Proton dissociation from nigericin at the membrane-water interface, the rate-limiting step of K⁺/H⁺ exchange on the bilayer lipid membrane

The rate of K⁺/H⁺ exchange through bilayer lipid membranes (BLM) induced by nigericin was measured by the method of pH gradient offset according to Antonenko, Yu.N. and Yaguzhinsky L.S. [(1990) Biochim. Biophys. Acta 1026, 236-240]. It was shown that under the conditions of high potassium ion concentration the rate of nigericin-mediated K⁺/H⁺ exchange increased with an increase in the concentrations of such buffer compounds as citric acid and MES. The concentration dependence was different for citrate and MES. The buffer concentration effect was absent at low potassium ion concentrations. Citrate increased the rate of K⁺/H⁺ exchange being added to the side of BLM where the K⁺ concentration was higher and had no effect at the opposite side. At High KCl and citrate concentrations, the rate of K⁺/H⁺ exchange was about 6 times lower in D₂O when compared to H₂O solutions. It is concluded that under certain experimental conditions the overall rate of the K⁺/H⁺ exchange induced by nigericin is determined by the rate of proton dissociation from nigericin at the membrane-water interface.