Non-ohmic proton conductance of mitochondria and liposomes

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Abstract
Direct measurements of the H+/H- 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.