Incorporation of the 1-pro-R and 1-pro-S hydrogen atoms of ethanol in the reduction of acids in the liver of intact rats and in isolated hepatocytes

Abstract

Ethanol oxidation causes redox effects. The coupling of this oxidation via NADH to intermediary metabolism was studied in order to reveal the underlying mechanisms. Isolated rat hepatocytes were incubated with [1,1-2H2]-, (1R)-[1-2H]- and (1S)-[1-2H]-ethanol and the 2H incorporation was measured in lactate, beta-hydroxybutyrate, fumarate, malate, succinate, alpha-oxoglutarate and citrate. The results differed in the following ways from results obtained in intact rats. Lactate became labelled to an increasing extent, and in more than one position, indicating labelling of pyruvate. A small and constant fraction of malate and fumarate was formed without access to [2H]coenzyme. Addition of aspartate increased this fraction, which was concluded to be formed in the mitochondria. Citrate was essentially unlabelled. The 2H from (1R)-[1-2H]ethanol contributed to malate to a larger extent and to beta-hydroxybutyrate to a smaller extent, and 2H from (1S)-[1-2H]ethanol contributed to lactate to a smaller extent. These results indicate that the exchange via shuttle system was less efficient in isolated hepatocytes than in intact rats. The 2H incorporation was independent of concentration of [1,1-2H2]ethanol when this was above 5mM. Additions known to increase ethanol elimination, and cyanamide, which decreases it, had no marked effect on the 2H incorporation. This indicates equilibration of the NADH bound to alcohol dehydrogenase with free NADH. Disulfiram and cyanamide caused a decrease in the relative incorporation from (1S)-[1-2H]ethanol into malate in liver of intact rats. Addition of cyanamide to incubations with hepatocytes resulted in a decrease of the contribution of 2H from (1S)-[1-2H]ethanol in lactate, beta-hydroxybutyrate and malate. This indicates that acetaldehyde was only oxidized in the mitochondrial compartment.