Kinetic properties of cation/H +-exchange: calcimycin (A23187)-mediated Ca 2+/2H +-exchange on the bilayer lipid membrane

Abstract

The calcimycin (A23187)-mediated electrically silent flux of hydrogen ions coupled with a counter transport of calcium or magnesium ions was measured by the method of local pH changes recording in the unstirred layers near the planar bilayer lipid membrane (BLM). It was shown that: (1) the pH dependence of calcimycin-mediated Ca 2+/2H + exchange had a maximum at pH 7; (2) the apparent Michaelis constant for the alkali earth cations were higher at acidic pH than the corresponding values at alkaline pH; (3) the apparent Michaelis constant for calcium was similar to that for magnesium ions in agreement with calcimycine cation binding constants; (4) the ratio of calcium and magnesium fluxes was independent of pH in the pH range from 5 to 8. (5) the flux was proportional to the calcimycin concentration at pH > 6.3 and proportional to the square of the carrier concentration at pH < 5; (6) the addition of calcium ion chelator EDTA increased the flux significantly. These data were discussed in terms of the model of cation/H +-exchange and it was concluded that the dissociation of the cation-carrier complex at the membrane/water interface played an important role in the process of calcimycine operation. The comparison of the kinetic properties of calcimycin with the previously described kinetics of nigericin (Antonenko and Yaguzhinsky (1988) Biol. Membr. (Russian) 5, 718–728) revealed much similarity. On the other hand, a significantldifference was found between the mechanism of the nigericin K/Na selectivity and calcimycin Ca/Mg selectivity.