Ethanol-induced increase in NADH-dependent monooxygenase activities of hepatic microsomal cytochrome P-450

Abstract

NADH-dependent hepatic microsomal monooxygenase activities were measured in the presence and absence of NADPH in material from adult male rats given ethanol in a liquid diet. Controls were given a liquid control diet (control group; lipid as substitute for ethanol) or rat chow (untreated group). Ethanol feeding elevated microsomal aniline hydroxylase activity and did not change ethylmorphine N-demethylase activity. NADH supported 21–24% of the NADPH-driven aniline hydroxylase activity in ethanol, control and untreated microsomes, but only about 6% of ethylmorphine N-demethylase activity. In the presence of NADPH, NADH gave 13–14% increase in aniline hydroxylase activity in microsomes from control and untreated rats, but only 3% in ethanol microsomes. In contrast, the presence of NADPH increased many times the effect of NADH on ethylmorphine N-demethylation with no striking difference between the groups. In another series of experiments, demethylation of 4-nitroanisole was elevated after ethanol feeding (4-fold with NADPH; 5-fold with NADH) and phenobarbital treatment (8-fold with NADPH, 2-fold with NADH). In the ethanol-induced activity, NADH and NADPH were less than additive. In the control and untreated and the phenobarbital-induced activities, NADH and NADPH were additive or possibly synergistic in driving the activity. Both ethanol and phenobarbital elevated cytochrome P-450; ethanol also elevated cytochrome b 5 measured as NADH-reducible cytochrome. NADPH caused oxidation of some cytochrome b 5 when added to NADH-reduced microsomes from ethanol-fed rats, but enhanced reduction of cytochrome b 5 when added to NADH-reduced microsomes from control or untreated rats or rats pretreated to induce either microsomal desaturases (fasted rats refed lipid-free high-carbohydrate diet) or monooxygenases (phenobarbital-treated rats). The finding suggests a tight connection between the ethanol-induced cytochrome P-450 and cytochrome b 5 and that ethanol-induced and phenobarbital-induced cytochrome P-450 differ with respect to communication with the NADH-dependent electron transport system which is usually of minor importance in delivering electrons to cytochrome P-450. These findings may be of some importance in the development of metabolic tolerance to ethanol in the liver.