IDENTIFICATION OF A REVERSIBLE COMPONENT IN THE IN-VITRO INHIBITION OF RAT HEPATIC CYTOCHROME-P450 2B1 BY PARATHION

Cytochrome P450 (P450) enzymes are inactivated in suicidal fashion dur ing microsomal parathion oxidation. In the present study, two distinct components of the inhibition of the phenobarbital (PB)-inducible P450 281 by parathion were characterized. Here we report for the first tim e that low concentrations of parathion potently and reversibly inhibit ed, but did not inactivate, 2B1. In contrast, the previously described inactivation process occurred only at considerably higher parathion c oncentrations, at which concentrations enzyme activity was already ext ensively inhibited. At low concentration, parathion was a competitive inhibitor of 2B1-mediated androstenedione 16 beta-hydroxylation (K-i = 0.44 +/- 0.07 mu M) and of 7-pentylresorufin O-depentylation (K-i = 0 .40 +/- 0.03 mu M) in microsomes from PB-pretreated rats and was simil arly effective against androstenedione 16 alpha- and 16 beta-hydroxyla tion catalyzed by purified 2B1. Although preincubation of higher conce ntrations of parathion (>5 mu M) With NADPH-supplemented microsomes fr om PB-pretreated rat liver decreased hoto-P450, heme loss was not obse rved near the K-i values. Instead, half-maximal loss of P450 occurred at 6 mu M and at 9 mu M parathion in PB-pretreated microsomes and in t he reconstituted system, respectively. Parathion metabolism was effici ent in PB-microsomes (K-i values for 4-nitrophenol and paraoxon format ion were 13 mu M and 10 mu M, respectively) and in the reconstituted s ystem (corresponding K-i values were 19 mu M and 14 mu M). Thus the co nstants for P450 inactivation and for parathion metabolism were simila r and were at least 15-fold greater than the K-i values for the revers ible process. These findings illustrate that competitive inhibition an d irreversible inactivation are distinct, concentration-dependent proc esses by which parathion impairs P450 2B1 function. Potent effects on drug elimination, unrelated to P450 inactivation, could occur after ex posure to small quantities of these agents.