MANGANESE TOXICITY TOWARDS SACCHAROMYCES-CEREVISIAE - DEPENDENCE ON INTRACELLULAR AND EXTRACELLULAR MAGNESIUM CONCENTRATIONS

Inhibition of the growth of Saccharomyces cerevisiae was evident at co ncentrations of 0.5 mM Mn2+ or higher, but a tolerance to lower Mn2+ c oncentrations was observed. The inhibitory effects of 2.0 mM Mn2+ were eliminated by supplementing the medium with excess Mg2+ (10 mM), wher eas addition of excess Ca2+ and K+ had negligible effect on Mn2+ toxic ity. Growth inhibition by Mn2+, in the absence of a Mg2+ supplement, w as attributed to Mn2+ accumulation to toxic intracellular levels. Mn l evels in S. cerevisiae grown in Mg2+-supplemented medium were severalf old lower than those of cells growing in unsupplemented medium. Mn2+ t oxicity was also influenced by intracellular Mg, as Mn2+ toxicity was found to be more closely correlated with the cellular Mg:Mn ratio than with cellular Mn levels alone. Cells with low intracellular levels of ME were more susceptible to Mn2+ toxicity than cells with high cellul ar Mg, even when sequestered Mn2+ levels were similar. A critical Mg:M n ratio of 2.0 was identified below which Mn2+ toxicity became acute. The results demonstrate the importance of intracellular and extracellu lar competitive interactions in determining the toxicity of Mn2+.