An electrogenic sodium pump as a possible factor leading to the concentration of amino acids by mouse ascites-tumour cells with reversed sodium ion concentration gradients.

According to the so-called ion gradient hypothesis, the spontaneous movements of Na+ and, possibly, K+ across the plasmalemma of the mammalian cell provide the energy that is needed to concentrate various amino acids in the cellular phase (reviewed by Schultz & Curran, 1970). One important approach has been the demonstration that the ion gradients may apparently drive the amino acid pump in the absence ofATP (Eddy, 1968). Nevertheless, recent work both with intestinal preparations (Kimmich, 1970; Newey et al., 1970) and with mouse ascites-tumour cells (Potashner & Johnstone, 1971; Schafer & Heinz, 1971) has been interpreted to mean that during respiration only a minor fraction of the driving force may be provided in that way. The tumour preparations used by Schafer & Heinz (1971) accumulated 2-aminoisobutyrate against its own concentration gradient, even when the combined concentration gradients ofNa+ andK+ might have caused the amino acid to be expelled from the tumour cells. Schafer & Heinz (1971) estimated that a force ofat least4kJ/mol, corresponding to a concentration factor of about 4.7-fold, acted in series with the gradients ofNa+ and K+ driving the amino acid pump, and they suggested that the additional force might be provided by the direct coupling of amino acid transport to cellular metabolism. We havenowre-examined the arguments leading to that conclusion with particular reference to the role of the membrane potential. Let the extracellular phase be denoted by subscript