The conversion of rabbit skeletal muscle phosphorylase b (1,4-α-d-glucan: orthophosphate α-glucosyltransferase, EC 126.96.36.199) to phosphorylase α by phosphorylase b kinase has been investigated. In agreement with previous reports it was established that during the interconversion a partially phosphorylated hybrid phosphorylase is formed. Hybrid phosphorylase was detected both by the incorporation of 32P from γ-labelled ATP and by a new assay of phosphorylase activity. This assay is based upon the observation that the activity of hybrid phosphorylase is equally increased by AMP in the presence or the absence of caffeine, whereas AMP-induced activity of phosphorylase b is inhibited by caffeine. The extent of phosphorylation is controlled by the relative amount of phosphorylase b and of phosphorylase b kinase. Increasing the concentration of phosphorylase b or decreasing the concentration of phosporylase b kinase lead to the formation of hybrid phosphorylase.
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