The Role of Pyruvate Carboxylase and P-enolpyruvate Carboxykinase in Rat Adipose Tissue

Summary Recent studies have shown that two enzymes, pyruvate carboxylase and P-enolpyruvate carboxykinase, normally considered to play a key role in the regulation of hepatic gluconeogenesis, are involved in the metabolism of adipose tissue, “a non-gluconeogenic” tissue. Pyruvate carboxylase is present in isolated adipose tissue mitochondria at a specific activity three times that found in liver mitochondria. This high activity is consistent with an important role of pyruvate carboxylase in providing oxaloacetate to support the rapid formation of citrate for lipogenesis. The overall rate of pyruvate carboxylation by mitochondria isolated from rat adipose tissue as measured by the incorporation of 14 C-bicarbonate into acid stable intermediates is of the same general magnitude as that found in mitochondria from rat liver and is somewhat higher than in rat kidney and guinea pig liver mitochondria. Citrate and malate were the main radioactive products, with citrate containing about two-thirds of the radioactivity fixed. Furthermore, pyruvate carboxylase and P-enolpyruvate carboxykinase of adipose tissue are part of a metabolic sequence for the synthesis of glyceride-glycerol (glyceroneogenesis). P-enolpyruvate carboxykinase is present in adipose tissue cytosol and has similar immunochemical properties to the enzyme in rat liver cytosol. Adipose tissue P-enolpyruvate carboxykinase is increased in activity by fasting, during diabetes, and unlike the liver enzyme, by adrenalectomy. Evidence is presented indicating that hormonal and dietary regulation of this enzyme in adipose tissue is closely correlated with the rate of glyceride-glycerol synthesis from pyruvate and that this pathway may play a role in the regulation of fat mobilization. Gluconeogenesis has been intensively studied over the past ten years and has been found to be confined in mammals, to liver and kidney cortex (Scrutton and Utter 1968, Krebs 1963). The integrative nature of this process as well as the specific controls regulating the interrelationship of gluconeogenesis with fatty acid oxidation in the mitochondria and with lipogenesis in the cytosol are, at the present time, only partly understood. One of the most fundamental aspects of the regulation of gluconeogenesis concerns those factors which influence what have been called the key gluconeogenic enzymes, glucose-6-phosphatase, fructose-1, 6-diphosphatase, pyruvate carboxylase and P-enolpyruvate carboxykinase. As was pointed out by Scrutton and Utter (1968) the maximal capacities of both pyruvate carboxylase and P-enolpyruvate carboxykinase in rat liver are only two to three-fold greater than the observed maximal rates of gluconeogenesis from various 3 and 4-carbon precursors. If these two enzymes are of prime importance in regulating hepatic glucose synthesis, we would expect that their intracellular location, kinetic and physical properties and adaptive behavior should provide information regarding their role in this process. But what is the function of these two enzymes in tissues which do not synthesize glucose? Our studies on adipose tissue have indicated that both pyruvate carboxylase and P-enolpyruvate carboxykinase are not only present in that tissue, but are also key enzymes in processes unrelated to the synthesis of glucose.