Reaction Mechanism of the Ca2+-dependent ATPase of Sarcoplasmic Reticulum from Skeletal Muscle

We measured the time-course of P-incorporation from Pi into the SR-ATPase (EP-formation) in medium consisting of 0.1 M KC1 and 50 mM Tris-maleate, pH 7.0 at 5°C, using SR pre-loaded with Ca2⁺ ions. The amount of EP formed increased linearly with time, after a short lag phase. Therefore, the rate of P-incorporation (EP-formation), vt, was measured from the linear part of the plot of [EP] versus time. The dependence of the rate of EP-formation, vt, on the concentrations of Mg2⁺ ions and Pi and the Ca2⁺-gradient was found to be given by $vr=Vr(1+KP[Pi])(1+KMg[Mg2+]){1+KCa([Ca0][Ca1])2}$ Under the experimental conditions used, the values of Vt, Kp, Kus, and Kca were 1.7moles/10⁶g/sec, 18mM, 9.6mM, and 6.7×10⁸, respectively.