Interaction of monovalent cations with Rb+ and Na+ uptake in yeast

The concentration dependence of both Rb+ uptake and Na+ uptake by yeast can be described by a quadratic rate equation. This equation is derived for translocation of cations via a two-site translocation system. In accordance with predictions made for such a two-site translocation system the shape of the uptake isotherm depends both upon the substrate cation species and upon the concentration of other added competing cations. On plotting the rate of Rb+ uptake against the quotient of that rate and the Rb+ concentration concave, convex and also linear curves are found depending upon the type and the concentration of added monovalent cations. The Na+ uptake isotherm plotted in a similar way shows a shift from a concave curve to a straight line on adding increasing amounts of Rb+ to the yeast suspension. Decreasing the pH of the medium leads to a more pronounced convex uptake isotherm for Rb+ uptake. This can be ascribed to an indirect effect of the surface potential upon Rb+ uptake and to occupation of the binding sites by protons, which are replaced again by Rb+ at increasing Rb+ concentrations. Replacement of one sodium ion from one of the two sites of the monovalent cation transport system by either K+ OR Rb+ leads to an increase in the rate of Na+ translocation across the yeast cell membrane.