Fatty acid synthesis and Krebs' cycle activity in heart mitochondria

Abstract

The relationship between [1- 14C]acetate incorporation into fatty acids (FA) and the concomitant [1- 14C]acetate flux through citrate synthase (CS) was studied in isolated rabbit heart mitochondria during state 4 (Chance) respiration. The flux through CS was calculated from the 14C content of 14CO 2, Krebs' cycle acids, and glutamate; the latter were extracted and separated by ion exchange chromatography. Both succinate in low concentration and l-malate effected low NADH NAD ratios and increased flux through CS without stimulating incorporation into FA. Succinate in high concentration effected high NADH NAD ratios, reduced flux through CS, and stimulated incorporation into FA. In the presence of succinate both rotenone and l-carnitine inhibited incorporation into FA but not flux through CS despite high NADH NAD ratios. Maximal [2- 14C]pyruvate flux through CS approximated that of [ 14C]acetate. However, with the high NADH NAD ratio effected by succinate, the [2- 14C]pyruvate flux through CS was greater than that of [ 14C]acetate and the [2- 14C]pyruvate incorporation into FA was only about one-tenth that of [ 14C]acetate. Studies with the isolated perfused rabbit heart showed that hypoxia increased five-to-nine fold the net [U- 14C]glucose incorporation into FA. We conclude that in heart mitochondria a high NADH NAD ratio not only stimulates [ 14C]acetate incorporation into FA but also concomitantly slows flux through citrate synthase by decreasing the oxalacetate level. The data suggest an acetyl CoA pool available for FA synthesis different from that for CS. During myocardial hypoxia mitochondrial FA synthesis may supplement lactate production as a mechanism for NADH oxidation.