Autophosphorylation of phosphorylase kinase and its regulatory function in the dephosphorylation of phosphorylase A

Autophosphorylation of phosphorylase kinase was measured under conditions that favoured autoactivation. Heparin and troponin C stimulated the autophosphorylation of phosphorylase kinase at pH 6.8 in a Ca2+-dependent manner. The concentration required for the half-maximal stimulation of autophosphorylation for calcium ions was 2 microM in the absence of effectors, whereas 0.7 microM and 0.1 microM in the presence of troponin C and heparin, respectively. Calmodulin increased the rate of autophosphorylation of the alpha subunit only, resulting in a slight increase in the rate of autoactivation of phosphorylase kinase. Troponin C, heparin and polybrene enhanced the rate of autophosphorylation of both alpha and beta subunits. The increased autophosphorylation coincided with an enhancement of kinase activity. Neither of these stimulatory macromolecules had significant influence on the total number of phosphate groups incorporated into the alpha or beta subunits by autophosphorylation. Thio-autophosphorylated form of phosphorylase kinase behaved as an inhibitor in the dephosphorylation of phosphorylase a by the catalytic subunits of phosphatase-1 or phosphatase-2A and by the latent form of phosphatase-2A. Concentration of phosphorylase kinase needed to 50% inhibition was in the range of 0.05-0.08 microM.