Activating enzyme of phosphorylase b in the fat body of the silkworm,Bombyx mori

Larval fat body of the silkworm, Bombyx mori, was shown to contain phosphorylase a and b activity. The relative activity of phosphorylase a in crude extracts increased by the addition of both ATP and magnesium to the reaction mixture, but the total phosphorylase activity did not change. Among the two forms of phosphorylase (a and b), only phosphorylase b activity was inhibited by the addition of either EDTA or sodium fluoride.Phosphorylase b kinase was demonstrated in this tissue directly. The enzyme was partially purified by acid precipitation and its specific activity, calculated as phosphorylase a, found to be 2 nmol/min per mg of protein. Its activity is dependent on both ATP and magnesium (optimal concentrations of 0.4 and 4 mM, respectively), but not on inorganic phosphate.