Muscle-type MM creatine kinase is specifically bound to sarcoplasmic reticulum and can support Ca2+ uptake and regulate local ATP/ADP ratios.

Highly purified fractions of sarcoplasmic reticulum (SR) were prepared from chicken pectoralis muscles (Saito, A., Seiler, S., Chu, A., and Fleischer, S. (1984) J. Cell Biol. 99, 875-885) and analyzed for the presence of creatine kinase (CK). Vesicles derived from longitudinal SR contained 0.703 +/- 0.428 IU of CK/mg of (SR) protein. Immunogold localization of muscle-type MM-CK on ultrathin cryosections of muscle, after removal of soluble CK, revealed relatively strong in situ labeling of M-CK remaining bound to the M band as well as to the SR membranes. In addition, purified SR vesicles were also labeled by anti-M-CK antibodies, and the peripheral labeling was similar to that observed with anti-Ca2(+)-ATPase antibodies. Only some particulate CK enzyme was released from isolated SR membranes by EDTA/low salt buffer, and CK was resistant to extraction by 0.6 M KCl. Thus, some of the MM-CK present in muscle displays strong associative behavior to the SR membranes. The SR-bound CK was sufficient to support, in the presence of phosphocreatine plus ADP, a significant portion of the maximal in vitro Ca2+ uptake rate. The ATP regeneration potential of SR-bound CK was similar to the rate of Ca2(+)-stimulated ATP hydrolysis of isolated SR vesicles. Thus, CK bound to SR may be physiologically relevant in vivo for regeneration of ATP used by the Ca2(+)-ATPase, as well as for regulation of local ATP/ADP ratios in the proximity of the Ca2+ pump and of other ATP-requiring reactions in the excitation-contraction coupling pathway.