RAPID DIFFUSION OF THE LIPID PHOSPHORUS OF PHOSPHATIDYLGLYCEROL LIPOSOMES THROUGH POLYCARBONATE MEMBRANES IS CAUSED BY THE OXIDATION OF THEUNSATURATED FATTY-ACIDS

The lipid phosphorus of phosphatidylglycerol liposomes was found to di ffuse extensively, after a lag time of 1 to 2 days, through a 0.1 mu m pore size polycarbonate membrane in a two compartment system. Diffusi on occurred when either multilamellar or large unilamellar vesicles we re studied, even if they were sedimented to eliminate any smaller part icles. The lipid of liposomes prepared under sterile conditions also d iffused extensively. Diffusion appeared to be related to the age of th e vesicles, and could be eliminated by incorporating antioxidants into the liposomes, or by using liposomes prepared from saturated phosphol ipids (C14 or larger). This indicated that diffusion was caused by pho spholipid oxidation, which was confirmed by HPLC analysis. Phospholipi d phosphorus that diffused through a membrane appeared more polar, as indicated by its capacity to distribute into the upper phase of a two phase extraction. Phospholipid phosphorus diffusion was preceded by th e complete loss of liposomes contents, indicated by the complete diffu sion of encapsulated carboxyfluorescein through the membrane. Oxidatio n of the lipid could be prevented by inclusion of either butylated hyd roxytoluene or alpha-tocopherol in the membrane, The best retention of liposomal contents was achieved when both antioxidants and cholestero l were included in the liposome preparation. The antioxidant incorpora ted in the liposomes remained effective in protecting the phospholipid s upon storage at 4 degrees C for 2 months. The inclusion of EDTA in t he suspension medium retarded the rapid oxidation, suggesting that the presence of trace amounts of heavy metal ions in the buffer catalyzed the oxidation. Phospholipid oxidation was most effectively inhibited by the presence of serum or chemically defined medium, suggesting that oxidation of liposomal lipids in a biological environment may be mini mized if appropriate steps are taken.