The Regulation of Gluconeogenesis in Mammalian Liver

The significance of mitochondrial phosphoenolpyruvate formation for gluconeogenesis was evaluated in isolated, perfused livers from rats, guinea pigs, and rabbits. In guinea pig liver, 0.2 mm octanoate decreased by 40 and 30% the rate of gluconeogenesis from lactate and alanine, respectively, but not from pyruvate. In rat liver, gluconeogenesis from all three substrates was increased. The inhibition of gluconeogenesis in guinea pig liver was associated with an increased level of reducing equivalents in the mitochondria as indicated by the NAD+:NADH ratios in freeze-clamped livers before and during infusion of octanoate. Infusion of β-hydroxybutyrate also increased gluconeogenesis from pyruvate and lactate in rat liver but inhibited glucose formation from lactate in guinea pig liver. Low concentrations of the artificial electron acceptor, phenazine methosulfate, reversed the inhibitory effects of excess mitochondrial reducing equivalents. Aminooxyacetate, which inhibits aminotransferases, totally abolished glucose formation from lactate (but not from pyruvate) in perfused rat liver but reduced gluconeogenesis by only 50 to 60% in guinea pig liver. This inhibition was further increased by β-hydroxybutyrate infusion. In livers from fed rabbits, aminooxyacetate had no effect, but in livers from fasted rabbits in which the activity of cytosolic P-enolpyruvate carboxykinase is induced, the inhibition by aminooxyacetate was quantitatively similar to that in the guinea pig liver. The data show that in the guinea pig and rabbit liver both the cytosolic and mitochondrial forms of P-enolpyruvate carboxykinase actively function during gluconeogenesis from lactate and alanine. The gluconeogenic flux via the mitochondrial enzyme accounts for at least one-half of the over-all rate of glucose formation from lactate. It is concluded that the regulation of gluconeogenesis in species containing a mitochondrial activity of P-enolpyruvate carboxykinase is significantly different from that operating in rat liver and reflect to a large extent the compartmentation of P-enol-pyruvate carboxykinase.