Enhancement of the Binding of O-Ethyl O-p-Nitrophenyl 3henylphosphonate (EPNoxon) to Microsomal Carboxylesterase by NAD in vitro

Abstract

Abstract-Inhibition of rat liver microsomal carboxylesterase(CEase) by 0-ethyl 0-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

NAD-Coupled Enzymatic Oxidation of O-Ethyl O-p-Nitrophenyl Phenylphosphonothioate (EPN) to Its Oxygen Analog with Liver Microsomes of Rats

“…Namely, binding of oxygen analogs of these com pounds to CEase, a nonvital enzyme may, reduce the anti -chol1nergic action of organophosphorus insecticides. In conclusion, taken together with our previous papers (1,2) and unpublished observation (S. Sugiyama et al), the present results strongly suggest that an enzymatic conversion of NAD to NADH by microsomal dehydrogenase(s) may be, at least in part, involved in binding of the organophosphate insecticides formed in vivo to CEase, by which, in turn, toxicities of these insecticides are reduced.…”

“…So, involvement of NAD-dependent dehy drogenase (s) in liver microsomes in the NAD-effect was studied. We have already reported that addition of NEM to the incubation mixture containing liver micro somes, EPN and NAD caused a remarkable decrease of the NAD-effect (2) . In this study, degree of formation of EPNoxon from EPN in the incubation mixture was decreased ( Table 2).…”

“…In order to clarify the NAD-effect, NAD-mediated formation of EPNoxon from EPN was studied when the supernatant of the microsomes solubilized by sodium cholate was used as the enzyme source. In the inhibition studies, NEM was used as a sulfhydryl-blocking reagent (2). Phenylmethylsulfonyl fluoride (PMSF) (8) and EDTA (8,9) were used to inhibit binding of EPNoxon to both CEase and A type-esterase (10) which was responsible for hydrolysis of EPNoxon.…”

“…Addition of suIf l ydryl-blocking reagents such as NEM and PCMB diminished the NAD-effect (2). Further, there is a close relation between the chemical structure of organophosphorothioate insecticides and oc currence of the NAD-effect (S. Sugiyama et al, unpublished observation).…”