Potentiation of ethyl para-nitrophenyl phenyl-phosphonothioate (EPN)-induced inhibition of liver microsomal carboxylesterase by NAD in vitro in rats.

The higher inhibition of liver microsomal carboxylesterase (CEase) by EPN, as compared to that of acetylcholinesterase (AchE) may be, at least in part, explained by the present findings that NAD potentiated the anti-CEase, but not anti-AchE, action of EPN. This phenomenon was referred to as "NAD-effect" in this paper. NAD-effect was not due to the increased formation of oxygen analog of EPN (EPN=O) by NAD addition through liver microsomal cytochrome P-450 catalyzed monoxygenase, because the amounts of EPN=O formed during incubation in the presence and absence of NAD were not significantly changed as shown by gaschromatography-mass spectrometric estimations. In addition, HAD-effect could be observed in the experiments even under carbon monoxide atmosphere. Such NAD-effect was observed only when NAD, EPN and an unidentified component bound to liver microsomal membrane were co-existent in the incubation mixture.