In vitro studies have established that cytochrome P450 (P450) is deactivated by the electrophilic sulfur atom released during the enzymic oxidation of parathion to paraoxon. However, in vivo studies in rats have been unable to demonstrate significant P450 loss. This study evaluated the possibility that there may be alternate pathways of parathion biotransformation in liver, other than those mediated by P450 and supported by NADPH. Initial experiments confirmed that parathion administration did not decrease microsomal P450 or testosterone hydroxylation activities. Subsequent in vitro experiments identified an NADH-dependent pathway of parathion biotransformation, and MS was used to confirm that paraoxon and 4-nitrophenol were the products of both the NADH- and NADPH-dependent reactions. The Michaelis constants of the NADH-dependent formation of paraoxon and 4-nitrophenol (26 +/- 6 microM and 53 +/- 10 microM, respectively) were approximately 3-fold greater than those for the NADPH-supported reactions (9 +/- 1 microM and 18 +/- 3 microM, respectively). Induction of male rats with phenobarbital and dexamethasone, but not beta-naphthoflavone, produced similar increases in the rates of NADH- and NADPH-mediated parathion metabolism. Rates of NADH- and NADPH-dependent metabolism were highly correlated in linear relationships. An anti-NADPH-cytochrome P450 reductase (NADPH-P450 reductase) antibody partially inhibited microsomal parathion oxidation mediated by either cofactor, and the P450 inhibitor clotrimazole was similarly effective against the NADH- and NADPH-supported oxidation of parathion. Finally, a reconstituted system containing P450 2B1, NADPH-P450 reductase, and phospholipid supported parathion oxidation mediated by NADH.(ABSTRACT TRUNCATED AT 250 WORDS)