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 Patterns

Abstract

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 (E^ADP_P) in the case of Ca²⁺_-ATPase, but that P₁ is released before ADP in the case of Na⁺, K⁺_-ATPase.