Regulation of gluconeogenesis and lipogenesis: Inhibition of pyruvate carboxylation in rat kidney mitochondria by malonate and malonyl CoA

Abstract

Rat kidney mitochondria in the presence of ATP, P i and magnesium ions, converted pyruvate and bicarbonate to malate, fumarate and citrate. Pyruvate carboxylase activity was estimated by measuring the incorporation of bicarbonate- 14C into tricarboxylic acid cycle intermediates. The addition of malonate decreased utilization of pyruvate and strongly inhibited bicarbonate- 14C incorporation into the organic acids. This inhibition of bicarbonate- 14C incorporation into organic acids in the presence of malonate was reversed by carnitine, oxalacetate, malate, fumarate, succinate and α-ketoglutarate, but not by citrate, isocitrate, aspartate, glutamate or acetylcarnitine. The ratio of concentration of carnitine to concentration of malonate determined the degree of reversal of this inhibition. Malonate-1- 14C was metabolized by rat kidney mitochondria to acetyl CoA and 14CO 2. Carnitine stimulated malonate-1- 14C decarboxylation presumably by removing acetyl CoA from the system and forming acetylcarnitine. In the presence of tricarboxylic cycle intermediates the incorporation of bicarbonate- 14C into malate was greatly increased. The inhibition of 14CO 2 incorporation into organic acids in the presence of malonate appears to be caused by malonyl CoA. Externally added malonyl CoA was shown to be a potent inhibitor of 14C0 2 incorporation into organic acids. This inhibition by malonyl CoA was partially reversed by carnitine. The conversion of pyruvate to dicarboxylic acids is one of the key steps in the pathway of gluconeogenesis. The inhibition of this step by malonate may exercise regulatory effects on gluconeogenesis. It is postulated in this paper that changes in the ratio of concentration of acetyl CoA to malonyl CoA may play a regulatory role, since malonyl CoA is a key intermediate in the pathway of lipogenesis this mechanism may be of physiologic significance.