Liposomal membranes. XX. Autoxidation of unsaturated fatty acids in liposomal membranes.

Autoxidation of egg phosphatidylcholines and polyunsaturated fatty acids, arachidonic and linoleic acids, has been investigated in homogeneous and liposomal membrane systems. In order to monitor the very initial stage of the radical chain mechanism of the autoxidation, an improved method using 1,1-diphenyl-2-picrylhydrazyl radical was newly implemented and used with the regular thiobarbituric acid test to determine the peroxides of polyunsaturated fatty acids. Autoxidation of polyunsaturated fatty acids or lipids was significantly enhanced in liposomal bilayers compared to that in a bulk homogeneous solution. In liposomal bilayers, the reaction could be controlled by membrane fluidity, which was confirmed by the fluorescence polarization technique using 1,6-diphenylhexatriene and dansylhexadecylamine. Even vitamin E esters such as the acetate and the pivalate could depress effectively the autoxidation of egg phosphatidylcholines in bilayer systems, which supported Lucy's proposal (Diplock, A.T. and Lucy, J.A. (1973) FEBS Lett. 29, 205-210) about the importance of the side-chain of vitamin E as an antioxidant.