The effect of ethanol on N-demethylation of aminopyrine in rat liver slices and in the microsomal fraction and on microsomal hydroxylation of pentobarbital and aniline was studied. With liver slices N-demethylation of aminopyrine was stimulated by 35–40% at low ethanol concentrations (2mm), whereas no stimulation occurred at high concentrations (100mm). With the liver microsomal fraction, an inhibitory effect was observed only at high ethanol concentrations (100mm). This was also observed with the other drugs studied. In agreement with these results, only at a high concentration did ethanol interfere with the binding of drug substrates to cytochrome P-450. Further, as previously reported, ethanol produced a reverse type I spectral change when added to the liver microsomal fraction. Evidence that this spectral change is due to removal of substrate, endogenously bound to cytochrome P-450, is reported. A dual effect of ethanol is assumed to explain the present findings; in liver slices, at a low ethanol concentration, the enhanced rate of drug oxidation is the result of an increased NADH concentration, whereas the inhibitory effect observed with the microsomal fraction at high ethanol concentration is due to the interference by ethanol with the binding of drug substrates to cytochrome P-450.

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