The electrochemical H⁺ gradient in the yeastRhodotorula glutinis

Abstract

The electrochemical gradient of protons,$\Delta \tilde \mu _{H + }$, was estimated in the obligatory aerobic yeastRhodotorula glutinis in the pH₀ range from 3 to 8.5. The membrane potential, ΔΩ, was measured by steady-state distribution of the hydrophobic ions, tetraphenylphosphonium (TPP⁺) for negative ΔΩ above pH₀ 4.5, and thiocyanate (SCN⁻) for positive ΔΩ below pH₀ 4.5. The chemical gradient of H⁺ was determined by measuring the chemical shift of intracellular P_i by³¹P-NMR at given pH₀ values. The values of pH_i increased almost linearly from 7.3 at pH₀ 3 to 7.8 at pH₀ 8.5. In the physiological pH₀ range from 3.5 to 6,$\Delta \tilde \mu _{H + }$ was fairly constant at values between 17–18 KJ mol⁻¹, gradually decreasing at pH₀ above 6. In deenergized cells, the intracellular pH_i decreased to values as low as 6, regardless of whether the cell suspension was buffered at pH₀ 4.5 or 7.5. There was no membrane potential detectable in deenergized cells.