INTERACTIONS OF LIPOSOME BILAYERS COMPOSED OF 1,2-DIACYL-3-SUCCINYLGLYCEROL WITH PROTONS AND DIVALENT-CATIONS

Bilayer liposomes were prepared by using pure DOSG (1,2-dioleoyl-3-suc cinylglycerol) or DPSG (1,2-dipalmitoyl-3-succinylglycerol) at pH 7.4 or above. These liposomes undergo destabilization upon incubation with acid, When calcein was used as an entrapped aqueous marker, half maxi mal content leakage was observed between pH 5.8-6.3. Differential scan ning calorimetry showed that at pH 7.4, the chain-melting temperature (T-m) of DPSG was 60.4 degrees C, and increased with decreasing pH (T- m = 57.0 degrees C and 62.7 degrees C at pH 8.9 and 6.7, respectively) . Below pH 6.7, extensive phase separation occurred as the major chain melting peak split into three peaks. These three peaks coalesced into one peak below pH 5. Freeze fracture electron micrographs of DOSG lip osomes at pH 4 showed the formation of non-bilayer as well as hexagona l phase structures. The effects of divalent cations, such as Ca2+ and Mg2+, On the destabilization of DASG bilayers have also been studied. Differential scanning calorimetry studies of bilayers composed of DPSG showed that both Ca2+ and Mg2+ could increase the T-m of DPSG with in creasing concentrations. However, under identical conditions Mg2+ was more effective than Ca2+ in increasing the T-m of DPSG. X-ray diffract ion indicated that both Ca2+ and Mg2+ could induce DPSG bilayers to un dergo a complete lamellar to hexagonal phase transition. There was a s ize-dependency on the plasma stability of DOSG liposomes. DOSG liposom es that were smaller in size were more stable in plasma than the large r ones. After incubation with plasma, DOSG liposomes became less acid- sensitive. DOSG immunoliposomes entrapping diphtheria toxin A chain we re used as a model for cytoplasmic delivery of the novel pH-sensitive liposomes. The delivery activity was comparable to that of the convent ional pH-sensitive liposomes containing unsaturated phosphatidylethano lamine. Our data indicate that the mechanism of liposome destabilizati on involves extensive bilayer phase separation as well as the formatio n of non-bilayer structures.