The ion selectivity of nonelectrogenic ionophores measured on a bilayer lipid membrane: nigericin, monensin, A23187 and lasalocid A.

A new method for measuring ion selectivity of nonelectrogenic ionophores on a planar bilayer lipid membrane (BLM) has been developed. This method is based on the phenomenon of formation of local gradients of ion concentration arising in the unstirred layers near the BLM in the course of transmembrane ion fluxes. The method gives rise to reproducible values of ion selectivity, independent of the conditions employed. In the case of equal charges of cations, measurable selectivity is determined by a combination of the binding and the translocation rate constants for two cations. The following values have been obtained for cation selectivity: nigericin, K(+)/Na (+) 25 +/- 4; monensin, Na(+)/K(+) 16 +/- 4; A23187, Ca(2+) /Mg (2+) 14 +/- 2; lasalocid A, K(+) /Na (+) 12 +/- 1, Ca(2+) /Mg (2+) 17+/- 2. The transport sequence of lasalocid A for cations is K(+) >Na(+) >Ca(2+) > Mg (2+). The data obtained show that, for monovalent cations, the selectivity of the antibiotic used depends on the cation binding constants on the membrane surface. For the transport of divalent cations by ionophores A23187 and lasalocid A, their selectivity values must also depend on the translocation rate constants of ionophore-cation complex.