Regulation of Hepatic Glycogen Synthetase

Glycogen synthetase in an in vitro liver system was activated by Sepharose-bound insulin as well as by free insulin. The incorporation of uniformly C14-labeled glucose into glycogen was also stimulated by either Sepharose-bound or free insulin. These results suggest that activation of glycogen synthetase by insulin is mediated by the interaction of insulin with the external cellular membrane and does not require the entry of the hormone into the cell. Furthermore, it is shown that activation of glycogen synthetase by insulin requires the maintenance of intact cells. Thus, the hormone had no effect on glycogen synthetase activity when added to either broken cell suspensions or cell-free homogenates.