A KINETIC STUDY OF THE Na PUMP IN RED CELLS: ITS RELEVANCE TO THE MECHANISM OF ACTIVE TRANSPORT *

One of the possible approaches t o understand better the mechanism responsible of the active movements of Na or K catalyzed by the Na pump is t o look at the dependence of these movements with the concentration of cations in the intraor extracellular media. This sort of study has been greatly favored by the development of procedures that allow the intracellular composition of red cells to vary within a wide range.' The study of the kinetics of cation fluxes through the Na pump can be approached in two rather different ways. The first is t o look at the flux vs. cation concentration curves when cation concentration is varied at one of the surfaces and kept constant at the opposite surface of the cell membrane. From the shape of these curves we can obtain indirect information on the number of cation binding sites, their apparent affinities, their interactions, and the like.2y3 But, however useful these studies may be, they are hampered by the fact that almost always more than one kinetic scheme will fit the experimental results. The distinction between alternative models lies beyond the realm of kinetics and probably will become possible only when procedures t o measure the binding of cations t o purified ATPase preparations become available. Another way of using the kinetic approach to study the mechanism of the Na pump is to look at the effect that changes in the cation composition a t one of the surfaces of the cell membrane have on the shape of the curves relating cation flux with cation composition at the opposite surface of the cell membrane.4 We would like t o comment now on the results of experiments performed in human red cells in which the second approach was employed, and t o show that they place a set of rather stringent restrictions on the hypothetical schemes of the Na pump.