Relationship of Cation Influxes and Effluxes in Yeast

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.