title: Occlusion of calcium in the ADP-sensitive phosphoenzyme of the adenosine triphosphatase of sarcoplasmic reticulum.
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.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D002122:Calcium Chloride / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D008433:Mathematics; D009132:Muscles / Q000201:enzymology; D010750:Phosphoproteins / Q000378:metabolism; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D010750:Phosphoproteins; D015636:Magnesium Chloride; D004533:Egtazic Acid; D000244:Adenosine Diphosphate; D000251:Adenosine Triphosphatases; D008274:Magnesium; D002122:Calcium Chloride