Simultaneity, occlusion and the sodium pump

Abstract

GARRAHAN and Garay^1,2 have shown that for the sodium pump of the human red blood cell, the half-saturation concentration for transport of a particular cation from one face of the membrane is not affected by the type or concentration of cation at the opposite face. This finding is consistent with previous observations on the squid axon³ and the sheep red blood cell⁴. The simplest explanation of these results, according to Garrahan and Garay², is that the pump binding sites are at equilibrium with cation and that transport occurs only when cations are bound simultaneously at both faces of the membrane. Their view is that previously proposed “sequential” models⁵ must be rejected and replaced by models in which cation binding sites are present simultaneously on both sides of the membrane. In addition, they show that the transport data are sufficient to reject a model (Fig. 1c) if the pump exists for a substantial time in a conformation in which bound cations are occluded. Yet there is biochemical evidence⁶ that, at least at 0° C, the pump does nonetheless exist in an occluded state for an appreciable time. Here we show that this apparent paradox is resolved by the tetrameric internal transfer model⁷ of Fig. 1d.