1. 1. The limited proteolysis of rabbit skeletal muscle phosphorylase a was undertaken with subtilisin BPN' immobilized to Sepharose 4B. The effect of substrates, activators and inhibitors of phosphorylase a was investigated by monitoring the changes in phosphorylase activity in the SDS gel electrophoretic pattern and in the 32P-content of 32P-labeled phosphorylase a. 2. 2. Phosphorylase a loses its activity upon subtilisin treatment. All ligands tested protect phosphorylase a activity against subtilisin action, probably by inducing structural changes in the tower loop of the enzyme. 3. 3. Glucose-6-P significantly accelerates [32P]peptide release from phosphorylase a through altering the structure of the N-terminal tail segment. 4. 4. The two subunits of dimeric phosphorylase a are held together by strong interactions-deduced from the correlation of the rate of proteolysis and the disappearance of catalytic activity.
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