Evidence for the coordinate control of activity of liver glycogen synthase and phosphorylase by a single protein phosphatase

Homogeneous rabbit liver phosphorylase phosphatase dephosphorylates glycogen synthase b. During purification, phosphorylase phosphatase and glycogen synthase phosphatase co purified with a constant ratio of activities. The two activities co migrated on disc gel electrophoresis. Both substrates competed with each other for the phosphatase, and both phosphatase activities were inhibited by lysine ethyl ester. It is concluded that liver phosphorylase phosphatase and glycogen synthase phosphatase have a common identity and that coordinate regulation of the phosphatase catalyzed activation of glycogen synthase and inactivation of phosphorylase occurs in vivo. This provides a parallel and opposing mechanism to that mediated by adenosine 3':5' monophosphate dependent protein kinase, which coordinately inactivates glycogen synthase, and, via phosphorylase kinase, activates phosphorylase. Maximal glycogen synthase phosphatase activity was observed near neutrality. Mg²⁺ and glucose 6 P activated the glycogen synthase phosphatase reaction and this activation was pH dependent. The K(m) for glycogen synthase b was 0.12 μm.