Activation/dephosphorylation of muscle glycogen synthase phosphorylated by phosphorylase kinase.

1. Glycogen synthase from rabbit skeletal muscle was phosphorylated by phosphorylase kinase to yield synthase b2. 2. Dephosphorylation and activation of synthase b2 by the catalytic subunits of protein phosphatase-1 (PP-1c) and protein phosphatase-2A (PP-2Ac) was studied. The apparent Km of PP-1c and PP-2Ac were 3.3 microM and 6.2 microM, respectively. The apparent Vmax of PP-1c was about two times larger than that of PP-2Ac. 3. Ligands with phosphate moiety (AMP, glucose-6-P at high concentration) caused an inhibition in dephosphorylation by both phosphatases. Spermine inhibited the dephosphorylation by PP-1c and stimulated the action of PP-2Ac. Therefore it can be employed to distinguish the phosphatases using synthase b2 as substrate.