Hepatic microsomal ethanol-oxidizing system (MEOS): dissociation from reduced nicotinamide adenine dinucleotide phosphate oxidase and possible role of form I of cytochrome P-450.

The activity of the hepatic microsomal ethanol-oxidizing system (MEOS) was compared with the content of three forms of cytochrome P-450. Measurements were also made of the activity of microsomal reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, the enzyme which generates H2O2 in microsomes and which has been considered by some to be the rate-limiting step of MEOS activity. Ethanol feeding to rats for 4 to 5 weeks significantly enhanced the activities of MEOS and NADPH oxidase by 102 and 62%, respectively. Concomitantly, form I of cytochrome P-450 was increased by 88% (P less than .001). Acute administration of a large dose of ethanol to animals pretreated chronically with ethanol enhanced MEOS activity by 21% (P less than .05), whereas NADPH oxidase activity remained unchanged. In addition, an acute dose of ethanol enhanced form I of cytochrome P-450 by 20% (P less than .05); thus its increase was comparable to that of MEOS activity. Pretreatment of rats with phenobarbital increased the specific activity of microsomal NADPH oxidase by 40% (P less than .05) but not that of MEOS. By contrast, CCl4 administration to rats diminished MEOS activity by 33% (P less than .01), whereas NADPH oxidase activity remained unchanged. The CCl4 treatment was found to decrease significantly all three forms of cytochrome P-450: form I by 45%, form II by 56% and form III by 24%. These results suggest that in the presence of NADPH microsomes oxidize ethanol to acetaldehyde by a process which involves, at least in part, the form I of cytochrome P-450 and in which H2O2 generation by NADPH oxidase is not the rate-limiting step.