Ammonium Toxicity and Potassium Limitation in Yeast

Abstract

DNA microarray analysis of gene expression in steady-state chemostat cultures limited for potassium revealed a surprising connection between potassium and ammonium: potassium limits growth only when ammonium is the nitrogen source. Under potassium limitation, ammonium appears to be toxic for Saccharomyces cerevisiae. This ammonium toxicity, which appears to occur by leakage of ammonium through potassium channels, is recapitulated under high-potassium conditions by over-expression of ammonium transporters. Although ammonium toxicity is well established in metazoans, it has never been reported for yeast. To characterize the response to ammonium toxicity, we examined the filtrates of these cultures for compounds whose excretion might serve to detoxify the ammonium (such as urea in mammals). Using liquid chromatography–tandem mass spectrometry to assay for a wide array of metabolites, we detected excreted amino acids. The amounts of amino acids excreted increased in relation to the severity of growth impairment by ammonium, suggesting that amino acid excretion is used by yeast for ammonium detoxification.

Ammonia-induced autophagy is independent of ULK1/ULK2 kinases

“…Comparable observations have been reported in yeast, where treatment with ammonia results in a cellular growth defect and massive secretion of amino acids including alanine (29). It has been reported recently that expression of Myc in cancer cells leads to increased glutamine metabolism by increasing the level of glutamine transporters and the enzymes involved in glutaminolysis in certain cancer cells (27,28).…”

Saccharomyces cerevisiae Multidrug Resistance Transporter Qdr2 Is Implicated in Potassium Uptake, Providing a Physiological Advantage to Quinidine-Stressed Cells

“…The indirect (or direct) role of Qdr2p in promoting this apparently nonspecific efflux of amino acids may become crucial to avoid abnormal large amounts of amino acids resulting from deregulated metabolism that may be deleterious to cell physiology, as is the case during growth arrest due to leucine limitation. Remarkably, a recently published work has established a link between K ϩ limitation, ammonium toxicity, and amino acid excretion (15). According to the indication of this microarray analysis, complemented by metabolite profiling, under limiting potassium concentrations, toxic amounts of ammonium might enter yeast via potassium channels, as observed in other organisms.…”

“…According to the indication of this microarray analysis, complemented by metabolite profiling, under limiting potassium concentrations, toxic amounts of ammonium might enter yeast via potassium channels, as observed in other organisms. When experiencing ammonium toxicity, yeast responds by excreting amino acids which may constitute a rudimentary ammonia detoxification mechanism in the yeast cell (15).…”

A Genomewide Screen for Tolerance to Cationic Drugs Reveals Genes Important for Potassium Homeostasis in Saccharomyces cerevisiae

“…Growth was visually evaluated and recorded after 48 h. Clones that showed strong, weak, or no macroscopic growth in the supplemented media after 48 h were considered putative positive clones. In order to confirm their sensitivity, these clones were recovered from the original 96-well plates, diluted with YPD medium to an OD 600 of 0.05, and evaluated for sensitivity by a drop test on YPD plates supplemented with different concentrations of each toxic cation (20,30,40,50, and 60 g/ml hygromycin B; 0.3, 0.4, 0.5, 0.6, and 0.7 mM spermine; and 0.2, 0.3, 0.4, 0.5, and 0.6 M TMA). The intensity of the phenotype was scored from 1 to 10 (most to least sensitive; for the wild-type strain the value was 6), on the basis of the lowest concentration of each toxic cation at which the strain showed no or marginal growth after 48 h.…”