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-la ctic acid (0.5%. v/v, at pH 5.0) exhibited Michaelis-Menten kinetics f or labelled L-lactic acid transport with the following parameters at p H 5.0: V-max, 0.38 nmol of total L-lactic acid s(-1) per mg dry weight of cells and K-m, 0.05 mM total L-lactic acid. Furthermore, evidence was available indicating that a proton symport for the charged form of the acid wits involved. D-lactic, acetic, propionic, pyruvic and form ic acids were competitive inhibitors of labelled L-lactic acid transpo rt, suggesting that these acids used the same transport system. The ab ility of T. delbrueckii IGC 4478 to grow with acetic acid as the carbo n 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 (p H 6.0), the transport of labelled acetic acid followed a Michaelis-Men ten kinetics with the following parameters at pH 5.0: V-max 2.93 nmol of total acetic acid s(-1) per mg dry weight of cells and K-m, 0.55 mM total acetic acid. The system also displayed a behavior consistent wi th a proton symport mechanism. However, the specificity of this carrie r was distinct from that observed for the monocarboxylate transport in DL-lactic acid grown cells. While propionic and formic acids were com petitive inhibitors of the labelled acetic acid transport, DL-lactic a nd pyruvic acids did not exhibit any inhibitory effects on that transp ort. Moreover, under the same conditions, no uptake was observed when the transport was measured with labelled L-lactic acid. Both systems w ere inducible and subjected to repression by glucose, fructose or sucr ose. Accordingly, diauxic growth was observed in a medium containing a mixture of any of these sugars plus lactic, pyruvic or acetic acid. W hile the induction of the acetate proton-symport appeared to be exclus ively associated with acetic acid, the lactate proton-symport could be induced by either lactic or pyruvic acid but not by acetic acid. Besi des, glucose repressed cells were still permeable to the undissociated form of the acids which entered the cells by simple diffusion. Furthe rmore, 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.