The effect of ultraviolet light on the sodium and potassium composition of resting yeast cells

The Na(+) and K(+) content of non-metabolizing yeast cells was determined before and after monochromatic ultraviolet (UV) irradiation. UV facilitated the uptake of Na(+) into and the loss of K(+) from the cells (net ion flux); the effect is greatest for the shortest wavelength employed (239 mmicro) and is partly dependent upon the presence of oxygen. The UV effect on net ion flux persists for at least 90 minutes during which tests were made and it occurs following dosages which are without measurable effect on colony formation. The UV effect on net ion flux is decreased by acidity and promoted by alkalinity. Addition of calcium ions in sufficient amount prevents the usual net ion flux changes observed in irradiated yeast. Increase in concentration gradient between the inside and the outside of the cell increases the net ion flux of irradiated yeast, Na(+) uptake leading K(+) loss in all cases. UV appears to act by disorganizing the constituents of the cell surface, permitting K(+) to leave the cell in exchange for Na(+). At low intensities of UV this ionic exchange approaches equivalence, but at higher intensities more Na(+) is taken up than K(+) is lost. Some evidence suggests that the Na(+) in excess over that exchanged for K(+) is adsorbed to charged groups produced by the photochemical effect of UV on the cell surface.