Transport of lactate and other short-chain monocarboxylates in the yeast Candida utilis

Summary Lactic acid grown cells of the yeast Candida utilis transported lactate by an accumulative electroneutral proton-lactate symport with a proton-lactate stoicheiometry of 1:1. The accumulation ratio at pH 5.5 was about twenty. The symport accepted the following monocarboxylates (K _svalues at 25°C, pH 5.5 in brackets): d-lactate (0.06 mM), l-lactate (0.06 mM), pyruvate (0.03 mM), propionate (0.05 mM) and acetate (0.1 mM). The system was inducible and was subject to glucose repression. The affinity of the symport for lactate was not affected by pH over the range 3–6, while the maximum transport velocity was strongly pH dependent, its optimum pH being around pH 5. Undissociated lactic acid entered the cells by simple diffusion. The permeability for the undissociated acid increased exponentially with pH, the diffusion constant increasing 35-fold when the pH was increased from 3 to 5.5.