Inhibition of the sodium pump by inorganic phosphate in resealed red cell ghosts.

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Abstract
Ouabain-sensitive Rb influx was measured into K-free resealed [human] red cell ghosts. The effects of Pi were examined. Phosphate decreased the magnitude of the influx. Increasing Pi lowered the apparent affinity for both ATP and external Rb ions. The effects of Pi on the affinity for external Rb were greatest at low ATP concentrations. In the nominal absence of phosphate, increasing ATP from 1 to 100 .mu.M had little effect on the Rb influx from solutions of low (1 .mu.M) Rb concentrations. In the presence of Pi, ATP increased Rb uptake markedly, even from solutions of low Rb concentration. The above interactions between Pi, ATP and external Rb are consistent with a consecutive scheme for the Na pump in which phosphate is released after K binds at the external surface and before ATP binds to release K ions to the solution. Previous failures to find an effect of P on either the affinity for ATP or that for external K (Rb) ions were equally consistent with the model. The lack of change of apparent affinity results from the restricted range of concentrations used in these previous experiments.