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

Area of Science:

• Microbiology
• Yeast Physiology
• Cellular Biophysics

Background:

• Understanding intracellular pH (pHin) and membrane potential is crucial for yeast physiology.
• Accurate measurements require appropriate buffering systems and knowledge of optimal pH ranges.

Purpose of the Study:

• To estimate intracellular pH (pHin) and membrane potential in Saccharomyces cerevisiae K, Rhodotorula gracilis, and Lodderomyces elongisporus.
• To determine the optimal pH range for yeast transport measurements.
• To evaluate the suitability of different buffer systems for these measurements.

Main Methods:

• Yeast species were incubated in four different buffers: triethanolamine--phthalic acid (TEPA), citric acid--trisodium citrate (CASC), acetic acid--NaOH (AANA), and MES.
• Intracellular pH (pHin) and membrane potential were measured across an extracellular pH (pHout) range of 3.5-7.5.
• Data analysis focused on the relationship between pHout, pHin, membrane potential, and protonmotive force.

Main Results:

• pHin showed consistent patterns across all buffers, with stable values below pHout 5 and above pHout 7.
• Membrane potential decreased linearly with decreasing pHout.
• The apparent protonmotive force increased as pHout decreased.
• All tested yeast species exhibited pHin = pHout between 5.0 and 6.0.
• The TEPA buffer demonstrated the smoothest pHin dependence on pHout and is metabolically inert.

Conclusions:

• The pH range of 5.0-6.0 is recommended for yeast transport measurements due to pHin and pHout equalization.
• Triethanolamine--phthalic acid (TEPA) buffer is the preferred choice for its stability and inertness in yeast studies.
• These findings provide essential parameters for future research on yeast transport mechanisms.