Order of Release of ADP and Pi from Phosphoenzyme with Bound ADP of Ca2+-Dependent ATPase from Sarcoplasmic Reticulum and of Na+, K+-Dependent ATPase Studied by ADP-Inhibition Patterns1

The inhibition of Ca²⁺-dependent ATPase from SR [EC 3.6.1.3] by ADP was of mixed type under both low Ca²⁺ and high Mg²⁺ concentrations and high Ca²⁺ and low Mg²⁺ concentrations. On the other hand, the inhibition of Na⁺, K⁺-dependent ATPase [EC 3.6.1.3] by ADP was of competitive type in the presence of low and high K⁺ concentrations. These results suggest that ADP is released before P1 from the phosphoenzyme with bound ADP ($EPADP$) in the case of Ca²⁺-ATPase, but that P₁ is released before ADP in the case of Na⁺, K⁺-ATPase.