Interaction of lucensomycin with cholesterol in membranes: kinetic and structural studies

The variations of optical density and fluorescence of lucensomycin are good indices of the binding of this polyenic antibiotic to membranes. The former parameter reflects more generally the binding to any site present in the membrane, while the latter is more specific for binding to cholesterol. Equilibrium titration experiments performed in the presence of an excess of membrane-bound cholesterol suggest that lucensomycin self-associates and that the binding and polymerization sites of the antibiotic are identical or quasi-identical; hence polymerization leads to a loss of binding sites and vice versa. The non-steroidal components of the membrane (such as proteins and lipids) seem to affect the rate of the individual reaction leading to the formation of the cholesterol-lucensomycin complexes, rather than the ratio among these heterologous aggregates at equilibrium. Polyene concentrations, which induce detectable proton release from unilamellar vesicles, are at least two orders of magnitude higher than those necessary to perform a spectroscopic titration of membrane cholesterol.