Interaction of the calcium antagonist fantofarone with phospholipids: electrostatic effects

The binding of fantofarone, a novel calcium channel antagonist, to cytoplasmic membranes and lipid vesicles has been studied by means of its fluorescence. The binding characteristics (dissociation constant Kd and total number of binding sites Bmax) were determined using saturation isotherms. In brain synaptic and cardiac sarcolemmal membranes, fantofarone binds to a single site with a Kd value of approximately 1.4 x 10(-6) M and a Bmax value of approximately 13 fantofarone molecules bound per 100 lipid molecules. Using vesicles made from egg phosphatidylcholine (PC), fantofarone was shown to possess a Kd approximately 7.5 x 10(-6) M and a Bmax approximately 15. When other classes of naturally-found lipids were incorporated into PC vesicles, a decrease in Kd with no modification in Bmax was observed for all acidic lipids studied. The decrease in Kd was inhibited by sodium and calcium. None of the experimental conditions modified the spectral properties or lifetimes of fantofarone. We conclude that an electrostatic interaction between fantofarone and negatively charged lipids takes place at the surface of the membrane and this interaction explains the decreased Kd value (increase in affinity) observed in cytoplasmic membranes.