Relationship between intracellular distribution of phosphoenolpyruvate carboxykinase, regulation of gluconeogenesis, and energy cost of glucose formation.

Abstract

Gluconeogenesis was studied in isolated perfused livers from pigeons, guinea pigs and rats in order to evaluate the role of intramitochondrial formation of phosphoenolpyruvate and the rate and importance of “futile cycling” of carbon for the regulation of glucose formation. The net formation of glucose from lactate (20 mM) by pigeon liver is about twice as high as in guinea pig liver and about 3 to 4 times higher than in rat liver, although the intracellular ATP/ADP ratio in pigeon liver is extremely low (< 1) in the presence and absence of gluconeogenesis.In contrast to experiments with rat livers, but in accordance with those with guinea pig livers, oleate (2 mM) failed to stimulate gluconeogenesis from lactate in pigeon liver.With pyruvate (20 mM) there is no net formation of glucose by pigeon livers in the absence or presence of hexanoate, oleate or xylitol. In the presence of ethanol the rate of glucose formation from pyruvate increased but did not exceed 30% of the rate observed with lactate as precursor.In contrast to the results obtained in rat liver experiments the transaminase inhibitor (aminooxy)acetate did not inhibit gluconeogenesis from lactate in pigeon liver although soluble and mitochondrial glutamate oxaloacetate aminotransferase from pigeon liver were strongly inhibited by (aminooxy)acetate. On the other hand, n‐butylmalonate (5 mM) inhibited strongly gluconeogenesis from lactate in isolated perfused rat and pigeon livers. This inhibition was accompanied by an inhibition of oxygen uptake in pigeon but not in rat liver. Benzene 1,2,3‐tricarboxylate, an inhibitor of the tricarboxylate translocator, had no effect on gluconeogenesis from lactate in isolated perfused rat and pigeon livers. Gluconeogenesis from propionate (10 mM) decreased in the order guinea pig‐rat‐pigeon.The activity of phosphoenolpyruvate carboxykinase as well as the phosphoenolpyruvate carboxykinase/pyruvate kinase ratio in pigeon liver were significantly higher than in rat and guinea pig liver. The activities of phosphoenolpyruvate carboxykinase as well as pyruvate carboxylase in pigeon liver were almost completely located intramitochondrially. Malic enzyme activity in pigeon liver was about 33 times higher than in rat liver and about 260 times higher than in guinea pig liver. However, the sum of the activities of the 4 NADPH‐generating enzymes, malic enzyme + isocitrate dehydrogenase (NADP) + glucose‐6‐phosphate dehydrogenase + 6‐phosphogluconate dehydrogenase was similar in pigeon and guinea pig liver, but somewhat lower in rat liver.Measurement of oxygen uptake during gluconeogenesis from intraportally infused lactate+pyruvate (10/1) was performed with isolated perfused livers from pigeon, rats and guinea pigs. The ratio oxygen used/glucose formed was 2.47 in pigeon liver, 2.42 in rat liver, and 2.35 in guinea pig liver.These results are very close to the theoretically expected value of 2.The following conclusions have been made: In pigeon liver phosphoenolpyruvate formed intramitochondrially is used for gluconeogenesis. There is no correlation between the overall ATP/ADP ratio and the rate of glucose formation. The regulation of gluconeogenesis is strongly species‐dependent as can be demonstrated for instance by the inability of pigeon liver to form glucose from pyruvate. The rate of “futile cycling” of carbon between phosphoenolpyruvate and pyruvate cannot exceed 40% of the rate of pyruvate carboxylation. The significant species difference in the maximum capacity for glucose formation from lactate cannot be explained on the basis of different degrees of “futile cycling”.