M. Murray et Am. Butler, IDENTIFICATION OF A REVERSIBLE COMPONENT IN THE IN-VITRO INHIBITION OF RAT HEPATIC CYTOCHROME-P450 2B1 BY PARATHION, The Journal of pharmacology and experimental therapeutics, 272(2), 1995, pp. 639-644
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.