THE RELATIVE SIGNIFICANCE OF CO₂‐FIXING ENZYMES IN THE METABOLISM OF RAT BRAIN

Abstract

Abstract To evaluate the relative significance of CO 2 ‐fixing enzymes in the metabolism of rat brain, the subcellular distribution of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, NADP‐isocitrate dehydrogenase and NADP‐malate dehydrogenase, as well as the fixation of H 14 CO 3 − by the cytosol and the mitochondria was investigated. Pyruvate carboxylase and phosphoenol‐pyruvate carboxykinase are mainly localized in the mitochondria whereas NADP‐isocitrate dehydrogenase and NADP‐malate dehydrogenase are present in both the cytosol and the mitochondria. In the presence of pyruvate rat brain mitochondria fixed H 14 CO 3 − at a rate of about 170 nmol/g of tissue/min whereas these organelles fixed negligible amounts of H 14 CO 3 − in the presence of α‐ketoglutarate or phosphoenolpyruvate. Rat brain cortex slices fixed H 14 CO 3 − at a rate of about 7 nmol/g of tissue/min and it was increased by two‐fold when pyruvate was added to the incubation medium. The carboxylation of α‐ketoglutarate and pyruvate by the reversal of the cytosolic NADP‐isocitrate dehydrogenase and NADP‐malate dehydrogenase respectively was very low as compared to that by pyruvate carboxylase. The rate of carboxylation reaction of both NADP‐isocitrate dehydrogenase and NADP‐malate dehydrogenase was only about 1/10th of that of decarboxylation reaction of the same enzyme. It is suggested that under physiological conditions these two enzymes do not play a significant role in CO 2 ‐fixation in the brain. In rat brain cytosol, citrate is largely metabolized to α‐ketoglutarate by a sequential action of aconitate hydratase and NADP‐isocitrate dehydrogenase. The operation of the citrate‐cleavage pathway in rat brain cytosol is demonstrated. The data show that among four CO 2 ‐fixing enzymes, pyruvate carboxylase, an anaplerotic enzyme, plays the major role in CO 2 ‐fixation in the brain.