INTERACTION OF THE CALCIUM-ANTAGONIST FANTOFARONE WITH PHOSPHOLIPIDS - ELECTROSTATIC EFFECTS

The binding of fantofarone, a novel calcium channel antagonist, to cyt oplasmic membranes and lipid vesicles has been studied by means of its fluorescence. The binding characteristics (dissociation constant K-d and total number of binding sites B-max) were determined using saturat ion isotherms. In brain synaptic and cardiac sarcolemmal membranes, fa ntofarone binds to a single site with a K-d value of approximate to 1. 4 x 10(-6) M and a B-max value of approximate to 13 fantofarone molecu les bound per 100 lipid molecules. Using vesicles made from egg phosph atidylcholine (PC), fantofarone was shown to possess a K-d approximate to 7.5 x 10(-6) M and a B-max approximate to 15. When other classes o f naturally-found lipids were incorporated into PC vesicles, a decreas e in K-d with no modification in B-max was observed for all acidic lip ids studied. The decrease in K-d was inhibited by sodium and calcium. None of the experimental conditions modified the spectral properties o r lifetimes of fantofarone. We conclude that an electrostatic interact ion between fantofarone and negatively charged lipids takes place at t he surface of the membrane and this interaction explains the decreased K-d value (increase in affinity) observed in cytoplasmic membranes.