The Specificity of Ionophore A 23187 in Cation Transport across Lipid Membranes Studies with Lecithin Vesicles

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Abstract
Lecithin vesicles containing different cations were prepared by sonication and characterized. When the concentration of the divalent cations in the buffer was increased from 1 mᴍ to about 100 mᴍ the cation-concentration inside of the vesicles did not increase proportional to the outside concentration, but showed a saturation behaviour. The efflux of various cations mediated by ionophore A 23187 was measured and relative transport-rates were determined. The following sequence was obtained: Zn2+ > Mn2+ > Ca2+ > Mg2+ > Sr2+ ⪢ Ba2+ ≈ Li+ > Na+. The efflux of Ca2+ increased proportional to the square of the A 23187-concentration. To lecithin-vesicles containing ethylenediamine tetraacetate (EDTA) inside different cations were added on the outside. In the presence of A 23187 divalent cations are transported into the vesicles and bound there by EDTA. During the influx of Me2+-ions into vesicles a H+-efflux was observed. The resulting pH-decrease was measured. The rate of the pH-change depended on the Me2+-ion used. The sequence was: Cd2+ > Zn2+ > Ca2+ > Mn2+ > Mg2+ ⪢ Sr2+, Ba2+.