Utilization of short-chain monocarboxylic acids by the yeast Torulaspora delbrueckii: specificity of the transport systems and their regulation.

Cells of Torulaspora delbrueckii IGC 4478 grown in a medium with DL-lactic acid (0.5% v/v, at pH 5.0) exhibited Michaelis-Menten kinetics for labelled L-lactic acid transport with the following parameters at pH 5.0: Vmax, 0.38 nmol of total L-lactic acid s-1 per mg dry weight of cells and Km, 0.05 mM total L-lactic acid. Furthermore, evidence was available indicating that a proton symport for the charged form of the acid was involved. D-lactic, acetic, propionic, pyruvic and formic acids were competitive inhibitors of labelled L-lactic acid transport, suggesting that these acids used the same transport system. The ability of T. delbrueckii IGC 4478 to grow with acetic acid as the carbon source was dependent on the acid concentration and on the pH of the culture medium. When the cells were grown in 0.5% (v/v) acetic acid (pH 6.0), the transport of labelled acetic acid followed a Michaelis-Menten kinetics with the following parameters at pH 5.0: Vmax, 2.93 nmol of total acetic acid s-1 per mg dry weight of cells and and Km, 0.55 mM total acetic acid. The system also displayed a behavior consistent with a proton symport mechanism. However, the specificity of this carrier was distinct from that observed for the monocarboxylate transport in DL-lactic acid grown cells. While propionic and formic acids were competitive inhibitors of the labelled acetic acid transport, DL-lactic and pyruvic acids did not exhibit any inhibitory effects on that transport. Moreover, under the same conditions, no uptake was observed when the transport was measured with labelled L-lactic acid. Both systems were inducible and subjected to repression by glucose, fructose or sucrose. Accordingly, diauxic growth was observed in a medium containing a mixture of any of these sugars plus lactic pyruvic or acetic acid. While the induction of the acetate proton-symport appeared to be exclusively associated with acetic acid, the lactate proton-symport could be induced by either lactic or pyruvic acid but not by acetic acid. Besides, glucose repressed cells were still permeable to the undissociated form of the acids which entered the cells by simple diffusion. Furthermore, the activities of the lactate proton-symport and of the acetate proton-symport appeared not to be associated with the activity of the L-lactate (cytochrome) dehydrogenase.