Differences in oxidative dearylation and desulfuration of fenitrothion by cytochrome P-450 isozymes and in the subsequent inhibition of monooxygenase activity

Abstract

The organophosphorus insecticide, fenitrothion ( O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate), a substrate for the cytochrome P-450 ( P-450) monooxygenase system, yields the corresponding phosphate triester (fenitroxon) by oxidative desulfuration (an activation reaction) and 4-nitro- m-cresol by oxidative dearylation (a detoxication reaction). Four constitutive P-450 isozymes and the major isozymes induced by phenobarbital (PB) and β-naphthoflavone (BNF) were incubated with [ 14C]fenitrothion in reconstituted monooxygenase systems. Both metabolites were formed with all isozymes, with the ratio of oxon to cresol being characteristic of the individual P-450s. Cytochrome P-450 B2 and P-450 PB formed the highest percentage of oxon, 75 and 82%, respectively; while P-450 A1, B3, and BNF produced 55 to 60% oxon. By contrast, P-450 A2 formed the least amount of oxon, 38%, with 62% of the product being 4-nitro- m-cresol. Microsomes prepared from mice treated in vivo with organophosphates, such as fenitrothion, have reduced levels of cytochrome P-450 and associated monooxygenase activities. Presumably, this reduction is due to destruction of P-450 caused by the release of active sulfur occurring during desulfuration. To determine if some P-450 isozymes were preferentially inhibited by fenitrothion, purified isozymes were incubated with fenitrothion and subsequent p-nitroanisole O-demethylase activity was determined. Inhibition studies using purified P-450 isozymes indicated close to 100% inhibition by fenitrothion of P-450 PB, while enzyme activity of P-450 BNF and P-450 A2 was inhibited 85–90%.