Interaction of sodium and potassium ions with Na+, K+-ATPase. III Cooperative effect of ATP and Na+ on complete release of K+ from E2K.

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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.
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