Tributyltin-mediated exchange diffusion of halides in lipid bilayers

Abstract

This paper describes the effect of tributyltin (TBT) on the inorganic anion permeability of lipid bilayers. When this compound is added in micromolar concentrations to one or both sides of a phosphatidyl ethanolamine (PE) membrane formed in 0.1 M NaCl or KCl (pH 7), there is no change in the electrical conductance. Under these circumstances, the Cl self-exchange flux measured with 36Cl (MCl) increases from a value of ~ 10-12 mol·cm-2·s-1, to ~ 10-8 mol·cm-2·s-1. It was further found that the relation between chloride flux and [TBT] and [C1] can be described as: MCl = B[TBT] [Cl]. When chloride was replaced by an equimolar concentration of different univalent anions in the trans compartment, the heteroexchange flux of chloride followed the sequence: I> Cl>F>NO3. Under all experimental conditions tested, the chloride flux was always more than 103 times the maximum flux predicted from the value of the membrane conductance, and at least 100 times higher than the expected fluxes of ion pairs (TBT-Cl) diffusing across the unstirred layers. Thus, the mechanism by which tributyltin increases anion permeability in bilayers seems to be that of an obligatory exchange diffusion, with the reaction between tributyltin and the halides occurring at the membrane surface. Measurements of interracial potentials indicate that tributyltin chloride lowers the positive intrinsic dipole potential of PE membranes by ~ 70 mV (at a TBT concentration of 30 µM) without substantial alteration of other parameters of the bilayer. The estimated adsorption coefficient of TBT-Cl was found to be 3 × 10-4 cm. © 1979, Rockefeller University Press., All rights reserved.