Non-ohmic proton conductance of mitochondria and liposomes

Direct measurements of the proton/hydroxyl ion flux across rat liver mitochondria and liposome membranes are reported. H+/OH- fluxes driven by membrane potential (delta psi) showed nonlinear dependence on delta psi both in mitochondria and in liposomes whereas delta pH-driven H+/OH- flux shows linear dependence on delta pH in liposomes. In the presence of low concentrations of a protonophore the H+/OH- flux was linearly dependent on delta psi and showed complex dependence on delta 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 delta psi explains the nonlinear dependence of electrochemical proton gradient on the rate of electron transport in energy coupling systems.