Inhibition of Rb + uptake in yeast by Ca 2+ is caused by a reduction in the surface potential and not in the Donnan potential of the cell wall

Abstract

Enzymic removal of the yeast cell wall does not affect the kinetics of Rb + uptake by the cells, nor the inhibition of this uptake by Ca 2+. The maximum rate of Rb + uptake is not affected, whereas the apparent K m is increased by Ca 2+, in protoplasts as effectively as in intact cells. Comparison of this effect of Ca 2+ with that exerted on the interaction of the surface potential fluorescent probe 9-aminoacridine with the cells reveals that the inhibition of Rb + uptake is caused by a decrease in the interfacial Rb + concentration near the membrane surface rather than by competition of Rb + and Ca 2+ for binding to the translocator. Moreover, the present results show that the decrease in interfacial Rb + concentration is the consequence of a reduction in the negative surface potential of the membrane by Ca 2+ and is not due to the concomitant reduction in the Donnan potential of the cell wall.