Stoichiometry and Localization of Adenosine Triphosphate-dependent Sodium and Potassium Transport in the Erythrocyte

Abstract

Membrane transport may be effected by specific enzymatic systems. In a search for an enzymatic basis for sodium transport, Skou discovered an adenosine triphosphatase activity in particles from crab nerve which required both sodium and potassium ions together (1, 2). A similar activity was later identified in broken erythrocyte membranes as a part of the active transport system for sodium and potassium ions in intact red cells ((3, 4); for reviews, see References 5-10). Both adenosine triphosphatase activity and active transport require sodium and potassium ions together, not separately, and both are inhibited by cardiac glycosides such as ouabain. A stoichiometric relationship between the active transport of sodium ions outward and potassium ions inward in human erythrocytes was proposed by Harris (ll), substantiated by Glynn (12), and measured by Post and Jolly (13) and Gill and Solomon (14). In this paper, evidence is presented on the stoichiometry between active transport and adenosine tripbosphate hydrolysis and on the intracellular localization of the active site of the transport adenosine triphosphatase activity. Intracellular localization of the site for combination with sodium ion and extracellular localization of the site for combination with potassium ion have already been demonstrated by Whittam (15) and Glynn (16). The design of these transport/-P1 ratio experiments came about in the following way. First, normal potassium-rich cells in sodium-rich plasma actively transport sodium ions out and potassium ions in at the same rate as these ions leak through the membrane passively (17). Inhibition of active transport with a cardiac glycoside, such as ouabain, leads to sodium accumulation and potassium loss, and should produce measurable changes in metabolic activity. Experiments in other laboratories showed small effects at best (18, 19). We therefore began with sodium-rich, cold-stored cells where the active transport rate is 4 to 6 times larger. The transport rate is also constant with respect to sodium and potassium ion concentrations over a useful range of concentrations (3, 13). Glucose was withheld