LOW-AFFINITY AND HIGH-AFFINITY TRANSPORT-SYSTEMS FOR CITRIC-ACID IN THE YEAST CANDIDA-UTILIS

Citric acid-grown cells of the yeast Candida utilis induced two transport systems for citric acid, presumably a proton symport and a facilitated diffusion system for the charged and the undissociated forms of the acid, respectively. Both systems could be observed simultaneously when the transport was measured at 25-degrees-C with labelled citric acid at pH 3.5 with the following kinetic parameters: for the low-affinity system, V(max), 1.14 nmol of undissociated citric acid s-1 mg (dry weight) of cells-1, and K(m), 0.59 mM undissociated acid; for the high-affinity system, V(max), 0.38 nmol of citrate s-1 mg (dry weight) of cells-1, and K(m), 0.056 mM citrate. At high pH values (above 5.0), the low-affinity system was absent or not measurable. The two transport systems exhibited different substrate specificities. Isocitric acid was a competitive inhibitor of citric acid for the high-affinity system, suggesting that these tricarboxylic acids used the same transport system, while aconitic, tricarballylic, trimesic, and hemimellitic acids were not competitive inhibitors. With respect to the low-affinity system, isocitric acid, L-lactic acid, and L-malic acid were competitive inhibitors, suggesting that all of these mono-, di-, and tricarboxylic acids used the same low-affinity transport system. The two transport systems were repressed by glucose, and as a consequence diauxic growth was observed. Both systems were inducible, and not only citric acid but also lactic acid and malic acid may induce those transport systems. The induction of both systems was not dependent on the relative concentration of the anionic form(s) and of undissociated citric acid in the culture medium. Undissociated citric acid entered the cells slowly by simple diffusion in glucose-grown cells. The permeability of the cells to undissociated acid by simple diffusion increased with pH, with the diffusion constant increasing 100-fold between pHs 3.0 and 5.0.