Ratio of hydrolysis and synthesis of ATP by the sarcoplasmic reticulum ATPase in the absence of a Ca2+ concentration gradient.

The reversibility of the reaction catalyzed by purified (Ca2+ + Mg2+)-dependent ATPase of sarcoplasmic reticulum was studied by measuring the rates of synthesis and hydrolysis of ATP observed during the ATP in equilibrium Pi exchange reaction. The ratio between the velocities of hydrolysis and synthesis of ATP was found to vary between 700 and 1.2 depending on the concentrations of ATP, Pi, Mg2+, and Ca2+ in the medium. Raising the magnesium concentration increases in parallel the calcium concentrations required for half-maximal activation of both the hydrolysis and the synthesis of ATP. The apparent Km of Pi for the ATP in equilibrium Pi exchange reaction varies, depending on the Mg2+ concentration of the medium, being lower with higher Mg2+ concentration. The ratio between the velocities of hydrolysis and synthesis of ATP decreases when the Mg2+ and Pi concentrations are increased and the ATP concentration is decreased.