Urea transport in Saccharomyces cerevisiae

Allantoate uptake appears to be mediated by an energy-dependent active transport system with an apparent Michaelis constant of about 50 pM. Cells were able to accumulate allantoate to greater than 3,000 times the extracellular concentration. The rate of accumulation was maximum at pH 5.7 to 5.8. The energy source for allantoate uptake is probably different from that for uptake of the other allantoin pathway intermediates. The latter systems are inhibited by arsenate, fluoride, dinitrophenol, and carboxyl cyanide-m-chlorophenyl hydra-zone, whereas allantoate accumulation was sensitive to only dinitrophenol and carboxyl cyanide-m-chlorophenyl hydrazone. Efflux of preloaded allantoate did not occur at detectable levels. However, exchange of intra- and extracellular allantoate was found to occur very slowly. The latter two characteristics are shared with the allantoin uptake system and may result from the sequestering of intracellular allantoate within the cell vacuole. During the course of these studies, we found that, contrary to earlier reports, the reaction catalyzed by allantoinase is freely reversible