Energy sources for amino acid transport in animal cells.

Abstract

AbstractThe existence of an electrogenic Na+ pump in Ehrlich cells which substantially contributes to the membrane potential, previously derived from the distribution of the lipid soluble cation tetraphenylphosphonium (TPP+), could be confirmed by an independent method based on the quenching of fluorescence of a cyanine dye derivative, after the mitochondrial respiration had been suppressed by appropriate inhibitors. The mitochondrial membrane potential, by adding to the overall potential as measured in this way is likely to cause an overestimation of the membrane potential difference (p.d.). But since this error tends to diminish with increasing pump activity, the true p.d. of the plasma membrane should easily account for the driving force to drive the active accumulation of amino acids in the absence of an adequate Na+ concentration gradient. Accordingly, the F2‐aminoisobutyric acid (AIB) uptake rises linearly with the distribution of TPP+ at constant Na+ concentrations, suggesting that each responds directly to membrane potential. There is evidence that the electrogenic (free) movement of Cl− is slow, at least at normal p.d., whereas a major part of the Cl− movement across the cellular membrane appears to occur by an electrically silent Cl−‐base exchange mechanism. By such a mode Cl−, together with an almost stoichiometric amount of K+, may under certain conditions move into the cell against a high adverse electrical potential difference. This “paradoxical” movement of K+Cl− contributing to the deviation of the Cl− distribution from the electrochemical equilibrium distribution, is not completely understood. It is insensitive towards ouabain but can almost specifically be inhibited by furosemide. As a likely explanation a H+–K+ exchange pump was previously offered, even though unequivocal evidence of such a pump is so far lacking. According to available evidence the electrogenic movement of free Cl− is too small, at least at normal orientation of the p.d., to significantly shunt the electrogenic pump potential so that the establishment of such a potential is plausible. The evidence presented is considered strong in favor of the gradient hypothesis since even in the absence of an adequate Na+ concentration gradient, the electrogenic Na+ pump will contribute sufficient extra driving force to actively transport amino acid into the cells.