Rabbit skeletal muscle glycogen synthase. II. Enzyme phosphorylation state and effector concentrations as interacting control parameters.

The effects of several inhibitors (ATP, ADP, AMP, UDP, and P1) and activators (Mg2+, glucose-6-P) of rabbit muscle glycogen synthase (UDP-glucose:glycogen 4-alpha-glucosyltransferase, EC 2.4.1.11) were studied in relation to the phosphorylation state of the purified enzyme. All the modifiers had increasing effects with enzyme of increasing alkali-labile phosphate content. In experiments where combinations of effectors were present, it was apparent that (a) concentrations of modifiers in the physiological range could be significant in determining enzymic activity and (b) the sensitivity of the reaction rate to changes in phosphorylation state was critically dependent on the concentration of the small molecules. Changes in the phosphorylation of the enzyme corresponding to changes in the %I activity reported in the literature for studies in vivo were capable of producing large alterations in glycogen synthase activity. Because the magnitudes of such changes were dependent on the effector concentrations, there may be an integration of local cellular control, through small molecule effects, with hormonal control, through the phosphorylation state of glycogen synthase.