Influences of intracellular pyridine nucleotide redox states on fatty acid synthesis in isolated rat hepatocytes.

The regulation of fatty acid synthesis, measured by ³H₂O incorporation into fatty acids, was studied in hepatocytes from rats meal-fed a high carbohydrate diet. Ca²⁺ increased fatty acid synthesis, which became maximal at physiological concentrations of Ca²⁺. Ethanol markedly inhibited fatty acid synthesis. Maximum inhibition was reached at 4 mm ethanol. However, ethanol did not decrease lipogenesis in the presence of pyruvate. dl-3-Hydroxybutyrate increased fatty acid synthesis. Acetoacetate decreased lipogenesis when used alone and reversed the effect of dl-3-hydroxybutyrate when both were added. dl-3-Hydroxybutyrate moderately decreased flux through the pyruvate dehydrogenase system and markedly inhibited citric acid cycle flux. By measurement of glycolytic intermediates, two ethanol-induced crossover points were observed: one between fructose 6-phosphate and fructose 1,6-diphosphate and the other between glyceraldehyde 3-phosphate and 1,3-diphosphoglycerate. The concentrations of pyruvate and citrate were decreased by ethanol and increased by dl-3-hydroxybutyrate. Aminooxyacetate and l-cycloserine inhibited fatty acid synthesis and these effects were overcome by dl-3-hydroxybutyrate. Results indicate that in hepatocytes in a metabolic state favoring a high rate of lipogenesis, production of reducing equivalents in the cytosol via ethanol metabolism inhibits fatty acid synthesis from glucose by inhibition of both phosphofructokinase and glyceraldehyde 3-phosphate dehydrogenase and by promoting reduction of pyruvate to lactate. Production of reducing equivalents in the mitochondria via dl-3-hydroxybutyrate enhances fatty acid synthesis in liver cells by altering the partition of citrate between oxidation in the citric acid cycle and conversion to fatty acids in favor of the latter pathway. These interactions indicate the importance of the intracellular pyridine nucleotide redox states in the rate control of hepatic fatty acid synthesis.