Are calcium‐dependent protein kinases involved in the regulation of glycolytic/gluconeogenetic enzymes?

Abstract

Changes in glycolytic flux have been observed in liver under conditions where effects of cAMP seem unlikely. We have, therefore, studied the phosphorylation of four enzymes involved in the regulation glycolysis and gluconeogenesis (6-phosphofructo-1-kinase from rat liver and rabbit muscle; pyruvate kinase, 6-phosphofructo-2-kinase and fructose-1,6-bisphosphosphatase from rat liver) by defined concentrations of two cAMP-independent protein kinases: Ca2+/calmodulin-dependent protein kinase and Ca2+/phospholipid-dependent protein kinase (protein kinase C). The results were compared with those obtained with the catalytic subunit of cAMP-dependent protein kinase. The following results were obtained. Ca2+/calmodulin-dependent protein kinase phosphorylates 6-phosphofructo-1-kinase and L-type pyruvate kinase at a slightly lower rate as compared to cAMP-dependent protein kinase. 6-Phosphofructo-1-kinase is phosphorylated by the two kinases at a single identical position. There is no additive phosphorylation. The final stoichiometry is 2 mol phosphate/mol tetramer. The same holds for L-type pyruvate kinase except that the stoichiometry with either kinase or both kinases together is 4 mol phosphate/mol tetramer. Rabbit muscle 6-phosphofructo-1-kinase is phosphorylated by cAMP-dependent protein kinase but not by Ca2+/caloculin-dependent protein kinase. Fructose-1,6-bisphoaphatase from rat but not from rabbit liver is phosphroylated at the same position but at a markedly lower rate by Ca2+/calmoculin-dependent protein kinase when compared to the phosphorylation by cAMP-dependent protein kinase. 6-Phosphofructo-2-kinase is phosphorylated by Ca2+/calmodulin-dependent protein kinase only at a negligible rate. Protein kinase C does not seem to be involved in the regulation of the enzymes examined: only 6-phosphofructo-2-kinase became phosphorylated to a significant degree. In contrast to the phosphorylation by cAMP-dependent protein kinase, this phosphorylation is not associated with a change of enzyme activity. This agrees with our observation that the sites of phosphorylation by the two kinases are different. The results indicate that Ca2+/calmodulin-dependent protein kinase but not protein kinase C could be involved in the regulation of hepatic glycolytic flux under conditions where changes in the activity of cAMP-dependent protein kinases seem unlikely.