Insulin and Glycogen Synthase

Abstract

The present status of the molecular nature of the enzyme glycogen synthase from rabbit skeletal muscle is reviewed. It is shown that per 90,000 Dalton subunit there are six phosphorylatable sites and six sulfhydryl groups. No pyridoxal phosphate is present. The enzyme has a hexagonal subunit visible in the electron microscope which associates to tetramers and higher forms as well. Insulin brings about a shift from dependent (D) to independent (I) form in all sensitive tissues. This is accompanied by a shift in the kinase from an I to D form. Although under certain conditions a decreased cyclic AMP concentration may be observed, under other conditions no change is observed. The combination of an altered kinase sensitivity to cyclic AMP together with a potential decrease in cyclic AMP concentration suggests a decreased kinase activity and explains the D to I shift in synthase. The question of a new intermediate related to cyclic AMP whose formation is stimulated by insulin is introduced as a hypothesis.