Synergy in K+/H+ exchange across phospholipid vesicle membranes with combinations of valinomycin and chlorophenols

Abstract

The protonophoric mobilities of mixtures of chlorophenols A₁ and A₂, inferred from a study of the decay of the pH difference across soybean phospholipid vesicular membrane (=ΔpH), have shown that there is a significant synergy for certain combinations of A₁ and A₂ in the presence of valinomycin (VAL). The results have shown that in addition to a term proportional to [VAL]₀[A_i]₀, a term proportional to the product of the concentrations [VAL]₀[A₁]₀[A₂]₀ (with A₁ = A₂ giving a quadratic term in the “two species case”) contributes to the ΔpH relaxation rate (=1/τ). The magnitudes of these terms depend on the choice of chlorophenols. The former term has been assigned to the well-known mechanism in which the anion A⁻ and the metal ion K⁺ are back transported across the membrane in the form of the ternary complex VAL–K⁺–A⁻. The latter term has been assigned to the less known catalyzed transport mechanism in which A₁⁻ is transported from the polar region of the membrane to the non-polar region in the form of the electro-neutral complex VAL–K⁺–A₁⁻ in a fast step followed by translocation through the non-polar region as A₁⁻ after dissociation from the ternary complex assisted by a complexing A₂⁻ from the polar region. The translocation of VAL–K⁺ across the membrane facilitates the compensating charge flux of K⁺ ions in this mechanism. The processes characterized in this work are useful in improving our understanding of the toxicity of uncouplers and uncoupler mixtures and may also help in the development of appropriate analytical tools.