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The role of cytochrome P450 (CYP) and the CYP isoform involved in the activation of the widely used pesticide methyl-parathion (MePA) were investigated in rat brain extracts by measuring the effect of different CYP inhibitors on acetylcholinesterase (AChE) inhibition by MePA. Brain extracts provide a useful tool to study the activation mechanisms of organophosphorus compounds (OP) since they contain both the activating enzyme(s) and the molecular target for OP toxicity. As expected, in incubations of rat brain extract supplemented with NADPH, AChE activity was non-competitively inhibited by the presence of MePA, indicating that MePA was activated to its reactive metabolite methyl-paraoxon (MePO). Indeed, Vmaxapp decreased from 13.4 to 8.7 mol thionitrobenzoic acid (TNB)/min per mg protein. MePA activation by rat brain extracts, as measured by the AChE inhibition produced by the presence of the pesticide in the incubation, was fully prevented by previously bubbling the incubation mix with CO, by the presence of monoclonal anti-rat CYP2B1/2B2 antibodies and by the addition of phenobarbital (PB), a CYP2B substrate. Interestingly, MePA showed a greater affinity for CYP2B than PB. CYP1A1 antibodies showed no effect on MePA activation. The presence of cytochrome P450 2B (CYP2B) in the rat brain extracts was confirmed by immunoblotting. These results demonstrate indisputably the responsibility of CYP2B in MePA activation in the rat brain in vitro, suggesting that metabolic activation of OP compounds in situ might be crucial for their organ specific toxicity to the central nervous system also in vivo. © 2001 Elsevier Science Ireland Ltd. All rights reserved.

The effect of 0, 1, 5, 25 and 50 mg/kg body weight (b.w.) lindane, administered in corn oil by gavage, on cytochrome P450 (CYP) phenotype was investigated in the liver of ovariectomized (ovx), sham operated (sham-ope) and nonovariectomized (n/ovx) adult Wistar rats. Total hepatic microsomal CYP content and the O-dealkylation of 7-ethoxy- (EROD), 7-methoxy- (MROD), 7-pentoxy- (PROD) and 7-benzyloxy-resorufin (BROD) were assayed. In addition, CYP1A1, 2B1/2B2, 2C11 and 3A2 proteins were determined by western blot using specific anti-rat antibodies. Protein bands were visualised by chemiluminescence and their intensities were compared among groups. A statistically significant, dose-dependent increase of all parameters studied was observed in all three animal groups after lindane administration. Ovx rats, however, responded differently to lindane administration than n/ovx or sham-ope animals. At the highest doses in ovx rats, the relative liver weight was more increased and the total CYP content was less increased than in n/ovx or sham-ope animals. Moreover, the degree of induction of PROD and BROD activities was higher and that of EROD activity was lower in ovx than in either n/ovx or sham-ope animals. Accordingly, CYP2B1/2B2 protein showed the largest increase in ovx rats, whereas CYP1A protein increased more in n/ovx or sham-ope animals. CYP2C11, a cytochrome predominantly expressed in males, was also more strongly induced in ovx than in n/ovx or sham-ope animals. CYP3A2 was slightly expressed in ovx but not in n/ovx non-treated rats, although the effect of induction was clearly greater in the latter. These results, overall, indicate that ovariectomy significantly affects, both qualitatively and quantitatively, CYP expression following induction by lindane and support the anti-estrogenic effect of lindane in rats. The pathophysiological and toxicological relevance of liver CYP induction by lindane and possibly other organochlorine xenobiotics in females with a lack or deficiency of estrogen supply remains to be investigated.