Hormonal effects on the phosphorylation of glycogen synthase in rat hepatocytes

The effects of epinephrine and vasopressin on the phosphorylation state of glycogen synthase were studied using rat hepatocytes incubated with 32P. After the incubation with hormones, 32P-labeled glycogen synthase was isolated using antibodies against rat liver enzyme. The immunoprecipitate showed a single radioactive band ( Mapp 88 kDa) when subjected to SDS-gel electrophoresis. Both epinephrine and vasopressin inactivated the enzyme and increased the 32P content of glycogen synthase. Cleavage of the immunoprecipitate with CNBr yielded two major 32P-labeled fragments of Mapp approximately 27 and 12 kDa. Both hormones increased the 32P content of both fragments. These results prove that epinephrine and vasopressin increase the phosphate content of the enzyme promoting its phosphorylation at multiple sites.