Rabbit muscle nonactivated phosphorylase kinase (EC 184.108.40.206) is converted to thiophosphate-activated phosphorylase kinase by cyclic AMP dependent protein kinase, Mg2+ and ATP-γ-S /adenosine-5′-O-(s-thiotriphosphate)/. The formation of thiophosphate-activated phosphorylase kinase was also observed in the protein-glycogen complex from skeletal muscle. This new form of kinase is resistant to the action of phosphatase and behaves as a competitive inhibitor in the dephosphorylation of phosphorylase a by phosphorylase phosphatase (Ki = 0.04 mg per ml). The fact that the inhibitory effect of thiophosphate-activated phosphorylase kinase is 3 times higher than in the case of nonactivated kinase, may explain the transient inhibition of phosphorylase phosphatase in the protein-glycogen complex. The use of activated (phosphorylated) phosphorylase kinase supports this assumption since it causes a delay in the dephosphorylation of phosphorylase a, i.e. the conversion of phosphorylase a into b could start only after the dephosphorylation of activated phosphorylase kinase.
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