Figure 10. Model for Ammonium Toxicity and Detoxification in Yeast.

We believe the results of this paper support the following model. If ammonium is present in high concentrations in the environment, then ammonium ions can enter the cell unregulated via potassium channels. Although most of the ammonium can be taken up into new biomass (if excess carbon and other nutrients are available), the unregulated flux creates an excess of internal ammonium that becomes toxic. To reduce internal ammonium levels, amino acids are excreted (most likely through the SPS amino acid transporters). The nitrogen affixed to amino acids will not be taken up through the potassium channels and is thus detoxified with respect to the cell.