Interaction of Sodium and Potassium Ions with Na⁺, K⁺-ATPase. III. Cooperative Effect of ATP and Na⁺ on Complete Release of K⁺ from E₂K

Abstract

The effects of Na⁺ and ATP on the K⁺ binding to Na⁺, K⁺-ATPase were investigated by the centrifugation method with radioactive K⁺ in the absence of Mg²⁺. In the presence of 10μM⁴³KCl, 0.6 and 10mM Na⁺ decreased the amount of bound K⁺ to one-half and zero, respectively. On the other hand, 10μM and 10mM ATP decreased the amount of K⁺ to 60 and 25-40%, respectively. When the combined effect of ATP and Na⁺ was tested, 10μM ATP decreased the Na⁺ concentration giving half-maximal inhibition of the K⁺ binding to one-third, showing synergistic inhibition by both ligands, though increase in ATP concentration seemed to depress the inhibitory effect of Na⁺. The synergistic inhibition by ATP and Na⁺ suggests that the release of K⁺ from E₂K is not completed by the binding of ATP alone but is completed by the binding of Na⁺ in addition to ATP during the cycle of Na⁺, K⁺-dependent ATP-hydrolysis as well as ion-transport.