A slow interconversion between active and inactive states of the (Na-K)ATPase.

We have examined slow changes in the rate of ATP hydrolysis for purified dog kidney Na+ and K+ stimulated adenosine triphosphatase [(Na-K)ATPase] at various concentrations of free Mg2+, Mg-ATP, K+, and Na+. The effect of these ligands on the rate of ATP hydrolysis is explained by a rapid binding step determining the initial rate of turnover followed by a slow conformational change. Inactivation of enzyme stored in the presence of ethylenediaminetetraacetic acid occurs upon adding free Mg2+, Mg-ATP, and K+; reactivation may be achieved if the concentration of these ligands is reduced. Because of the slow conformational change, the affinities for ligands affecting inactivation are time dependent. A model is presented to explain the effects of free Mg2+ and Ma-ATP on (Na-K)ATPase activity.