Am. Butler et M. Murray, BIOTRANSFORMATION OF PARATHION IN HUMAN LIVER - PARTICIPATION OF CYP3A4 AND ITS INACTIVATION DURING MICROSOMAL PARATHION OXIDATION, The Journal of pharmacology and experimental therapeutics, 280(2), 1997, pp. 966-973
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.