Proton and divalent cations induce synergistic but mechanistically different destabilizations of pH-sensitive liposomes composed of dioleoyl phosphatidylethanolamine and oleic acid.

Protons and divalent cations show synergistic effects on the destabilization of liposomes composed of unsaturated phosphatidylethanolamine and oleic acid (Düzgünes et al., Biochemistry (1985) 24, 3091). We have extended these observations and investigated the effects of Ca2+ and Mg2+ on the proton-induced destabilization of dioleoyl phosphatidylethanolamine/oleic acid (DOPE/OA) (4:1 molar ratio) liposomes. Temperature-induced aggregation was measured by 90 degrees light scattering. Lipid mixing was used to monitor vesicle destabilization and freeze-fracture electron microscopy was used to examine the structures formed from DOPE/OA vesicles in the presence of Ca2+ and/or protons. Both Mg2+ and Ca2+ shift the pH required for 50% lipid mixing to higher values. Temperature-induced vesicle aggregation occurs at lower temperatures in the presence of divalent cations and/or protons, indicating that intervesicular repulsions are decreased. Freeze-fracture electron micrographs show that the structures formed from DOPE/OA in the presence of Ca2+ differ significantly from those found in the presence of protons. In general, protons induce the formation of hexagonal phase, while the presence of Ca2+ leads to the formation of extensive regions of lamellar sheets with numerous lipidic particles. The synergistic effect of divalent cations and proton may be important for the maximal biological activity of DOPE/OA liposomes.