Regulation of Pyruvate Metabolism in Rat‐Liver Mitochondria by Adenine Nucleotides and Fatty Acids

Abstract

Fluxes of pyruvate through the pyruvate dehydrogenase and the pyruvate carboxylase reactions in intact rat liver mitochondria were measured under various experimental conditions. When amounts of pyruvate not limiting for pyruvate dehydrogenase were present in the incubation medium, pyruvate carboxylase was found to be regulated essentially by the ATP:ADP ratio within the mitochondrial matrix. No direct correlation between pyruvate carboxylase activity and the level of intramitochondrial acetyl‐CoA could be observed under these conditions. When the ATP:ADP ratio within the matrix was decreased by elevation of the ADP concentration in the medium, a strong inhibition of the pyruvate carboxylase was observed. After addition of oleate to the medium this low ATP:ADP ratio within the matrix was increased and the inhibition of pyruvate carboxylase was partially reversed. It is demonstrated that this effect of oleate is probably caused by an inhibition of the adenine nucleotide translocase. The effects of oleate on pyruvate carboxylase were compared to those of octanoate. In contrast to oleate, octanoate diminished the pool size of the exchangeable adenine nucleotides, ATP and ADP, within the matrix and oxidative phosphorylation was stimulated. The reduced level of intramitochondrial ADP and the increased rate of oxidative phosphorylation elevated the ATP:ADP ratio within the matrix and partially reversed the inhibition of the pyruvate carboxylase by ADP. Thus, both fatty acids reversed the inhibition of pyruvate carboxylase by ADP via changes in the intramitochondrial ATP:ADP ratio but they did this by differing mechanisms. Octanoate was found to be much more rapidly activated and oxidized within the matrix than oleate. Furthermore, octanoate strongly inhibited pyruvate oxidation whereas with oleate this effect was much smaller. The physiological implications of these findings are discussed in relation to the stimulation of gluconeogenesis by fatty acids in the liver.