Binding and transport of calcium ions by synthetic analogues of ionomycin

Abstract

The transport of calcium ions across an organic barrier by a series of synthetic analogues of ionomycin was measured in a cylindrical glass cell using chloroform as the artificial membrane. The presence of a β-diketone and carboxyl group in the same molecule and a sufficient lipid solubility of the compounds were shown to be necessary and sufficient conditions for Ca2+ transport. Small and no transport of Ca2+ were found for analogues 5 and 6, respectively, due to the low lipid solubility of these compounds. The Ca2+ transport rate for analogues 8 and 13-15 followed the order of 8 > 13 > 14 > 15, which demonstrated that optimal Ca2+ transport was achieved when the β-diketone was separated from the carboxyl group by seven methylene units, identical to that found in ionomycin. Analogues 8-10 were comparable to ionomycin and calcimycin in terms of Ca2+ transport. Ca2+ transport by the analogues was found to be a saturable process that obeyed Michaelis-Menten kinetics. It was dependent on the pH in the aqueous source phase and independent of the pH in the receiving phase. Both the carboxyl and the β-diketo groups were ionized in the transport of Ca2+, indicating that the stoichiometry of Ca2+ complex in transport was 1:1. The pK(a) of the β-diketone of the analogues was determined to be in the range of 10.90-11.16 in 80% MeOH-H2O. The pK(a) values were related to the lipid solubility of the compounds and the hydrocarbon chain length between the β-diketone and the carboxyl group. The binding constants of the analogues with Ca2+ and Mg2+ in 80% MeOH-H2O were determined to be in the order of 102 M-1 and 103 M-1, respectively, using the pK(a) method. The stoichiometry of Mg2+ binding was found to be 1:1 by the mole ratio method. The selectivity in binding and in transport followed the same order, being Mg2+ > Ca2+ >> Na+, K+.