Casal M, Leão C
Department of Biology, University of Minho, Braga, Portugal.
Biochim Biophys Acta. 1995 Jun 20;1267(2-3):122-30. doi: 10.1016/0167-4889(95)00067-3.
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.
在含有DL-乳酸（0.5% v/v，pH 5.0）的培养基中生长的德氏有孢圆酵母IGC 4478细胞，对标记的L-乳酸转运表现出米氏动力学，在pH 5.0时具有以下参数：Vmax为每毫克细胞干重0.38 nmol总L-乳酸·s⁻¹，Km为0.05 mM总L-乳酸。此外，有证据表明该过程涉及酸的带电形式的质子同向转运。D-乳酸、乙酸、丙酸、丙酮酸和甲酸是标记的L-乳酸转运的竞争性抑制剂，这表明这些酸使用相同的转运系统。德氏有孢圆酵母IGC 4478以乙酸作为碳源生长的能力取决于酸浓度和培养基的pH值。当细胞在0.5%（v/v）乙酸（pH 6.0）中生长时，标记的乙酸转运遵循米氏动力学，在pH 5.0时具有以下参数：Vmax为每毫克细胞干重2.93 nmol总乙酸·s⁻¹，Km为0.55 mM总乙酸。该系统也表现出与质子同向转运机制一致的行为。然而，这种载体的特异性与在DL-乳酸生长的细胞中观察到的单羧酸转运不同。虽然丙酸和甲酸是标记的乙酸转运的竞争性抑制剂，但DL-乳酸和丙酮酸对该转运没有任何抑制作用。此外，在相同条件下，用标记的L-乳酸测量转运时未观察到摄取。这两种系统都是可诱导的，并且受到葡萄糖、果糖或蔗糖的阻遏。因此，在含有这些糖中的任何一种与乳酸、丙酮酸或乙酸的混合物的培养基中观察到了二次生长。虽然乙酸质子同向转运的诱导似乎仅与乙酸有关，但乳酸质子同向转运可由乳酸或丙酮酸诱导，而不能由乙酸诱导。此外，葡萄糖阻遏的细胞对通过简单扩散进入细胞的未解离形式的酸仍然具有通透性。此外，乳酸质子同向转运和乙酸质子同向转运的活性似乎与L-乳酸（细胞色素）脱氢酶的活性无关。
