Transport of malic acid and other dicarboxylic acids in the yeast Hansenula anomala.

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RESUMO

DL-Malic acid-grown cells of the yeast Hansenula anomala formed a saturable transport system that mediated accumulative transport of L-malic acid with the following kinetic parameters at pH 5.0: Vmax, 0.20 nmol.s-1.mg (dry weight)-1; Km, 0.076 mM L-malate. Uptake of malic acid was accompanied by proton disappearance from the external medium with rates that followed Michaelis-Menten kinetics as a function of malic acid concentration. Fumaric acid, alpha-ketoglutaric acid, oxaloacetic acid, D-malic acid, and L-malic acid were competitive inhibitors of succinic acid transport, and all induced proton movements that followed Michaelis-Menten kinetics, suggesting that all of these dicarboxylates used the same transport system. Maleic acid, malonic acid, oxalic acid, and L-(+)-tartaric acid, as well as other Krebs cycle acids such as citric and isocitric acids, were not accepted by the malate transport system. Km measurements as a function of pH suggested that the anionic forms of the acids were transported by an accumulative dicarboxylate proton symporter. The accumulation ratio at pH 5.0 was about 40. The malate system was inducible and was subject to glucose repression. Undissociated succinic acid entered the cells slowly by simple diffusion. The permeability of the cells by undissociated acid increased with pH, with the diffusion constant increasing 100-fold between pH 3.0 and 6.0.

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