Phosphoprotein formation and ADP-ATP exchange of cardiac sarcoplasmic reticulum

Membranes of cardiac sarcoplasmic reticulum (SR) incorporate the terminal phosphate of gamma-[32P]ATP in the presence of Ca2+ and Mg2+ (0.85-1.3 nmoles/mg of protein). In the absence of Ca2+, or in the absence of Ca2+ and Mg2+, a value of about 0.3 nmoles/mg of protein was obtained. The Ca2+-dependent membrane phosphorylation is inhibited by ADP, NEM, and salyrgan, but not affected by dinitrophenol (DNP), azide, or ouabain. [32P]Phosphoprotein formed in the presence of Ca2+ is rapidly dephosphorylated by EGTA and/or ADP. The cardiac SR catalyzes a Ca2+-dependent [32P]ADP-ATP exchange and [32P]ATP formed = about 0.3 mumol/mg of protein X min at 25 degrees C, which is inhibited by NEM and salyrgan, but unaffected by DNAP, azide or ouabain. The demonstrated ADP-ATP exchange and the phosphorylated intermediate of the Ca2+-dependent ATPase would agree with a Ca2+ translocation mediated by the ATPase molecule, as proposed for skeletal sarcoplasmic reticulum.