Potassium limitation, ammonium toxicity and amino acid excretion in yeast
As a single-celled eukaryote organism, the yeast strain S. cerevisiae has some limitations in terms of how it can be used as a model for more complex multicellular eukaryotes. However, in an article published online this week in the open access journal PLoS Biology, David Hess, David Botstein, and colleagues demonstrate that a stress response to ammonium toxicity (linked with limited potassium) in yeast is dealt with by amino acid excretion and is likely to be a yeast equivalent of urea excreted by mammals in urine.
Using microarrays, the authors found that, in potassium-limiting conditions, many genes involved in nitrogen metabolism showed altered activity compared with unstressed cells. These expression patterns suggested that some attempt was being made by the cell to deal with a toxic influx of nitrogen in the cell--intriguing as nitrogen toxicity was thought to be limited to multicellular organisms. Using a chemostat, the authors monitored the responses of cells exposed to different levels of ammonium and potassium. In low potassium, cell numbers decreased dramatically (suggesting a toxic effect of ammonium when potassium is limited). Indeed, using alternate nitrogen-rich sources in place of ammonium, the authors saw that this was not a general nitrogen response, but one specific to ammonium. Tests across different yeast strains found this to be not a quirk on one strain of S. cerevisiae. Further as a metabolic fingerprint of this adverse reaction to ammonium, the researchers observed high levels of amino acids in the cellular environment of these potassium-limited cells, as measured by liquid chromatography tandem mass spectrometry in collaboration with the Rabinowitz lab at Princeton
To investigate further the link between potassium concentration and ammonium toxicity, the authors hypothesized that ammonium might leak into cells through potassium channels when they are not occupied by potassium. Indeed, in an engineered strain in which ammonium influx could be increased without stimulating innate ammonium concentration regulatory mechanisms, the cells engineered to let in more ammonium showed greater mortality even in high potassium concentration, supporting the idea that ammonium is the root of the problem. Also in these engineered cells, growth was limited even though potassium was not, and again, a high level of amino acid excretion was seen.
It seems, therefore, that S. cerevisiae experiences ammonium toxicity under potassium-deprived conditions and that it uses a primitive detoxification system involving the production and excretion of amino acids in an attempt to deal with it.
Citation: Hess DC, Lu W, Rabinowitz JD, Botstein D (2006) Ammonium toxicity and potassium limitation in yeast. PLoS Biol 4(11): e351. DOI: 10.1371/journal.pbio.0040351.
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