Control of β-hydroxybutyrate and acetoacetate oxidation by inorganic phosphate and adenosine 5′-diphosphate in heart mitochondria

Abstract

The oxidation of d(−)β-hydroxybutyrate and acetoacetate in beef-heart mitochondria is controlled by inorganic phosphate, ADP, and ATP through the reactions of substrate-level phosphorylation. These compounds affect the mitochondrial concentration of succinyl coenzyme A, which is required for activation and subsequent oxidation of acetoacetate. Thus, in the presence of dinitrophenol-treated mitochondria, phosphate causes a severe inhibition of acetoacetate oxidation (50% at 10 m m phosphate), and AMP, ADP, and GDP enhance this effect. Through the reactions of substrate-level phosphorylation, these compounds can shift the utilization of succinyl-CoA toward the synthesis of ATP and make it unavailable for acetoacetate activation. Since all the reactions of substrate-level phosphorylation are reversible, addition of ATP reverses the phosphate inhibition. Added succinyl-CoA is a potent activator of acetoacetate oxidation, even when 90% of the activity is inhibited by phosphate and AMP. In addition to the above results, an interesting competition between phosphate and arsenate is presented. Thus, 0.17 m m arsenate inhibits acetoacetate oxidation by 92%, but comparable amounts of phosphate reverse this inhibition. The reason for the contrasting activator-inhibitor effect of phosphate is explained. Moreover, the implications of the above results are discussed in relation to ketone body metabolism as controlled by the respiratory state of mitochondria, especially during starvation and diabetic ketosis.