We have examined vanadate inhibition of the Na,K pump in the presence of external Na (Nao). Nao protects against inhibition of the Na,K pump by vanadate, but not against inhibition by phosphate or arsenate. Protection by Nao is reversed by external K (Ko). Although the site at which Na exerts its protective effect has properties similar to the two transport sites for K at the outside of the pump, it is not one of the transport sites. The data can be qualitatively accounted for if it is postulated that there is a protective site, separate from the transport sites, at which Nao and Ko compete. When the site is empty or bound to K, vanadate combines with high affinity with pumps that have two K ions bound to the transport sites, but not with pumps that have Na bound to the protective site, even if K is bound to the transport sites. The protective site has a high affinity for both Na and K; the apparent K 1/2 for external Na is less than 2 mM, which is similar to that of a previously described site at which Nao inhibits a number of the partial reactions of the pump. Nao protects against vanadate inhibition of the K-K exchange in the absence of cell Na, and against vanadate inhibition of p-nitrophenylphosphatase activity of the pump in the absence of ATP. The protective site is a manifestation of an E2 conformation of the pump. The protective effect of Nao is not changed by altering the intracellular Mg2+ concentration.

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