Reactivity of Sulfhydryl Groups of the Catalytic Subunits of Rabbit Skeletal Muscle Protein Phosphatases 1 and 2A

Chemical modification of the sulfhydryl residues of the catalytic subunits of protein phosphatases 1 and 2A was studied. Both enzymes were inactivated by a variety of thiol group reagents. Mercurial compounds were the most effective inactivators. Of the alkylating agents the maleimides were more effective than iodoacetate or iodoacetamide which were relatively slow reacting. Both enzymes were also inactivated by disulfides, including glutathione disulfide, 5,5′-dithiobis(2-nitrobenzoic acid), and 4,4′-dipyridyl disulfide. The latter two were much more reactive than glutathione disulfide and, in addition, displayed a significant differential reactivity toward phosphatase 1. The apparent second-order rate constants for the inactivation of phosphatase 1 were 20- to 70-fold higher than for phosphatase 2A with 5,5′-dithiobis(2-ni-trobenzoic acid) and 4,4′-dipyridyl disulfide. Kinetic analysis indicated that inactivation of both enzymes could be correlated with the modification of one cysteine per one mole of enzyme.