Sarcoplasmic reticulum vesicles and glycogen-protein particles in microsomal fraction of skeletal muscle.

1. Heavy microsomal fraction (HM) of rabbit skeletal muscle obtained by differential centrifugation between 8 000-30 000 g and consisting of sarcoplasmic reticulum (SR) vesicles contains variable amounts of glycogen and reveals some activity of phosphorylase b. The monomer of this enzyme of mol. wt. about 100 000 co-migrates in SDS-polyacrylamide gel electrophoresis with the main SR protein--Ca2+,Mg2+--dependent ATPase. 2. The highest specific activity of phosphorylase and the highest content of glycogen is present in the light microsomal (LM) fraction (30 000-100 000 g). 3. Contrary to the ATPase, phosphorylase b is released from the microsomal fraction by treatment with EDTA and is resistant to trypsin. 4. Both HM and LM fractions can be further fractionated on continuous sucrose density gradient at high speed. Main fraction of HM consists of highly purified SR vesicles. The second, small fraction of HM is identical with the main fraction of LM and consists of two populations: vesicles of structure and properties different from those of SR vesicles, and the particles of a complex of glycogen with some glycolytic enzymes.