Hormone-induced changes in pyruvate kinase activity in isolated hepatocytes. II. Relation to the hormonal regulation of gluconeogenesis
Van Berkel, T.J.; Kruijt, J.K.; Koster, J.F.
Biochimica et Biophysica Acta 500(2): 267-276
Incubation of isolated hepatocytes with glucagon (10-6 M) or dibutyryl cyclic[dbc]AMP (0.1 mM) causes a decrease in pyruvate kinase [PK] activity of 50%, measured at suboptimal substrate (phosphoenolpyruvate) concentrations and 1 mM .**GRAPHIC**. The magnitude of the decrease in activity is not influenced by the applied extracellular concentrations of lactate (1 and 5 mM), glucose (5 and 30 mM) or fructose (10 and 25 mM). With all 3 substrates comparable inhibition percentages are induced by glucagon or dbc AMP. The extent of inhibition of pyruvate kinase induced by incubation of hepatocytes with glucagon or dbc AMP is not influenced by the extracellular Ca2+ concentration nor by the presence of 2 mM EGTA. The reactivation of PK seems to be inhibited by a high concentration of extracellular Ca2+ (2.6 mM) as compared to a low concentration of extracellular Ca2+ (0.26 mM). Incubation of hepatocytes in a Na+-free, high K+-concentration medium does not influence the magnitude of the PK inhibition induced by dbc AMP. The reactivation reaction is stimulated under these incubation conditions. Incubation of hepatocytes with dbc GMP (0.1 mM) leads to 25% decreased in PK activity. The magnitude of the inhibition by dbc GMP is not influenced by the presence of pyruvate (1 mM) or glucose (5 mM and 30 mM). The relative insensitivity of the PK inhibition induced by glucagon, dbc AMP and dbc GMP to the extracellular environment leads to the conclusion that the hormonal regulation of PK is not the only site of hormonal regulation of glycolysis and gluconeogenesis. Hormonal regulation of PK activity is exerted by changes in the degree of (de)phosphorylation of the enzyme reflecting acute hormonal control and by changes in the concentration of the allosteric activator fructose 1,6-diphosphate. The latter depends at least in part on the hormonal control of the phosphofructokinase-fructose-1,6-phosphatase cycle.