Enhancement of transmembrane proton conductivity of protonophores by membrane-permeant cations

Abstract

The rate of protonophore-mediated decay of pH gradient across lipid vesicular membranes was found to be enhanced by orders of magnitude by valinomycin-K⁺. Experiments in the presence of gramicidin have shown that the observed rate enhancement by valinomycin-K⁺ is not due to collapse of the diffusion potential alone. The enhancement of the rate showed hyperbolic dependence on the concentration of valinomycin. Rate enhancement was observed in the presence of the membrane permeant cation tetraphenylphosphonium (TTP⁺) also. Several factors which might enhance the intrinsic H⁺ conductivity of protonophores were analyzed. The level of partitioning of the protonophore into the membrane and the pK of membrane-bound protonophores were measured. Valinomycin-K⁺ did not alter both these parameters significantly. TPP⁺ increased the partitioning of protonophores and decreased the pK values of membrane-bound protonophores. However, these changes were too small to explain the observed rate enhancements. We suggest that valinomycin-K⁺ and TPP⁺ enhance the H⁺ conductivity of protonophores by increasing the permeability of the ionized form of protonophores by forming an ion pair.