Binding of propranolol and gentamicin to small unimellar phospholipid vesicles: Contribution of ionic and hydrophobic forces

Abstract

Binding of propranolol and gentamicin to small unilamellar phospholipid vesicles having different surface charges was studied at pH 4.4 using an ultra-centrifugation method, and the results were analyzed by an equation describing the Langmuir adsorption isotherms. Gentamicin, a polycationic drug, bound to negatively-charged small unilamellar vesicles composed of 60% phosphatidylcholine and 40% of either phosphatidylinositol, phosphatidylglycerol or phosphatidylserine in a manner consistent with a single class of binding sites but did not bind at all to small unilamellar vesicles of phosphatidylcholine alone. In contrast, propranolol bound readily to both neutral and negatively-charged liposomes in a manner consistent with two types of binding sites. Based on the binding parameters calculated from replots, it is suggested that the high-affinity site is probably at the surface of the liposome and that ionic forces are primarily responsible for this binding. The low-affinity, high-capacity binding site for propranolol was demonstrated with both neutral and negatively-charged liposomes and appeared to be independent of the surface charge. Gentamicin, which is not hydrophobic, did not bind to the low-affinity site. It is hypothesized that hydrophobic interactions are the driving force for propranolol binding to the low-affinity site which may be the interior of the lipid bilayer.