Metabolism and Metabolic Actions of Ethanol

Abstract

It has been known for a long time that metabolism of ethanol is faster in the fed than in the fasted state. The mechanism of this effect has recently been clarified by the definition of some rate-limiting factors in the oxidation of ethanol in a study by Meijer et al. (1975). Ethanol oxidation in liver via its main alcohol dehydrogenase (ADH) pathway (Fig. 1) is limited by the rate of reoxidation of cytosolic NADH, not by the activity of ADH. Reoxidation of cytosolic NADH requires transport of reducing equivalents into the mitochondria via substrate shuttles and reoxidation of the reducing equivalents by the mitochondrial respiratory chain. There have been few attempts to determine whether the rate-limiting factor in ethanol oxidation is the activity of the shuttles in transporting reducing equivalents, or the actual oxidation of these reducing equivalents by the respiratory chain. Data from Meijer et al. (1975) now suggest that each of these factors may be a rate-limiting step in ethanol oxidation, depending on whether rats were fed or starved prior to the experiment. Ethanol oxidation was studied in isolated rat liver cells. In cells from starved animals, the addition of the components of the malate-aspartate shuttle, e. g., glutamate or malate, resulted in a marked stimulation of ethanol oxidation; this effect was sensitive to inhibition by the transaminase inhibitor cycloserine. These results indicate that the malate-aspartate cycle does not operate at full capacity. Addition of dihydroxyacetone stimulated ethanol uptake by 45% and caused a 3-fold increase in glycerol-3-phosphate content. Since ethanol uptake under these conditions is insensitive to cycloserine, it may be assumed that hydrogen transport into mitochondria occurs by the glycerol-3-phosphate cycle. The increase of ethanol oxidation by the addition of components of the malate-aspartate cycle (and its inhibition by transaminase inhibitors) or of the glycerol-3phosphate cycle suggests that in the fasting state, the oxidation of ethanol by isolated liver cells is limited by the rate of transfer of reducing equivalents from the cytosol (where they are generated by ADH) to the mitochondria, and that this transport is itself regulated by the intracellular concentrations of the intermediates of the shuttles. Inhibition of the electron transport chain by the addition of amytal resulted in a significant reduction of ethanol oxidation, suggesting that the flux through the respiratory chain also regulates the rates of ethanol metabolism. However, the actual reoxidation of reducing equivalents by the respiratory chain is not limiting under these conditions, since uncoupling agents, which stimulate oxygen consumption, did not stimulate ethanol oxidation.