Regulation in vitro and in vivo of Adenosine 3′:5′‐monophosphate‐Dependent Inactivation of Rat‐Liver Pyruvate Kinase Type L

The cyclic‐AMP‐dependent inactivation of pyruvate kinase L has been studied in a crude Sephadex filtrate of isolated hepatocytes. This inactivation requires the presence of Mg‐ATP (apparent K m = 0.1 mM) and a half‐maximal rate of inactivation was obtained in the presence of 0.15 μM cyclic AMP. It was inhibited by physiological concentrations of phospho enol pyruvate and by micromolar concentrations of fructose bisphosphate and these inhibitory effects were counteracted by Mg‐ATP and by several l ‐form amino acids such as cysteine, alanine, serine, phenylalanine, which are also ligands of pyruvate kinase. As a rule, it appears that effectors that increase the activity of the enzyme are also those which prevent its cyclic‐AMP‐dependent inactivation and vice‐versa . Considering the potentially important role of phospho enol pyruvate concentration in the control of gluconeogenesis, experiments were performed to check if the inhibitory action of this metabolite on the inactivation of pyruvate kinase was of importance in vivo . We found that the concentration of phospho enol pyruvate was higher than normal in the livers of anesthetized rats in conditions like fasting and diabetes in which the rate of gluconeogenesis is known to be higher and the activity of pyruvate kinase lower than normally. The reverse was true upon sucrose feeding. Inactivation of pyruvate kinase was induced in vivo by the administration of glucagon. This inactivation was less important in the livers with high concentrations of phospho enol pyruvate and vice‐versa . It is proposed that phospho enol pyruvate exerts an important buffering effect in the variations of the rate of gluconeogenesis.