Regulation of gluconeogenesis and lipogenesis. The regulation of mitochondrial pyruvate metabolism in guinea-pig liver synthesizing precursors for gluconeogenesis

1. The carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase in guinea-pig liver mitochondria was determined by measuring the amount of 14C from H14CO3− fixed into organic acids in the presence of pyruvate, ATP, Mg2+ and Pi. The main products of pyruvate carboxylation were malate, fumarate and citrate. Pyruvate utilization, metabolite formation and incorporation of 14C from H14CO3− into these metabolites in the presence and the absence of ATP were examined. The synthesis of phosphoenolpyruvate from pyruvate and bicarbonate is minimal during continued oxidation of pyruvate. Larger amounts of phosphoenolpyruvate are formed from α-oxoglutarate than from pyruvate. Addition of glutamate, α-oxoglutarate or fumarate did not appreciably increase formation of phosphoenolpyruvate when pyruvate was used as substrate. With α-oxoglutarate as substrate addition of fumarate resulted in increased formation of phosphoenolpyruvate, whereas addition of succinate inhibited phosphoenolpyruvate formation. In the presence of added oxaloacetate guinea-pig liver mitochondria synthesized phosphoenolpyruvate in amount sufficiently high to play an appreciable role in gluconeogenesis. 2. Addition of fatty acids of increasing carbon chain length caused a strong inhibition of pyruvate oxidation and phosphoenolpyruvate formation, and greatly promoted carbon dioxide fixation and malate, citrate and acetoacetate accumulation. The incorporation of 14C from H14CO3−, [1−14C]pyruvate and [2−14C]pyruvate into organic acids formed was examined. 3. It is concluded that guinea-pig liver pyruvate carboxylase contributes significantly to gluconeogenesis and that fatty acids and metabolites play an important role in its regulation.