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Saccharomyces cerevisiae IGC4072 grown in lactic acid medium transported lactate by an accumulative electroneutral proton-lactate symport with a proton-lactate stoichiometry of 1:1. The accumulation ratio measured with propionate increased with decreasing pH from ca. 24-fold at pH 6.0 to ca. 1,400-fold at pH 3.0. The symport accepted the following monocarboxylates (Km values at 25 degrees C and pH 5.5): D-lactate (0.13 mM), L-lactate (0.13 mM), pyruvate (0.34 mM), propionate (0.09 mM), and acetate (0.05 mM), whereas apparently a different proton symport accepted formate (0.13 mM). The lactate system was inducible and was subject to glucose repression. Undissociated lactic acid entered the cells by simple diffusion. The permeability of the plasma membrane for undissociated lactic acid increased exponentially with pH, and the diffusion constant increased 40-fold when the pH was increased from 3.0 to 6.0.

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Summary. Cells of Candida shehatae repressed by growth in glucose-or D-xylose-medium produced a facilitated diffusion system that transported glucose ( K s + 2 m M , Vm,x--+2.3 mmoles g-1 h-l), D-xylose (Ks + 125 mM, Vmax + 22.5 mmoles g-1 h-1) and D-mannose, but neither D-galactose nor L-arabinose.Cells derepressed by starvation formed several sugar-proton symports. One proton symport accumulated 3-0-methylglucose about 400-fold and transported glucose (Ks+0.12 mM, V m a x + 3 . 2 mmoles g-1 h-1) and D-mannose, a second proton symport transported D-xylose (Ks+ 1.0 mM, Vmax 1.4 mmoles g-1 h-1) and D-galactose, while L-arabinose apparently used a third proton symport. The stoicheiometry was one proton for each molecule of glucose or D-xylose transported. Substrates of one sugar proton symport inhibited non-competitively the transport of substrates of the other symports.Starvation, while inducing the sugar-proton symports, silenced the facilitated diffusion system with respect to glucose transport but not with respect to the transport of D-xylose, facilitated diffusion functioning simultaneously with the D-xylose-proton symport.

Several authors have shown that the halotolerant yeast Debaryomyces hansenii, when growing exponentially in glucose medium in the presence of sodium chloride, maintains osmotic balance by establishing sodium and glycerol gradients of opposite signs across the plasma membrane. Evidence is presented here that the two gradients are linked through a sodium‐glycerol symport that uses the sodium gradient as a driving force for maintaining the glycerol gradient. The symporter also accepts potassium ions as co‐substrate. The kinetic parameters at 25°C, pH 5·0 were the following: Vmax, decreasing from over 500 to less than 40 μmol g−1 per h over a concentration range of 0–3 M extracellular sodium chloride; Km (glycerol) 0·40–0·6 mM over the same range; Km (sodium ions) 16·0 ± 3·21μM; Km, (potassium ions) 10·4 ± 3·6μM. Furthermore, it was observed that glycerol uptake was accompanied by proton uptake when extracellular sodium chloride was present and that the protonophore carbonylcyanide‐M‐chlorophenylhydrazone induced collapse of the glycerol gradient, supporting earlier proposals by others that the sodium gradient is maintained by an active sodium‐proton exchange mechanism.