Purification and properties of glycogen phosphorylase isolated from bovine skeletal muscles

Glycogen phosphorylase isolated from bovine skeletal muscles was found to be homogeneous during polyacrylamide gel electrophoresis. The enzyme phosphorylation by phosphorylase kinase is accompanied by the incorporation of one mole of labeled phosphate per protein dimer; therefore the enzyme is represented by a partly phosphorylated form. The presence of a phosphate group prevents the removal of the protein-bound pyridoxal phosphate. The partly phosphorylated bovine phosphorylase possesses a low affinity for AMP and is inactive in the presence of IMP. Bovine phosphorylase a obtained from the partly phosphorylated enzyme has a molecular mass corresponding to a dimer. Both forms of bovine phosphorylase exhibit high cooperativity towards the substrate. The mechanism of phosphorylase a activation by AMP and IMP is identical: the nucleotides increase the enzyme affinity for the substrate as well as the maximal rate of the enzymatic reaction. Study of the enzyme inhibition by caffeine revealed the cooperativity of caffeine-binding centers. The equilibrium between the active and inactive enzyme conformations in the presence of caffeine is markedly shifted towards the inactive (T) form of glycogen phosphorylase.