The asymmetrical stimulation of a membrane adenosine triphosphatase in relation to active cation transport

Abstract

A study has been made of the stimulation by Na+ and K+ ions of the adenosine triphosphatase in human erythrocyte membranes. Erythrocyte ghosts containing adenosine triphosphate and a range of sodium and potassium concentrations were prepared by the reversal of haemolysis. They had a low permeability to Na+ and K+ ions, the concentrations of which were varied independently both in the ghosts and in the incubation media. During incubation at 37[degree], the rate of liberation of orthophosphate from adenosine triphosphate within the ghosts was measured. Replacement of Na+ ions of the medium with either choline or K+ ions did not affect the rate but omission of K+ ions from the medium caused a fall of about 50%. Maximum stimulation was shown by 10 mM-K+ ions in the medium. The membrane adenosine triphosphatase was therefore stimulated by K+ ions, but not by Na+ ions, in the medium. The sodium and potassium concentrations in the ghosts were varied and it was shown that the adenosine triphosphatase is stimulated twofold by an increase in sodium concentration. Fivefold variations in potassium concentration were without effect. The membrane adenosine triphosphatase was therefore stimulated by Na+ ions inside the ghosts. The synergic stimulation of the adenosine triphosphatase by internal Na+ and external K+ ions was counteracted by ouabain. The membrane adenosine triphosphatase is stimulated asymmetrically by Na+ and K+ ions and spatial asymmetry is another property shared with the membrane ion pump for Na+ and K+ ions.