Effect of H+ on the kinetics of Na+-dependent amino acid transport in Ehrlich ascites tumor cells: evidence for H+ as an alternative substrate.

The effects of H+ on the kinetics of alpha-aminoisobutyric acid (AIB) influx in Ehrlich ascites tumor cells have been investigated at different external Na+ concentrations. Elevation of [H+] in the presence of both high (154 mEq/l) and low (10 mEq/l) external Na+ leads to decreases in the maximum influx (Jmax) and increases in the apparent Michaelis-Menten constant (Km) for influx of AIB. In the virtual absence of external Na+ (0.96 +/- 0.04 mEq/l), alterations in [H+] are without measurable effect on AIB flux. Furthermore, addition of AIB (10 mM) to cell suspensions (pH 5.90) stimulates H+ uptake by the cells in either the presence or absence of Na+. The data are consistent with two kinetic models for Na+-dependent amino acid transport: an order bireactant (Na+-binding necessary before AIB binding) system or a random bireactant system. Both models require that H+ serve as an alternative substrate for Na+. The consistency of the models was tested by fit to data from the present study (not used to evaluate the kinetic parameters) and by prediction of the pH dependence of Na+-dependent amino acid transport compared to earlier studies.