Specific mixed disulfide formation with purified bovine cardiac glycogen synthase I and glutathione.

Bovine cardiac glycogen-free glycogen synthase I reacts with oxidized glutathione at low temperature to partially inactivate the enzyme. Evidence is presented that a mixed disulfide between glutathione and the enzyme is formed in this reaction. A short incubation of the GSSG-treated enzyme with dithiothreitol restores full enzyme activity. The reaction with GSSG is pH dependent and the product is quite stable at neutral pH. Oxidation of one sulfhydryl group in glycogen synthase is associated with a loss of 60-70% of the enzyme activity. Further modification of protein sulfhydryls has less effect on the enzyme activity. Other low molecular weight disulfides also inactivate glycogen synthase and treatment with [35S]cystine to produce a 40% loss of enzyme activity gave rise to a single major radioactive peptide after cyanogen bromide digestion. Thus the GSSG-mediated inactivation of glycogen synthase apparently occurs through a single reactive sulfhydryl group that forms a mixed disulfide with low molecular weight disulfide molecules. Uridine 5'-diphosphate glucose and glycogen prevent the inactivation of glycogen-free glycogen synthase with GSSG, and glucose 6-phosphate retards the rate of inactivation. Reduction and reactivation of the GSSG-oxidized glycogen synthase is not affected by glycogen and it occurs readily at neutral pH with dithiothreitol, mercaptoethanol, or cysteamine. Oxidation of the reactive sulfhydryl group with GSSG has no effect on the rate of glycogen synthase phosphorylation by the catalytic subunit of cAMP-dependent protein kinase.