Non-ohmic proton conductance of mitochondria and liposomes

Abstract

Direct measurements of the proton/hydroxyl ion flux across rat liver mitochondria and liposome membranes are reported. H⁺/OH^- fluxes driven by membrane potential (Δψ) showed nonlinear dependence on delta psi both in mitochondria and in liposomes whereas Δ pH-driven H⁺/OH^- flux shows linear dependence on Δ pH in liposomes. In the presence of low concentrations of a protonophore the H⁺/OH^- flux was linearly dependent on Δψ and showed complex dependence on Δ pH. The nonlinearity of H⁺/OH^- permeability without protonophore is described by an integrated Nernst- Plank equation with trapezoidal energy barrier. Permeability coefficients depended on the driving force but were in the range 10^-3 cm/s for mitochondria and 10^-4-10^-6 cm/s for liposomes. The nonlinear dependence of H⁺/OH^- flux on Δψ explains the nonlinear dependence of electrochemical proton gradient on the rate of electron transport in energy coupling systems.