Molecular Mode of Action of the Antifungal β-Amino Acid BAY 10-8888

BAY 10-8888 is a cyclic β-amino acid that is related to cispentacin and that has antifungal activity. Candida albicans cells accumulated BAY 10-8888 intracellularly to a concentration about 200 that in the medium when grown in media with a variety of nitrogen sources. In complex growth medium, BAY 10-8888 transport activity was markedly reduced and was paralleled by a decrease in its antifungal activity. Uptake of BAY 10-8888 was mediated by an H(+)-coupled amino acid transporter with specificity for branched-chain amino acids (isoleucine, leucine, and valine) and showed a K(T) (Michaelis constant of the transport reaction) of 0.95 mM and a V(max) of 18.9 nmol × min(−1) × 10(7) cells(−1). Similar to the transport of natural amino acids in Saccharomyces cerevisiae, the transport of BAY 10-8888 into the cell was unidirectional. Efflux occurred by diffusion and was not carrier mediated. Inside the cell BAY 10-8888 inhibited specifically isoleucyl-tRNA synthetase, resulting in inhibition of protein synthesis and cell growth. Intracellular isoleucine reversed BAY 10-8888-induced growth inhibition. BAY 10-8888 was not incorporated into proteins. BAY 10-8888 inhibited isoleucyl-tRNA synthetase with the same concentration dependency as protein biosynthesis in intact cells assuming 200-fold accumulation.