Rabbit-Liver Glycogen Synthase: Properties and Interconversion by Phosphorylation and Dephosphorylation

Abstract

Studies of the enzymes of glycogen metabolism, largely concentrated on the enzymes of rabbit skeletal muscle, have revealed an elegant system of protein-phosphorylation reactions whereby glycogenolysis is initiated and glycogenesis is terminated. Thus, hormonally induced increases in cAMP lead to the activation of a protein kinase which phosphorylates and activates phosphorylase kinase. This in turn phosphorylates and activates phosphorylase; the same protein kinase phosphorylates and inactivates glycogen synthase. These reactions have not been clearly defined in liver, and additionally, less is known in any tissue of the reverse reactions of dephosphorylation. In this report are summarized studies on liver glycogen synthase, its interconversion by cAMP-dependent protein kinase (of heart or muscle), and by a liver protein phosphatase originally isolated as a phosphatase acting on muscle phosphorylase a and shown also to dephosphorylate the synthase.