Relevance of cation-exchange capacity of cell wall in detecting H+-symport in yeasts.

Addition of mono- or divalent cations to unbuffered yeast suspensions causes acidification of the medium. In some yeasts (Rhodotorula glutinis and Metschnikowia reukaufii) this represented a mediated H+ extrusion in exchange for cations (mostly K+). However, in some other yeasts (Pichia humboldtii and Candida albicans) the pH change was very rapid. The rapid acidification of the media by Pichia humboldtii was found to be due to high cation-exchange capacity of its cell wall. This cation-exchange was saturable, exhibited high affinity for divalent cations and was stoichiometric under appropriate experimental conditions. Results suggest that such cation exchange capacity of some yeasts could hamper the measurements related to H+-symport mechanisms.