Functional Compartmentation of Acetaldehyde Oxidation in Rat Liver

Abstract

Abstract Perfused rat liver and suspensions of isolated rat liver cells have been used to study the influence of the transaminase inhibitors aminooxyacetate and dl-cycloserine on the rate of acetaldehyde utilization, and the effects of acetaldehyde on the state of reduction of cytosolic and mitochondrial pyridine nucleotides. In the presence of 4-methylpyrazole to inhibit acetaldehyde reduction to ethanol, acetaldehyde removal was constant over the range from 0.1 to 0.4 mm at a rate of approximately 400 µmoles per g dry weight per hour. Between 0.4 and 10 mm acetaldehyde, the rate of acetaldehyde uptake increased by 60% with increasing acetaldehyde concentrations, with a half-maximum increment of uptake being achieved at about 1 mm acetaldehyde. dl-Cycloserine had no effect on acetaldehyde uptake at low concentrations of acetaldehyde but almost completely inhibited the stimulation of uptake observed at high concentrations. Neither dl-cycloserine nor aminooxyacetate had any inhibitory effect on the reduction of mitochondrial pyridine nucleotides induced by acetaldehyde. At high acetaldehyde concentrations (3 to 4 mm) pyruvate stimulated acetaldehyde uptake, but little effect was observed at low acetaldehyde concentrations (0.3 mm). Acetaldehyde uptake was inhibited by fatty acids, β-hydroxybutyrate, and amobarbital, and stimulated by acetoacetate. Oxidation of endogenous fatty acid was diminished by acetaldehyde. These data indicate that oxidation of acetaldehyde by rat liver occurs almost entirely in the mitochondrial compartment when the mean arterial-venous acetaldehyde concentration is below about 0.4 mm. At higher acetaldehyde concentrations, oxidation occurs also in the cytosol, and reducing equivalents generated in the cytosol by a high Km NAD-linked aldehyde dehydrogenase are transported to the mitochondria mainly by the malate-aspartate cycle. It may be concluded that acetaldehyde generated during ethanol metabolism is oxidized to acetate predominantly in the mitochondria, so that only 1 eq of NADH is generated in the cytosol per mole of ethanol oxidized via alcohol dehydrogenase.