Control of Phosphorylase Activity in a Muscle Glycogen Particle

A fraction containing a protein-glycogen complex was obtained from rabbit muscle by different procedures involving acid precipitation followed by differential centrifugation, direct differential centrifugation, and acetone fractionation. All three preparations were essentially identical in terms of their physical, chemical, and enzymatic characteristics strongly suggesting that the protein-glycogen complex represents a structural and functional unit of the cell rather than artifacts resulting from a given isolation procedure. Electron micrographs of the isolated fractions showed the presence of glycogen granules together with vesicles arising from fragments of the sarcoplasmic reticulum. Sedimentation velocity patterns obtained in the analytical ultracentrifuge, sucrose gradient centrifugation, and Sepharose 2B gel filtration indicated that the material can be separated into a light (approximately 120 S) and a heavy (approximately 600 S) fraction. The light fraction consists mainly of glycogen particles to which are associated phosphorylase, phosphorylase kinase, and phosphatase among other enzymes. The heavy fraction contains mainly the elements of the sarcoplasmic reticulum characterized by a strong ATPase activity and some lysosomes as indicated by the presence of traces of acid phosphatase and amylase activity. Contamination of the glycogen particles by this latter enzyme resulted in their slow degradation that prevented further purification.