Effects of ethanol and fat on the transport of reducing equivalents into rat liver mitochondria

Chronic ethanol intoxication in rats accelerated the rate of ethanol metabolism, but decreased the activity of alcohol dehydrogenase. Chronic consumption of ethanol decreased the activities of cytochrome oxidase and succinic dehydrogenase in mitochondria and in cell free homogenates, whereas total hepatic mitochondrial protein was not altered by ethanol feeding. Thus, total 'mitochondrial activity' was decreased and cannot account for the increased rate of blood ethanol clearance. The activities of mitochondrial enzymes and those of 'shuttle' systems for the transport of reducing equivalents into mitochondria were investigated. Reconstituted malate aspartate, fatty acid, and α glycerophosphate shuttles were equally effective in transporting reducing equivalents into the mitochondria from ethanol fed and control rats. The activities of enzymes involved in the shuttles, such as cytoplasmic and mitochondrial α glycerophosphate dehydrogenase and glutamic oxalacetic transaminase, were either decreased or unchanged by chronic ethanol consumption. The permeability of the mitochondria to NADH and to substrate anions which participate in the shuttles was also not altered by ethanol feeding. It is therefore unlikely that the increased rates of ethanol oxidation produced by chronic ethanol consumption are caused by enhanced transport of reducing equivalents into the mitochondria. A high fat diet alone (35% of total calories) increased both the endogenous and total rates of shuttle activity, compared with the rates obtained with mitochondria from rats which had consumed a low fat diet. However, the rates of blood ethanol clearance were comparable in rats consuming high fat and low fat diets. Therefore, the transport of reducing equivalents into the mitochondria under these conditions does not appear to be rate limiting for ethanol metabolism.