Control of Glycogen Synthase Phosphatase from Rat Heart

Abstract

Glycogen synthase phosphatase from rat heart has been partially purified by ultrafiltration techniques, and the properties of this enzyme have been examined by using an assay based on the conversion of glycogen synthase D into I. As observed in other tissues, the phosphatase from heart was activated by both Mg2+ and glucose-6-P. The specificity of this activation was examined and it was shown to be virtually identical with the specificity for activation of glycogen synthase D. In addition, Mg2+ decreased the requirement for glucose-6-P approximately 10-fold in both the phosphatase reaction and in the synthase D reaction, and both inorganic sulfate and phosphate inhibited the glucose-6-P activation of both reactions. These results were interpreted as evidence for the regulation of the phosphatase reaction by the conformational state of the substrate, glycogen synthase D. Glycogen was shown to be an inhibitor of the phosphatase and this inhibition was independent of the activation of this reaction by Mg2+ and glucose-6-P. These results indicate a complex pattern of interaction between various cellular metabolites in the regulation of glycogen synthase D dephosphorylation in heart tissue.