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.

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+ concentration 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 Pi from the phosphoenzyme with bound ADP (EPADP) in the case of Ca2+-ATPase, but that Pi is released before ADP in the case of Na+, K+-ATPase.