The Electrochemical Proton Gradient of <i>Saccharomyces</i>

Abstract

The electrochemical gradient of protons (ΔμΔH + ) across the plasma membrane of Saccharomyces × 17/17 has been measured as a function of external pH and the external potassium concentration. The proton gradient (ΔpH) has been determined from the distribution of the weak permeable [ 14 C]propionic acid across the plasma membrane. The accumulation of the lipophilic cation [ 3 H]tetraphenylphosphonium has been used to calculate the membrane potential (ΔΨ). It has been observed that yeast cells are able to maintain a ΔμH + whose values vary from −150 mV at pH 4.5 to −90 mV at pH 7.0. When considered independently ΔpH and ΔΨ have values which vary reciprocally depending on the external pH. At pH 4.5 δpH is the main component of ΔμH + having a value of −105 mV. Then its value decreases as the external pH increases reaching a value of −10 mV at pH 7.0. At this pH ΔΨ is the main cornponcnt of δμ,H+ + having a value of −80 mV, while at pH 4.5 its value is −45 mV. As the external concentration of potassium increases the value of ΔΨ becomes smaller. This decrease in ΔΨ is accompanied by an increase in the value of ΔpH. For any given concentration of potassium, the decrease in ΔΨ is larger than the increase in δpH the net result being, therefore, a decrease in the protonmotive force. In cells treated with the protonophores carbonylcyanide M‐chlorophenylhydrazone or dinitrophenol, δpH is drastically reduced at every pH while ΔΨ is not significantly changed. Upon treatment with dicyclohexylcarbodiimide ΔpH is reduced at any pH tested. This decrease is accompanied by an increase in the value of ΔΨ so that the total protonmotive force remains essentially constant. If the treatment with dicyclohexylcarbodiiniide is carried out in the presence of 200 mM potassium carbodiimide effect on ΔpH and ΔΨ is abolished. The results show that the movements of H + and K + are closely connected and play a central role in the establishment and maintenance of an energized state of the cell. They further suggest that δμH + regulates the fluxes of protons while ΔμH + regulates the establishment of a ΔμK + .