Protonmotive force in yeasts--pH, buffer and species dependence.

Using yeast species Saccharomyces cerevisiae K, Rhodotorula gracilis, and Lodderomyces elongisporus, their intracellular pH value and their membrane potential were estimated at pH 3.5-7.5 in four different buffers: triethanolamine--phthalic acid (TEPA), citric acid--trisodium citrate (CASC), acetic acid--NaOH (AANA) and MES. The pHin followed the same pattern in all buffers, with rather constant values below pHout = 5 and again above pHout = 7. The membrane potential decreased regularly with decreasing pHout. The apparent protonmotive force increased with decreasing pHout. It is seen that for all the yeast species pHin and pHout are the same between 5.0 and 6.0 which is thus the pH range of choice for various transport measurements. Of the four buffers, TEPA gives smoothest pHin dependences on pHout and, being metabolically inert, is the one to be recommended.