A comparative study on the transport of L (‐)malic acid and other short‐chain carboxylic acids in the yeast Candida utilis : Evidence for a general organic acid permease

Cells of the yeast Candida utilis grown in medium with short‐chain mono‐, di‐ or tricarboxylic acids transported L(‐)malic acid by two transport systems at pH 3·0. Results indicate that probably a proton symport for the ionized form of the acid and a facilitated diffusion for the undissociated form were present. Dicarboxylic acids such as succinic, fumaric, oxaloacetic and α‐ketoglutaric acids were competitive inhibitors of the malic acid for the high‐affinity system, suggesting that these acids used the same transport system. In turn, competitive inhibition uptake studies of labelled carboxylic acid in the low‐affinity range indicated that this system was non‐specific and able to accept not only carboxylic (mono‐, di‐ or tri‐) acids but also some amino acids. Additionally, under the same growth conditions, C. utilis produced two mediated transport systems for lactic acid: a proton symport for the anionic form which appeared to be a common monocarboxylate carrier and a facilitated diffusion system for the undissociated acid displaying a substrate specificity similar to that observed for the low‐affinity dicarboxylic acid transport. The mediated carboxylic acid transport systems were inducible and subjected to repression by glucose. In glucose‐grown cells the undissociated dicarboxylic acids entered the cells slowly by simple diffusion. Repressed glucose‐grown cells were only able to produce both transport systems if an inducer, at low concentration (0·5%, w/v), was present during starvation in buffer. This process was inhibited by the presence of cycloheximide indicating that induction requires de novo protein synthesis. If a higher acid concentration was used, only the low‐affinity transport system was detectable, showing that the high‐affinity system was also repressed by high concentrations of the inducer.