The formation of phosphoenzyme of sarcoplasmic reticulum. Requirement for membrane-bound Ca2+

Membrane-bound Ca or Mg of sarcoplasmic reticulum fragments were removed by treating the membrane with EDTA or an acidic solution, and the changes in the enzymatic activities of sarcoplasmic reticulum fragments induced by these treatments were examined. With the decrease in the amount of membrane-bound Ca below 1-3-10(-8) mol/mg protein, it was demonstrated that the activity of (Ca2+ + Mg2+)-ATPase transiently increased and then diminished, that the Ca-uptake and phosphoenzyme formation declined gradually, and that the activity of Mg2+-ATPase was affected to a less extent. Sodium dodecyl sulfate-gel electrophoretic patterns of peptides from the metal-deficient membranes were the same as those of the untreated material. The level of the phosphoenzyme formation of the metal-deficient membrane was restored by increasing the amount of membrane-bound Ca, but not by increasing the amount of membrane-bound Mg.