Time-dependent pseudo-activation of hepatic glycogen synthase b by glucose 6-phosphate without involvement of protein phosphatases

During a 30 min incubation at 25 °C in the presence of 5–10 mM glucose 6-phosphate, pure glycogen-bound glycogen synthase b from dog liver was progressively converted into a form that was fully catalytically active in the presence of 10 mM Na₂SO₄ plus 0.5 mM glucose 6-phosphate. The latter enzyme was unlike synthase a (which does not require glucose 6-phosphate for activity), and unlike synthase b (which is strongly inhibited by sulphate). The conversion was insensitive to various inhibitors of Ser/Thr-protein phosphatases and alkaline phosphatases, and was therefore termed ‘pseudo-activation’. Kinetically, pseudo-activation increased the V_max 4-fold without affecting the K_m for the substrate UDP-glucose. Pseudo-activation appeared to be an irreversible process, but several lines of evidence argue against a limited proteolysis. Pseudo-activation of glycogen synthase occurred also readily in a rat liver cytosol, but it was not observed with purified synthase from skeletal muscle. These observations have important implications for the assay of liver glycogen-synthase phosphatase; the possible physiological implications remain to be explored.