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ATP-dependent Ca2+ uptake, ATPase phosphorylation, and Pi production by sarcoplasmic reticulum vesicles were measured in rapid quench experiments, using trichloroacetic acid, H (b) a step related to hydrolytic cleavage of the phosphorylated intermediate is rate-limiting in the enzyme turnover; (c) the concentration of acid-labile enzyme l phosphate complex is negligible in the presence of ATP and Ca’+; (d) modulation of enzyme activity is dependent on occupancy of cation binding sites exposed to the outer or inner side of the membrane, consistent with a model of shifting site orientation to account for vectorial transport: (e) a state of very low permeability and minimal efflux of intravesicular Ca2+ is obtained only in the presence of ATP and Mg’+, when Ca2+ is dissociated from high affinity ATPase sites.
Chiesi et al. (Mon,) studied this question.