Mitochondrial phosphoenolpyruvate carboxykinase (GTP) and the regulation of gluconeogenesis and ketogenesis in avian liver

Chicken liver synthesizes glucose from lactate and dihydroxyacetone at high rates but pyruvate, glycerol, alanine, and other amino acids are poor glucose precursors. Despite its limited conversion to glucose, 1mM pyruvate completely suppressed ketone body synthesis from octanoate by perfused chicken liver, whereas lactate at this concentration had no effect on ketogenesis. Also, increasing the lactate concentration over a range of 0.5 to 10mM caused a marked decrease in ketone body formation. In hepatocytes from chicken liver, pyruvate (10mM) stimulated the oxidation of [1-¹⁴C] octanoate to ¹⁴CO₂ 5-fold, while decreasing net ketone body synthesis. Conversely, octanoate had no effect on the oxidation of [1-¹⁴C] pyruvate or [2-¹⁴C] pyruvate to ¹⁴CO₂. The regulation of pyruvate metabolism by chicken liver mitochondria differed markedly from guinea pig liver mitochondria. The conversion of [2-¹⁴C] pyruvate to either [¹⁴C] acetoacetate or ¹⁴CO₂ by isolated chicken liver mitochondria was not inhibited by octanoate oxidation to the same extent as was noted with mitochondria from guinea pig liver. Also, the decarboxylation of [1-¹⁴C] pyruvate to ¹⁴CO₂ by guinea pig liver mitochondria was more sensitive to inhibition by octanoate. Our results suggest that the pyruvate dehydrogenase complex in chicken liver is not as sensitive to the inhibitory effects of fatty acid oxidation as is the same enzyme complex in guinea pig liver. Measurement of intermediates in the pathway of gluconeogenesis from lactate, pyruvate, glycerol, and dihydroxyacetone was used to assess the points in the pathway where intermediates accumulate, especially after the simultaneous infusion of octanoate. With both lactate and pyruvate, octanoate infusion greatly increased the levels of citrate, α-ketoglutarate, and malate, with only a marginal effect on glycolytic intermediates, except for a decrease in the concentration of phosphoenolpyruvate. The most notable change was a 10-fold increase in oxalacetate concentration in chicken liver after perfusion with pyruvate and octanoate. The limitation in disposal of cytosolic NADH in chicken liver was apparent in experiments in which glycerol or glycerol plus octanoate were perfused simultaneously. In this experiment the concentration of α-glycerophosphate in the liver was greater than 5μmol/g. Thus, the mitochondrial location of P-enolpyruvate carboxykinase in chicken liver influences the pattern of regulation of gluconeogenesis and ketogenesis in this species.