Derks W J, Borst-Pauwels G W
Biochim Biophys Acta. 1980 Mar 13;596(3):381-92. doi: 10.1016/0005-2736(80)90125-x.
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.
酵母对铷离子（Rb⁺）和钠离子（Na⁺）的摄取浓度依赖性可用二次速率方程来描述。该方程是针对阳离子通过双位点转运系统的转运而推导得出的。根据对这种双位点转运系统的预测，摄取等温线的形状既取决于底物阳离子种类，也取决于其他添加的竞争性阳离子的浓度。绘制铷离子摄取速率与该速率和铷离子浓度的商的关系图时，根据添加的单价阳离子的类型和浓度，会发现凹形、凸形以及线性曲线。以类似方式绘制的钠离子摄取等温线显示，向酵母悬浮液中添加越来越多的铷离子时，曲线从凹形转变为直线。降低培养基的pH值会导致铷离子摄取的凸形摄取等温线更加明显。这可归因于表面电位对铷离子摄取的间接影响以及质子对结合位点的占据，在铷离子浓度增加时，质子会再次被铷离子取代。单价阳离子转运系统的两个位点之一中的一个钠离子被钾离子（K⁺）或铷离子取代会导致钠离子跨酵母细胞膜转运速率增加。
