Regulation of liver phosphorylase phosphatase by glutathione disulfide

The mechanism of inactivation of rabbit liver phosphorylase phosphatase by glutathione disulfide (GSSG) was investigated. The catalytic subunit of phosphorylase phosphatase was inactivated by GSSG and other disulfides. Inactivation by GSSG was a concentration-dependent process and resulted in the formation of an inactive, stable enzyme species. The inactivated enzyme could be reactivated by addition of various sulfhydryl compounds, including glutathione (GSH). Homogeneous phosphorylase phosphatase contains, per mol of catalytic subunit (Mr = 33,000), two sulfhydryl groups, one of which reacted with GSSG to form inactive enzyme. Binding studies with [glycine-2-3H]GSSG revealed simultaneous incorporation of 3H radioactivity into the catalytic subunit and stoichiometric loss of catalytic activity. Treatment of the 3H-labeled enzyme with GSH was accompanied by release of 3H radioactivity from the enzyme and restoration of enzyme activity. The results suggest that inactivation of phosphorylase phosphatase by GSSG results from the formation of a mixed disulfide between GSSG and one of the two sulfhydryl groups in the catalytic subunit.