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

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 co-workers (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 protons taking place at both interfacial space layers while the turnover of the mobile carrier is accomplished under asymmetrical conditions by a translocation process of the acidic grisorixin. Using the data of some previous studies for mixed ionophore-lipid 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.(ABSTRACT TRUNCATED AT 250 WORDS)