Non-identical behavior of the Ca2(+)-ATPase in the terminal cisternae and the longitudinal tubules fractions of sarcoplasmic reticulum.

The kinetics of Ca2+ dissociation from and binding to the Ca2(+)-ATPase and the coupled tryptophan fluorescence changes were compared in the heavy and the light sarcoplasmic reticulum (SR) fractions. It was found that in the light SR both the dissociation of Ca2+ from the Ca2(+)-ATPase and the coupled tryptophan fluorescence change took place in a biphasic fashion, but they were monophasic in the heavy SR. On the other hand, the time courses of both the Ca2+ binding and the coupled tryptophan fluorescence increase were biphasic, and virtually indistinguishable between the heavy and the light SR fractions. Submicromolar ruthenium red altered the kinetics of both the Ca2+ dissociation and the coupled fluorescence change in heavy, but not in light SR; the monophasic time course characteristic of the heavy fraction became biphasic, similar to that found for light SR in the absence of ruthenium red. Extraction of non-ATPase proteins from the heavy SR vesicles also changed the kinetics of the Ca2+ dissociation-coupled fluorescence decrease in the heavy SR from monophasic to biphasic. These results suggest that the difference between the heavy and light SR Ca2(+)-ATPase in the Ca2+ dissociation kinetics is most probably produced by a ruthenium-red-sensitive interaction of the Ca2(+)-ATPase with additional protein(s), which is/are present exclusively in the heavy SR, rather than being due to a difference inherent to the Ca2(+)-ATPase polypeptide itself.