The microsomal ethanol oxidizing system and its interaction with other drugs, carcinogens, and vitamins.

The interaction of ethanol with the oxidative drug-metabolizing enzymes present in liver microsomes results in a number of clinically significant side effects in the alcoholic. Following chronic ethanol consumption, the activity of the microsomal ethanol oxidizing system (MEOS) increases. This enhancement of MEOS activity is primarily due to the induction of a unique microsomal cytochrome P-450 isozyme, which has a high capacity for ethanol oxidation, as shown in reconstituted systems. Normally present in liver microsomes at low levels, this form of cytochrome P-450 increases dramatically after chronic ethanol intake in many species, including baboons. The in-vivo role of cytochrome P-450 in hepatic ethanol oxidation, especially following chronic ethanol consumption, has been conclusively demonstrated in deer-mice lacking liver ADH. Induction of microsomal cytochrome P-450 by ethanol is associated with the enhanced oxidation of other drugs as well, resulting in metabolic tolerance to these agents. There is also increased cytochrome P-450-dependent activation of known hepatotoxins such as carbon tetrachloride and acetaminophen, which may explain the greater susceptibility of alcoholics to the toxicity of industrial solvents and commonplace analgesics. In addition, the ethanol-inducible form of cytochrome P-450 has the highest capacity of all known P-450 isozymes for the activation of dimethylnitrosamine, a potent (and ubiquitous) carcinogen. Moreover, cytochrome P-450-catalyzed oxidation of retinol is accelerated in liver microsomes, which may contribute to the hepatic vitamin A depletion seen in alcoholics. In contrast to chronic ethanol consumption, acute ethanol intake inhibits the metabolism of other drugs via competition for shared microsomal oxidation pathways. Thus, the interplay between ethanol and liver microsomes has a profound impact on the way heavy drinkers respond to drugs, solvents, vitamins, and carcinogens.