Peroxyl radical trapping and autoxidation reactions of alpha-tocopherol in lipid bilayers.

A phospholipid liposome system was employed to model peroxyl radical trapping reactions of alpha-tocopherol (1) in biological membranes. Peroxyl radicals generated by thermolysis of 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN) at 37 degrees C oxidized 1 to 8a-[(2,4-dimethyl-1-nitrilopent-2-yl)dioxy]tocopherone (3a), 8a-(hydroperoxy)tocopherone (3b), alpha-tocopherol quinone (4), 4a,5-epoxy-8a-hydroperoxytocopherone (6), 2,3-epoxy-alpha-tocopherol quinone (7), and 5,6-epoxy-alpha-tocopherol quinone (8). The products were purified by high-performance liquid chromatography and characterized by UV-vis spectroscopy, mass spectrometry, and cochromatography with authentic standards. Products accumulated in approximately constant proportion as 1 was consumed. Tocopherones 3a/3b decomposed in the bilayer primarily by hydrolyzing to produce 4. Tocopherone decomposition also produced small amounts of epoxides 6-8, apparently by unimolecular tocopherone decomposition rather than by peroxyl radical dependent oxidation, since neither AMVN nor 1 affected the rate of 3a loss or the distribution of products. Epoxides 6-8 appear to be formed primarily by autoxidation reactions that compete with the peroxyl radical trapping reactions that form tocopherone 3a. Epoxide products may thus serve as biochemical markers for irreversible oxidation of 1 by peroxyl radicals in membranes.