Microsomal epoxidation of cyclodiene insecticides

Chlordane and heptachlor are metabolized enantioselectively by rat liver microsomes

Chlordane, heptachlor, and their metabolites are chiral persistent organic pollutants that undergo enantiomeric enrichment in the environment. This study investigated the enantioselective metabolism of both chlordane isomers and heptachlor, major components of technical chlordane, by liver microsomes prepared from male rats treated with corn oil (CO) or inducers of CYP2B (PB; phenobarbital) and CYP3A enzymes (DX; dexamethasone), isoforms induced by chlordane treatment. The extent of the metabolism of all three parent compounds was dependent on the microsomal preparation used and followed the rank order PB > DX > CO. The mass balances ranged from 49 to 130% of the parent compound added to the microsomal incubations. Both cis- and trans-chlordane were enantioselectively metabolized to oxychlordane (EF = 0.45-0.89) and 1,2-dichlorochlordene (EF = 0.42-0.90). Heptachlor was metabolized enantioselectively, with heptachlor epoxide B (EF = 0.44-0.54) being the only metabolite. Interestingly, the direction on the enrichment for oxychlordane, 1,2-dichlorochlordene, and heptachlor epoxide differed depending on the microsomal preparation. These findings demonstrate that the direction and extent of the enantioselective metabolism of both chlordane isomers and heptachlor is P450 isoform-dependent and can be modulated by the induction of P450 enzymes.

Inhibitors of CYP6D1 in house fly microsomes

CYP6D1 is a cytochrome P450 responsible for the metabolism of insecticides and other xenobiotics in the house fly (Musca domestica). Using a CYP6D1-specific monooxygenase activity and a non-CYP6D1-specific monooxygenase activity, 21 compounds were evaluated as inhibitors of CYP6D1 in house fly microsomes. CYP6D1 was strongly inhibited by xanthotoxin, chlorpyrifos, beta-naphthoflavone, piperonyl butoxide and 5-methoxypsoralen. The highest selectivity for inhibition of CYP6D1 was seen for 5-methoxypsoralen, xanthotoxin, beta-naphthoflavone, chlorpyrifos oxon, isosafrole and psoralen. The results clearly indicate that identification of isoform-selective inhibitors of P450s within an insect, and across species, is possible. In addition, psoralen and 5-methoxypsoralen stimulated ethoxycoumarin O-deethylation suggesting that these compounds were substrates for monooxygenases in house fly microsomes. Isosafrole was shown to be a potent synergist of pyrethroid insecticides in adult house flies.

Enhancement of the binding of O-ethyl O-p-nitrophenyl phenylphosphonate (EPNoxon) to microsomal carboxylesterase by NAD in vitro

Inhibition of rat liver microsomal carboxylesterase (CEase) by O-ethyl O-p-nitrophenyl phenylphosphonothioate (EPN) and binding of EPN oxygen analog to microsomal CEase were enhanced by addition of NAD or NADP. This was more prominent in addition of NAD than NADP. No potentiation of anti-CEase action of EPN by NAD was seen when pure esterase (E.C. 3.1.1.1) instead of liver microsomes was used as an enzyme source. This effect of NAD in microsomal CEase was significantly decreased when N-ethylmaleimide or p-chloromercuribenzoic acid was added. From these findings, it is strongly suggested that NAD-mediated potentiation of the anti-CEase action of EPN might be attributed to the increase in formation of NADH from NAD by microsomal dehydrogenase(s) containing a sulfhydryl group, leading to a subsequent increase in formation of the EPN oxygen analog from EPN, and in turn, CEase inhibition was enhanced.

Methylene-C14-dioxyphenyl compounds: metabolism in relation to their synergistic action

The methylene-C(14) group is hydroxylated yielding formate-C(14) in the microsome-reduced nicotinamide-adenine dinucleotide phosphate system in vitro and yielding expired C(14)O(2), in living mice and houseflies. Methylenedioxyphenyl compounds apparently serve as alternate substrates for this enzymatic hydroxylation system of microsomes, and thus reduce the rate of metabolism and prolong the action of certain drugs and insecticide chemicals.