Studies in rat liver have shown that cytochrome P450 (CYP) enzymes mediate the oxidative biotransformation of the phosphorothioate pesticide parathion to paraoxon and 4-nitrophenol. Transfer of the phosphorothioate thionosulfur atom to the CYP apoprotein results in amino acid modification and enzyme inactivation. Our study investigated the role of human hepatic CYP in parathion oxidation and their relative susceptibilities to inhibition and inactivation. Rates of parathion oxidation varied 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 implicated CYP3A4 in the reaction. Thus, parathion oxidation was correlated strongly with testosterone 6beta-hydroxylation (r2 = 0.95, n = 11), but not with activities mediated by CYP 1A2, 2C9 or 2E1. CYP 3A4 expressed in lymphoblastoid cell lines was an efficient catalyst of parathion oxidation, although CYP 1A2 and 2B6 also catalyzed the activity. The CYP3A4 inhibitors ketoconazole and triacetyloleandomycin decreased the observed rate of microsomal parathion oxidation, but chemicals known to interact preferentially with other human CYP were essentially noninhibitory. P450 was lost during parathion biotransformation in human hepatic microsomes. Thus, incubation (10 min) of parathion (25 microM) with NADPH-supplemented microsomes led to an apparent 19 +/- 4% decrease in holo-P450 content. Several CYP-specific oxidation reactions were inhibited and inactivated by parathion. Testosterone 6beta-hydroxylation (mediated by CYP3A4), 7-ethylresorufin O-deethylation (CYP1A2) and tolbutamide methyl hydroxylation (CYP2C9/10), but not aniline 4-hydroxylation (CYP2E1), were inhibited effectively by parathion. Preincubation of microsomes with parathion and NADPH intensified the extent of inhibition (i.e., elicited inactivation) of reactions mediated by 3A4 and 1A2 and, to a lesser extent, 2C9. In summary, these findings strongly implicate CYP 3A4 as the principal catalyst of parathion oxidation in human liver, although other CYP may play a lesser role. During parathion 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 elimination in humans.