Effect of pH on the activity of the Ca2+ + Mg2(+)-activated ATPase of sarcoplasmic reticulum

A kinetic model for the Ca2(+) + Mg2(+)-activated ATPase of sarcoplasmic reticulum was presented in a previous paper [Stefanova, Napier, East & Lee (1987) Biochem. J. 245, 723-730]. Here, that model is modified to account for the pH-dependence of ATPase activity and for the effects of Mg2+ on activity at high pH. It is shown that effects of Mg2+ on measurements of ATPase activity as a function of ATP concentration at pH 8.0 and pH 8.5 are consistent with binding of Mg2+ to the Ca2(+)-binding sites on the phosphorylated ATPase, such binding inhibiting dephosphorylation of the ATPase. It is also shown that slow dissociation of Ca2+ from the phosphorylated ATPase is consistent with the previously published model.