Sodium and potassium transport in the halophilic yeast Debaryomyces hansenii

Debaryomyces hansenii, a halophile yeast found in shallow sea waters and salty food products grows optimally in 0.6 M of either NaCl or KCl, accumulating high concentrations of Na(+) or K(+). After growth in NaCl or KCl, a rapid efflux of either accumulated cation was observed if the cells were incubated in the presence of KCl or NaCl, respectively, accompanied by a slower accumulation of the cation present in the incubation medium. However, a similar, rapid efflux was observed if cells were incubated in buffer, in the absence of external cations. This yeast shows a cation uptake activity of both (86)Rb(+) and (22)Na(+) with saturation kinetics, and much higher affinity for (86)Rb(+) than for (22)Na(+). The pH dependence of the kinetics constants was similar for both cations, and although K(m) values were higher at pH 8.0, there was also an increase in the V(max) values. The accumulation of (22)Na(+) was found to be increased in cells grown in the presence of 0.6 M NaCl. (86)Rb(+) was also accumulated more in these cells, but to a slightly greater extent. The inhibition kinetics of the uptake of (22)Na(+) by K(+), and that of (86)Rb(+) by Na(+) was found to be non-competitive. It can be concluded that Na(+) in D. hansenii is not excluded but instead, its metabolic systems must be resistant to high salt concentrations.