Metabolic Effects of Ethanol in Perfused Rat Liver

Abstract

Abstract In the present study, ethanol oxidation by the perfused rat liver has been used to investigate the interrelationships between the pathways of glucose metabolism, fatty acid oxidation, and the citric acid cycle. In the absence of exogenous fatty acids, the production of glucose from alanine was stimulated 2-fold by 10 mm ethanol, whereas, in the presence of 1 mm oleate, ethanol caused an inhibition of net glucose production. Measurements of the rates of ethanol utilization and acetate formation showed that over 80% of the ethanol metabolized was converted to acetate. The increased rate of generation of reducing equivalents in the cytosol during ethanol oxidation increased the oxidation-reduction state of pyridine nucleotides in both the intra- and extramitochondrial compartments. This fact was established by analyses of the tissue content of pyridine nucleotides and substrate couple ratios, and directly by surface fluorometry. Changes of flavin and pyridine nucleotide fluorescence intensity from the surface of the liver showed that the transfer of reducing equivalents from cytosol to mitochondria during ethanol oxidation was very rapid. Analyses of intermediates in the gluconeogenic pathway of livers perfused in the absence of fatty acids indicated an activational site at the glyceraldehyde-3-P dehydrogenase step upon ethanol addition. The stimulatory effect of ethanol on gluconeogenesis from alanine, therefore, results from the increased availability of the NADH in the cytosol. On the other hand, when ethanol was added to livers perfused in the presence of oleate, an inhibitory site was observed between fructose-1, 6-di-P and fructose-6-P. Measurement of the tissue levels of the known modifiers of phosphofructokinase and fructose diphosphate phosphatase indicated that this effect was caused primarily by deinhibition of phosphofructokinase resulting from a fall of the citrate content. Oxidation of NADH produced during ethanol metabolism inhibited the activity of the citric acid cycle. Sites of inhibition were identified at the citrate synthase and iso-citrate dehydrogenase steps. The relative strengths of the inhibitory interactions at these sites were dependent on the rate of β oxidation. It is proposed that a coordinated inhibition of citrate synthase and isocitrate dehydrogenase is mediated primarily by the increased state of reduction of intramitochondrial pyridine nucleotides.