Modulation of the hydrolysis rate of the ADP-insensitive phosphoenzyme of the sarcoplasmic reticulum ATPase by H+ and Mg2+.

Effects of H+ and Mg2+ on the hydrolysis rate of the ADP-insensitive phosphoenzyme intermediate (E2P) of the sarcoplasmic reticulum ATPase were investigated at 6 degrees C in the presence and absence of K+. In the absence of K+, the pH dependence of the E2P hydrolysis rate obtained in the absence of divalent cations showed a bell-shaped profile with an optimum at pH 9. At neutral pH, Mg2+ or other divalent cations accelerated the E2P hydrolysis while they strongly inhibited it at alkaline pH. The accelerating effect occurred on the cytoplasmic side of the membrane whereas the inhibitory effect occurred on the luminal side of the membrane, presumably at the low affinity calcium transport sites. The presence of Mg2+ or other divalent cations, therefore, shifted the pH activity profile to the acidic side while the magnitude of this shift and the activity obtained at the optimum pH depended on the species and the concentration of the divalent cation used. Simulation of a set of the pH activity curves obtained in 0 to 40 mM Mg2+ suggests that the marked activation of E2P hydrolysis by high Mg2+ observed at neutral pH is primarily caused by a Mg2+-induced increase in the dissociation constant of the ionizing group(s) rather than a markedly increased rate constant for E2P hydrolysis. In the presence of K+, the stimulatory effect of Mg2+ at pH 7 was less pronounced but its inhibitory effect at pH 9 was similar to that observed in the absence of K+. These effects of Mg2+ and other divalent cations should be taken into account when the role of H+ in the ATPase reaction is investigated.