Relationship of Cation Influxes and Effluxes in Yeast

Abstract

The Na+ efflux from Na+-rich yeast cells into a cation-free medium is largely balanced by the excretion of organic anions. In the presence of Rb+, K+, or high levels of H+ (pH 3–4), the Na+ efflux is increased and the organic anion excretion is suppressed so that stoichiometric cation exchanges occur. H+ participates in the exchanges, moving into or out of the cells depending on the external pH and on the concentration of external Rb+(K+). The total cation efflux is dependent on the external Rb+ concentration in a "saturation" relationship, but the individual cations in the efflux stream are not. The discrimination factor in the efflux pathway between H+ and Na+ is very large (of the order of 10,000), and between Na+ and K+ considerable (of the order of 50). For the latter pair, the recycling of K+ from the cell wall space is an important factor in the discrimination. In addition, the Na+ efflux as a function of Na+ content follows a sigmoidal curve so that the discrimination factor is increased at high levels of cellular Na+. Although the influx and efflux pathways behave as a tightly coupled system, the mechanism of coupling is not entirely clear. A single system with different cation specificities and kinetic behaviors on the inside and outside faces of the membrane could account for the data.