Inhibition of muscle phosphorylase a by natural components of the sarcoplasm

Using a reconstituted glycolytic enzyme system from muscle tissue, it was shown that phosphorylase activity was regulated by some process to provide only the required amount of glucose 1-phosphate, regardless of the percentage of phosphorylase in the a form. By carrying out phosphorylase a assays at high enzyme concentration (2 mg ml^?1), the same concentration as in the reconstituted system and comparable with in vivo, it was shown that (a) the K_m for phosphate was higher and V lower than at low enzyme concentration (2 μg ml^?1), (b) the presence of other glycolytic enzymes at 40 mg ml^?1 suppressed the activity a further threefold, and (c) phosphocreatine inhibited the enzyme. Taken together, these three effects were sufficient to explain the relative lack of activity of phosphorylase a in the reconstituted system. The inhibition by phosphocreatine is seen as a mode of feedback control on phosphorylase activity in vivo.