pH Lowering in Liposomal Dispersiond Induced by Phospholipid Peroxidation

Liposomes were prepared from egg phosphatidylcholine (PC) for investigation of their long-term stability. When egg PC liposomes were incubated at 40°C, lowering of the pH was observed with a corresponding change in appearance. This pH lowering was suggested to be induced by peroxidation of the unsaturated fatty acyl chains in egg PC because : 1) the pH lowering was prevented by incorporation of α-tocopherol into liposomes or bubbling nitrogen into the liposomal dispersion at neutral pH; 2) the liposomes prepared from completely hydrogenated soya PC (CH-Soya PC) showed no change in pH under the same conditions. The relationship between the pH lowering and lipid peroxidation was also supported by the formation of thiobarbituric acid-reactive substances (TBA-RS) and the accumulation of carbonyl compounds in liposomes. The amount of TBA-RS reached a maximum after about 5 days' storage and then decreased gradually, while the pH lowering and production of carbonyl compounds in the liposomal dispersion continued for 20d. These results indicate that production of TBA-RS does not directly correlate with pH lowering in the liposomal dispersion; rather TBA-RS might be decomposed into secondary peroxidation products, such as carbonyl compounds, which may affect the pH of the liposomal dispersion.