion. Preincu bation of microsomes with parathion and NADPH intensified the extent o f inhibition (i.e., elicited inactivation) of reactions mediated by 3A 4 and 1A2 and, to a lesser extent, 2C9. In summary, these findings str ongly implicate CYP 3A4 as the principal catalyst of parathion oxidati on in human liver, although other CYP may play a lesser role. During p arathion oxidation CYP3A4 undergoes significant inactivation. In view of the role of this enzyme in the oxidation of many therapeutic agents , exposure to phosphorothioate pesticides may adversely affect drug el imination in humans.