A Membrane-bound Creatine Phosphokinase in Fragmented Sarcoplasmic Reticulum

Fragmented sarcoplasmic reticulum was isolated from rabbit skeletal muscle and creatine phosphokinase activity was measured; 0.2 to 1.16 units of creatine-P kinase per mg of fragmented sarcoplasmic reticulum protein were found. The activity was inhibited by dinitrofluorobenzene, a known inhibitor of creatine-P kinase, and had Km values similar to those measured for crystalline enzyme. Since the activity diasppeared from the supernatant upon centrifugation, the creatinine-P kinase is bound to the membranous fraction of the fragmented microsomal preparation. However, it was not present on the mitochondrial fraction from the same muscle homogenate. The creatine-P kinase of the fragmented sarcoplasmic reticulum preparation represents approximately 1% of the total activity in muscle and may be significant as a localized source of ATP for calcium transport processes during muscle relaxation.