Abstract
Isolated rat‐liver parenchymal cells oxidized ethanol at a rate of 1.4, 1.7, 1.9 and 2.5μmol/min per ml packed cells at 4, 20, 40 and 65 mM ethanol, respectively. Between 40 and 65 mM ethanol an abrupt 30% increase in the ethanol oxidation rate was observed. The activity of the NAD‐dependent alcohol dehydrogenase in a homogenate of isolated cells corresponded to 1.25–5 U/ml packed cells, depending on the assay method used. Fructose or pyruvate enhanced the oxidation rate of ethanol by 1.4–2.0 μmol/min per ml packed cells independent of the ethanol concentration applied. No additive effect of the two compounds upon ethanol oxidation was observed. Pyrazole inhibited the NAD dependent alcohol dehydrogenase activity in a homogenate of isolated cells as well as the unstimulated and the fructose‐stimulated ethanol oxidation with a Ki‐value of 9–13 μM.The “fructose effect” was inhibited 100% by pyrazole concentrations, which inhibited the unstimulated ethanol oxidation only 30%, indicating that the “fructose effect” is mediated via alcohol dehydrogenase. mM pyrazole only slightly inhibited that part of the ethanol oxidation which was not catalyzed by alcohl dehydrogenase, whereas 18 mM pyrazole inhibited also this pathway significantly. 50 μ pyrazole had no effect at all upon the basal ethanol oxidation rate wheras the fructose‐stimulated ethanol oxidation was 30% inhibited. The results concerning the effect of fructose and pyruvate upon ethanol oxidation are interpreted in terms of the mechanism for the reaction catalyzed by alcohol dehydrogenase. At low concentrations of ethanol and in the absence of fructose, the rate‐limiting step in ethanol oxidation appears to be dissociation of the enzyme‐NADH complex, whereas, in the presence of fructose, the maximal activity of ethanol dehydrogenase may be rate‐limiting for the oxidation of ethanol. The results also suggest that enzyme systems other than alcohol dehydrogenase participate in ethanol oxidation at high concentrations of ethanol.
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