Transport of potassium ions across planar lipid membranes by the antibiotic, grisorixin: I. The equilibrium state and self-diffusion K+ fluxes

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Abstract
42K+ tracer flux and steady-state conductance measurements were carried out with bilayer lipid membranes containing grisorixin, a carboxylic polyether antibiotic. When the membranes are placed between two bulk aqueous solutions of identical composition, the exchange or self-diffusion transmembrane flux of potassium is measured by a method which allows the characterization of the bilayer K+ permeability at the equilibrium state. The K+ self-diffusion flux increases with the pH in the range pH 6 to pH 9 and reaches a constant value for values above 9. This can be directly related to the increase of the surface concentration of the 1∶1 complex formed by K+ and the deprotonated polyether at both bilayer membrane interfaces. The transport model initially proposed by Pressman and coworkers (Proc. Natl. Acad. Sci. USA 58:1949–1956, 1967) is again taken into consideration in the quantitative analysis of the flux data. The transmembrane transport of K+ results from the translocation of its neutral complex with grisorixin and the association-dissociation of the antibiotic with either potassium or conditions by a translocation process of the acidic grisorixin. Using the data of some previous studies for mixed ionophorelipid monolayers at the air/water interface and the present results for the self-diffusion flux measurements, it was possible to propose an evaluation of the more important parameters characterizing the transport; namely, the total surface concentration of grisorixin, the interfacial pK and the translocation rate constant of its potassium neutral complex. The method proposed could be extended easily to other carboxylic polyethers, which would lead to an interesting comparison of their ionophoric properties using model membrane systems.

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