Résumé : Aminoglycoside antibiotics such as gentamicin, which are fully hydrophilic, and cationic amphiphilic drugs such as bis(beta-diethylaminoethylether)hexestrol (DEH), are both known to inhibit lysosomal phospholipases and induce phospholipidosis. This enzymatic inhibition is probably related to the neutralization of the surface negative charges on which the lysosomal phospholipases A1 and A2 are dependent to express fully their activities (Mingeot-Leclerq et al., Biochem Pharmacol 37: 591-599, 1988). Using negatively charged liposomes, we show by 31P NMR spectroscopy that both gentamicin and DEH cause a significant restriction in the phosphate head mobility and, in sonicated vesicles, the appearance of larger bilayer structures. Both DEH and gentamicin increased the apparent size of sonicated negatively charged liposomes (but not of neutral liposomes) as measured by quasi-elastic light scattering spectroscopy. Examination of replicas from freeze-etched samples, however, revealed that gentamicin caused aggregation of liposomes, whereas DEH induced their fusion and the formation of intramembranous roundly shaped structures. Only DEH caused a significant decrease of the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, a fluorescent lipid-soluble probe. In addition, DEH, but not gentamicin, interfered with the bilayer to hexagonal phase transition occurring in dioleoyl- and dielaidoylphosphatidylethanolamine liposomes upon warming, and caused the appearance of an isotropic signal suggestive of the formation of inverted micelles. In computer-aided conformational analysis of the molecules at a simulated air-water interface, gentamicin was shown to display a largely-open crescent shape. When surrounded by phosphatidylinositol molecules, it remained as such at the interface which it locally mis-shaped, establishing close contact with the negatively charged phospho groups. In contrast, DEH could be oriented perpendicularly to the interface, with its two cationic groups associated with the phospho groups, and its phenyl- and diethylethandiyl moieties deeply inserted between and interacting with the aliphatic chains. Thus, although both agents cause lysosomal phospholipases inhibition, the differences in their interactions with negatively-charged bilayers is likely to result in a different organization of the phospholipids accumulated in vivo, which could lead to different toxicities.