Comparison of the liver glycogen synthase from normal and streptozotocin-induced diabetic rats

Glycogen synthase in the liver extracts of short-term (3 days) streptozotocin-induced diabetic rats is poorly activated by the endogenous synthase phosphatase as well as phosphatase(s) from the liver extracts of normal animals. However, synthase in the liver extracts of diabetic rats is readily activated by the 35,000 M_r rabbit liver protein phosphatase (H. Brandt, F. L. Capulong, and E. Y. C. Lee (J. Biol. Chem.250, 8038–8044 (1975)). The purified synthases from normal and diabetic animals respond differently after incubations with three different phosphatases. Both normal and diabetic synthase are activated by the 35,000 M_r protein phosphatase; however, the total activity of diabetic, but not the normal, synthase is significantly increased. Normal, but not the diabetic, synthase is activated by a synthase phosphatase from normal rats; this activation is accompanied by an increase in total synthase activity. Incubation of the diabetic synthase with calf intestinal alkaline phosphatase results in a reduction of the total synthase activity, whereas synthase activity of the normal is not significantly affected. The reduction in total activity of the diabetic synthase by treatment with alkaline phosphatase was prevented by prior dephosphorylation with 35,000 M_r rabbit liver protein phosphatase. In addition to their differences in responses to different phosphatases, the normal and diabetic synthases are also different in their molecular weights as determined by sucrose density gradient centrifugation (154,000 ± 17,000 (n = 6) for the normal and 185,000 ± 15,000 (n = 8) for the diabetic synthase) and their kinetic properties.