Vectorial aspects of adenosine-triphosphatase activity in erythrocyte membranes

Abstract

Directional effects of ions on a chemical mechanism are implied by the complex way in which active transport processes depend on the ionic composition of the fluids bathing the cell membranes. There appears to be an interdependence of cation movements, for an ion is transported in one direction only when another ion is moved in the opposite direction. The net extrusion of Na+ ions from erythrocytes requires the presence in the medium of K+ ions, which are simultaneously taken up (Harris & Maizels, 1951; Glynn, 1956; Post & Jolly, 1957). It is this coupled movement of Na+ and K+ ions that is inhibited by cardiac glycosides, probably from the external surface of the cell Other ions, notably Rb+, NH4+, Cs+ and Li+, may replace K+ ions both in being accumulated and in facilitating Na+ ion efflux (Solomon, 1952; Love & Burch, 1953; Post & Jolly, 1957; Kahn, 1962; McConaghey & Maizels, 1962). There is, in contrast, a strict requirement for internal Na+ ions, because no active transport occurs in their absence (McConaghey & Maizels, 1962). The Na+ ions are always moved outwards and the other alkali-metal ions inwards. A further example of the dependence of cation transport on the cell's ionic environment is the retardation by external Na+ ions of the uptake of K+ ions when the K+ ion concentration is low relative to that of Na+ ions It follows that Na+ ions affect K+ ion up- take differently, depending on where they are, since from inside the cell they promote K+ ion uptake and from outside they retard it. These effects illustrate the marked directionality of the cell membrane in the way it catalyses ion movements.