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Binding and transport of calcium by synthetic analogues of ionomycin Hu, Thomas Q.

Abstract

The syntheses of simple analogues of ionomycin (3), namely a series of f3-diketoco-carboxylic acids 4-6, 28-30 and 33-35, were achieved by consecutive alkylation of the dianion of 2,4-pentanedione (7) with appropriate bromides and subsequent oxidation of theP-diketo tau-alcohols. The transport of calcium across an organic barrier by these synthetic analogues of ionomycin was determined in a cylindrical glass cell using chloroform as the artificial membrane. The presence of the P-diketone and carboxyl groups within the same molecule and a sufficient lipid solubility of the compounds were shown to be necessary and sufficient conditions for calcium transport. Small and no transport of calcium was found for analogue 4 and 6 respectively due to the low lipid solubility of these compounds. The relative calcium transport rates for analogues 28 and 33-35 were 28 > 33 > 35 > 34, which demonstrated that optimum calcium transport was achieved when the P-diketone group was separated from the carboxyl group byseven methylene units identical to the backbone found in ionomycin. Analogues 28-30 were comparable to ionomycin and calcimycin in terms of calcium transport. The transport of calcium by synthetic analogues of ionomycin was found to be a saturable process which obeyed Michaelis-Menten kinetics. It was dependent of the pH in the aqueous source phase and independent of the pH in the receiving phase. Both the carboxyl and the 13-diketone groups were ionized in the transport of calcium, indicating the stoichiometry of calcium complex in transport was 1:1. Analogues 29 and 33 were found to cause transient calcium mobilization in cultured human leukemic cells. The pKa of the 0-diketone group of the analogues was determined to be in the range of 10.90 to 11.16 in 80% Me0H-H20 by UV spectrophotometric titration. The pKa values were 0 0 0^HO OH OH 3 HO directly related to the lipid solubility of the compounds and the hydrocarbon chain length between the 13-diketone and the carboxyl groups. The binding constants of the analogues with calcium and magnesium in 80% Me0H-H20were determined to be in the order of 102 M-1 and 103 M-1 respectively using the pKa method. The binding constants of the analogues with magnesium were also determined by the mole ratio method which established the 1:1 stoichiometry in the magnesium complex. The selectivity in binding was the same as the selectivity in transport, which was Mg2+ > Ca2+ >> Nat, K+. Dimethylation of the 13-diketone 42 proceeded with high diastereoselectivity. The major diastereomer (43b) had the two methyl groups trans to each other. The characterization of the calcium salt of analogue 28 showed that the stoichiometry of the calcium complex in the solid state was 1:1.

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