Structural changes in phosphorylase b as revealed by proteolysis with subtilisin BPN'

The proteolysis of rabbit skeletal muscle phosphorylase b was studied with Sepharose 4B bound subtilisin BPN' in the absence and presence of various ligands. The proteolysis was carried out at pH 7.0 and pH 8.5 and was followed by measuring phosphorylase b activity and by SDS gel electrophoresis. The effect of ligands proved to be qualitatively the same at both pH values. It was found that AMP and alpha-D-glucose-1-phosphate accelerated the inactivation of phosphorylase b by subtilisin, two main proteolytic products (Mr 70 000 and 30 000) were formed in the presence of these ligands. IMP and glycogen protected phosphorylase b against proteolytic attack. Subtilisin treatment in the presence of D-glucose, caffeine and D-glucose-6-phosphate produced a reproducible increase (about 20%) of phosphorylase b activity. This "activation" resulted in an increased Vmax of phosphorylase b though did not alter the subunit size, the aggregation state and the ligand binding capacity of the enzyme.