Calcium ions and glycogen act synergistically as inhibitors of hepatic glycogen-synthase phosphatase.
We investigated the inhibitory effect of Ca2+ in the micromolar range on the activation of glycogen synthase in crude gel-filtered liver extracts [van de Werve (1981) Biochem. Biophys. Res. Commun. 102, 1323-1329]. The magnitude of the inhibition was highly dependent on the glycogen concentration in the final liver extract. Ca2+ inhibited the activation of purified hepatic synthase b by the G-component of synthase phosphatase, as present in the isolated glycogen-protein complex. The cytosolic S-component was not inhibited. Maximal inhibition of the crude G-component occurred at 0.3 microM-Ca2+. The inhibition was not influenced by the addition of either calmodulin or calmodulin antagonists, or by various proteinase inhibitors. The use of purified G-component revealed that the inhibition by 0.3 microM-Ca2+ increased from 45% to 85% when the concentration of glycogen was raised from 1.5 to 20 mg/ml. Muscle glycogen synthase, extensively phosphorylated in vitro, was also used as substrate for purified G-component. Activation and dephosphorylation were similarly inhibited by 0.3 microM-Ca2+, but the magnitude of the inhibition was much greater with the hepatic substrate. No effect of 0.3 microM-Ca2+ was found on the activity of phosphorylase phosphatase in various liver preparations. We conclude that the inhibition of synthase activation by Ca2+ is one of the mechanisms by which cyclic AMP-independent glycogenolytic hormones promote the inactivation of glycogen synthase in the liver, especially in the fed state.
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002850:Chromatography, Gel; D004357:Drug Synergism; D004789:Enzyme Activation / Q000187:drug effects; D006003:Glycogen / Q000494:pharmacology*; D006006:Glycogen Synthase / Q000378:metabolism; D006007:Glycogen-Synthase-D Phosphatase / Q000037:antagonists & inhibitors*; D066298:In Vitro Techniques; D008099:Liver / Q000201:enzymology*; D008297:Male; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D011480:Protease Inhibitors / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013347:Subcellular Fractions / Q000201:enzymology
D011480:Protease Inhibitors; D006003:Glycogen; D006006:Glycogen Synthase; D010749:Phosphoprotein Phosphatases; D006007:Glycogen-Synthase-D Phosphatase; D002118:Calcium