Interactions of neomycin with monomolecular films of polyphosphoinositides and other lipids

The interactions of calcium and the aminoglycosidic antibiotic, neomycin, with various lipids were investigated in monomolecular films. Lipids were spread over a subphase of 0.05 M N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, pH 7.0, and NaCl to give an ionic strength of 0.2. Measurements of surface pressure (pi) were taken with a Wilhelmy balance. In the absence of Ca2+, 1 muM--1 mM neomycin in the subphase decreased pi (i.e. condensed films) of all acidic lipids tested. In the presence of 1 mM Ca2+, neomycin did not change pi of films of phosphatidylserine, phosphatidylinositol and phosphatidic acid while it lowered pi of cardiolipin and cerebroside sulfate films. A unique pattern of interaction was observed with polyphosphoinositide monolayers. In the absence of Ca2+, 1 muM neomycin decreased pi followed by an increase of pi at higher neomycin concentrations. Ca2+ (1 mM) condensed the film significantly more than did neomycin. However, as little as 1 muM neomycin induced expansion of the calcium/lipid film which at 1 mM neomycin reached the same pi as in the absence of Ca2+. Such expansion was observed at all pressures of the film including the collapse pressure indicating a strong 'complex' between the drug and polyphosphoinositide not antagonized by Ca2+. In the absence of possible hydrophobic interactions, both the condensation and the expansion of the film should be mediated by ionic forces. Combined in vivo and in vitro evidence is discussed to suggest the polyphosphoinositides as the physiological receptors for aminoglycosides in the mammalian cell membrane.