Ketogenesis in isolated rat-liver mitochondria. IV. Oxaloacetate decarboxylation: Consequences for metabolic calculations

Abstract

Oxaloacetate which is formed by isolated rat-liver mitochondria during oxidation of malate may be decarboxylated to pyruvate by the action of oxaloacetate decarboxylase (EC 4.1.1.3). The pyruvate so formed is rapidly oxidized to acetyl-CoA from which citrate is formed by condensation with a second molecule of oxaloacetate. The rate of this pathway has been measured in various energy states in the presence of fluorocitrate to block the oxidation of citrate. The rate of conversion of malate to acetyl-CoA was calculated either from a comparison of the disappearance of malate and the formation of the various products or from 14CO 2 production from l-[U- 14C]-malate. When fatty acids or pyruvate are oxidized together with malate, the conversion of malate to acetyl-CoA is decreased, especially in a high-energy state (State 4). In a low-energy state (State 3) this conversion decreases with increasing rates of β-oxidation; at the same time the concentration of oxaloacetate in the medium is lowered. It is concluded tentatively that the rate of decarboxylation of oxaloacetate is controlled by the amount of oxaloacetate available to oxaloacetate decarboxylase in the mitochondrial matrix. Pyruvate was found to exert a direct inhibitory effect upon the decarboxylase. The implications of our observations for metabolic calculations concerning the simultaneous oxidation of fatty acid and malate are discussed. It is pointed out that relative rates of ketogenesis, calculated from oxygen uptake and disappearance of substrate, are severely overestimated if the variable contribution of malate to the acetyl-CoA pool is not taken into account.