Ureidosuccinic acid permeation in Saccharomyces cerevisiae

Some strains of Saccharomyces cerevisiae exhibit a specific transport system for ureidosuccinic acid, which is regulated by nitrogen metabolism. Ureidosuccinic acid uptake occurs with proline but with ammonium sulfate as nitrogen source it is inhibited. The V for transport is 20-25 mumol/ml cell water per min. The apparent Km is 3-10(-5) M. For the urep1 mutant (ureidosuccinic acid permease less) the internal concentration never exceeds the external one. In the permease plus strain ureidosuccinic acid can be concentrated up to 10 000 fold and the accumulated compound remains unchanged in the cells. Energy poisons such as dinitrophenol, carbonyl cyanide-m-chlorophenyldrazone (CCCP) or NaN3 inhibit the uptake. No significant efflux of the accumulated compound occurs even in the presence of these drugs. The specificity of the permease is very strict, only amino acids carrying an alpha-N-carbamyl group are strongly competitive inhibitors. The high concentration capacity of the cells and lack of active exit of the accumulated compound support the hypothesis of a carrier mediated active transport system.