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Occlusion of calcium in the ADP-sensitive phosphoenzyme of the adenosine triphosphatase of sarcoplasmic reticulum

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Makinose,  Madoka
Max Planck Institute for Medical Research, Max Planck Society;

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Citation

Takisawa, H., & Makinose, M. (1983). Occlusion of calcium in the ADP-sensitive phosphoenzyme of the adenosine triphosphatase of sarcoplasmic reticulum. The Journal of Biological Chemistry, 258(5), 2986-2993. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/6219108.


Cite as: https://hdl.handle.net/21.11116/0000-0003-6D4C-F
Abstract
In order to characterize the form of the phosphorylated Ca2+-ATPase of sarcoplasmic reticulum which occludes the calcium bound in the enzyme (Takisawa, H., and Makinose, M. (1981) Nature (Lond.) 290, 271-273), a kinetic method was developed allowing quantitation of the amount of ADP-sensitive and ADP-insensitive phosphoenzyme. The relationships between occluded Ca2+ in the enzyme and the two forms of phosphoenzyme were studied at various concentrations of CaCl2 and MgCl2. The amount of tightly bound Ca2+ in the phosphoenzyme increases concordantly with the increase in the amount of ADP-sensitive phosphoenzyme, suggesting that occlusion of Ca2+ occurs in the ADP-sensitive phosphoenzyme. These results suggest that 1 mol of ADP-sensitive phosphoenzyme occludes 2 mol of Ca2+. Ca2+ is released from the enzyme under conditions which favor the formation of the ADP-insensitive phosphoenzyme (e.g. 5 mM MgCl2 and 50 microM CaCl2). Ca2+ release correlates approximately with the formation of the ADP-insensitive phosphoenzyme. The simulated time course of Ca2+ release, based on the Ca2+-binding properties of the two forms of phosphoenzyme, shows a good fit with the Ca2+ release curves observed, indicating that the ADP-insensitive phosphoenzyme binds no Ca2+ under these conditions.