Calcium transport by a beta-diketone in model membranes

The beta-diketone 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione (FOD) translocates calcium from an aqueous medium into an organic phase. FOD is less efficient than but acts synergistically with A23187 in causing calcium translocation. The FOD-mediated process of calcium translocation is inhibited by NaCl, although the translocation of sodium by FOD is two to three orders of magnitude lower than that of calcium, when expressed relative to the concentration of these cations in the aqueous medium. At pH 7.4, FOD mediates calcium exchange-diffusion in fluid liposomes as efficiently as A23187. The extent of exchange-diffusion depends on the rigidity and cholesterol content of the liposomes. Conformational analysis of the complex formed by two molecules of FOD and one calcium atom at a simulated membrane interface reveals the existence of several interconvertible, asymmetrical and more-or-less planar configurations. The efficiency of FOD-mediated calcium ionophoresis thus appears to be regulated in a multifactorial manner by such factors as the concentration of calcium and monovalent cations, chemical composition and fluidity of the membrane, availability of other ionophoretic molecules and spatial configuration of the calcium complex.