Regulation of Rat Liver Glycogen Synthetase: RELATIONSHIP OF THE HORMONAL ACTIVATION AND THE TIME-DEPENDENT IN VITRO ACTIVATION

Abstract

Abstract The activation of glycogen synthetase by insulin proceeds via the transformation of the less active D form to the active I form, with essentially no change in total activity. Although the enhancement of glycogen deposition has also been attributed to glucocorticoid activation of glycogen synthetase, insulin-induced activation is counteracted by hydrocortisone in diabetic and starved rats. The time-dependent activation of glycogen synthetase in crude liver extracts, which has been used as a model for activation of the enzyme by various agents, has now been demonstrated with more purified preparations. With the purified system the characteristics of the in vitro activation have been compared to the in vivo hormonally induced activation. In contrast to the in vivo case, the total activity of glycogen synthetase increased during activation in vitro in addition to the increase in the I activity. The effect of various cellular metabolites on the in vitro activation kinetics is profoundly altered by EDTA. Certain properties of the synthetase system, such as the ratio of D to I form, the Km for UDP-glucose, and the in vitro activation, are temperature-dependent.