PH DEPENDENT BINDING OF LIGANDS TO SERUM-LIPOPROTEINS

Purpose. The binding interactions of binedaline, nicardipine and darod ipine with lipoproteins (HDL, LDL, VLDL) were examined as a function o f pH in order to evaluate the role of lipoprotein components and ligan d protonation in the binding process. Methods. Binding studies were pe rformed by equilibrium dialysis with radiolabeled ligands and differen tial UV-visible spectroscopy. Results. Deprotonated ligands had a mark edly higher affinity for lipoproteins than the protonated forms, resul ting in a concomitant decrease in the pK(a) of bound ligands, i.e., a decrease in the basicity of the ligand in the bound state. The UV-visi ble difference spectra generated upon binding of auramine O to lipopro teins also showed that there was a contribution to the binding arising from the deprotonation of the ligand. Ligand binding was related to t he phospholipid and cholesteryl ester content and to a lesser degree t o the free cholesterol and protein content of lipoproteins, therefore to the surface monolayer components of lipoproteins. This relationship was even more accurate for the deprotonated, high-affinity, than for the protonated species. Conclusions, It is suggested that among other possible interactions, ligand binding to lipoproteins involves proton exchange between the reactants and that the high affinity ligand speci es interact more specifically with the phospholipids of the lipoprotei n surface monolayer.