Proton flux in large unilamellar vesicles in response to membrane potentials and pH gradients.

The transport of protons across liposomes composed of phosphatidylcholine in response to electrical potentials or pH gradients has been investigated. The results support three major conclusions. The first of these concerns the need for reliable measurements of electrical potentials and pH gradients. It is shown that the potential probe tetraphenylphosphonium and the pH probe methylamine provide accurate and self consistent measures of electrical potentials and pH gradients respectively in these systems. Second, it is shown by two independent techniques that the pH gradients induced in response to valinomycin and potassium dependent electrical potentials are significantly smaller than would be expected for electrochemical equilibrium. The pH gradients observed are stable over an 8 h time course and are sensitive to the ionic composition of the buffers employed, where the presence of external sodium results in the smallest induced pH gradients. These results are discussed in terms of current models of proton conductance across membranes. In a final area of investigation, it is shown that valinomycin and carbonyl cyanide m-chlorophenyl hydrazone (CCCP) can transport sodium ions in a synergistic manner.