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 pKa of the β-diketone of the analogues was determined to be in the range of 10.90−11.16 in 80% MeOH–H2O. The pKa 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 pKa 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+.[Formula: see text]