Pyruvate kinase is a key regulatory enzyme in hepatic carbohydrate metabolism. Studies from this laboratory were the first to demonstrate that stimulation of gluconeogenesis by glucagon and epinephrine is accompanied by inhibition and phosphorylation of pyruvate kinase. Insulin also regulates pyruvate kinase, favoriate dephosphorylation. The mechanisms by which insulin and glucose cause dephosphorylation of pyruvate kinase is not known. Recently, insulin was shown to cause activation of a phosphoprotein phosphatase in rat liver which dephosphorylates (and activates) pyruvate kinase. One hypothesis is that insulin binding to its receptor causes phosphorylation of a """"""""pyruvate kinase"""""""" phosphatase. A primary goal of the proposed research will be to test this hypothesis. The """"""""pyruvate kinase"""""""" phosphatase will be purified from rat liver and tested as a substrate for the protein kinase activity of the insulin receptor. A reconstituted system will be developed to test the influence of glucose and its metabolites on the dephosphorylation of pyruvate kinase. Finally, insulin regulation of the """"""""pyruvate kinase"""""""" phosphatase will be characterized in isolated rat hepatocytes and in liver from normal and diabetic rats. For this purpose, antibodies will be raised against the """"""""insulin-regulated"""""""" phosphatase. Success in the proposed studies will provide better insight into the mechanisms by which insulin and glucose modulate hepatic carbohydrate metabolism. Ultimately that knowledge may lead to new treatments for diabetics which will reduce the detrimental consequences of high rates of gluconeogenesis.and glucose excretion observed in those patients.
