SLOW TRANSITION OF PHOSPHOENZYME FROM ADP-SENSITIVE TO ADP-INSENSITIVE FORMS IN SOLUBILIZED CA2+,MG2+-ATPASE OF SARCOPLASMIC-RETICULUM - EVIDENCE FOR RETARDED DISSOCIATION OF CA-2+ FROM THE PHOSPHOENZYME

Solubilized Ca2+, Mg2+-ATPase of sarcoplasmic reticulum was phosphorylated with ATP without added MgCl2. The phosphoenzyme formed was ADP-sensitive. Ca2+ in the medium was chelated after phosphorylation. This induced a slow transition of the phosphoenzyme from ADP-sensitive to ADP-insensitive forms. The ADP-sensitivity was restored by subsequent addition of CaCl2. These results showed that the transition was caused by dissociation of Ca2+ bound to the phosphoenzyme. Further observations indicated that, when Ca2+ in the medium was chelated, Ca2+ bound to the phosphoenzyme was dissociated much more slowly than Ca2+ bound to the dephosphoenzyme. This suggests a possible formation of the occluded form of the Ca2+-binding site in the phosphoenzyme. © 1979.