Binding and transport of calcium ions by synthetic analogues of ionomycin
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 aqueou...