ANTIOXIDANT STOICHIOMETRY AND THE OXIDATIVE FATE OF VITAMIN-E IN PEROXYL RADICAL SCAVENGING REACTIONS

Oxidation of R,R,R-alpha-tocopherol (vitamin E; TH) by peroxyl radical s generated from the azo initiator azobis(2,4-dimethylvaleronitrile) i n acetonitrile, hexane, or in phospholipid liposomes yields 8a-(alkyld ioxy)tocopherone adducts, 8a-(hydroxy)tocopherone, and their hydrolysi s product alpha-tocopherolquinone. TH oxidation also yields 4a,5-epoxy - and 7,8-epoxy-8a-(hydroperoxy)tocopherones and their respective hydr olysis products 2,3-epoxy-alpha-tocopherolquinone and 5,6-epoxy-alpha- tocopherolquinone. Previous work indicates that the distribution of TH oxidation products varies with reaction environment. We investigated the dependence of antioxidant stoi chiometry on TH oxidation product d istribution for reactions in hexane, acetonitrile, and in phosphatidyl choline liposomes. Yields of 8a-substituted tocopherones were highest in hexane and lowest in phosphatidylcholine liposomes. In contrast, yi elds of epoxide products were highest in the liposome system and lowes t in hexane. Yields of alpha-tocopherolquinone were similar in all thr ee systems. Antioxidant stoichiometry, measured by the inhibited autox idation method, was approximately 2.0 peroxyl radicals trapped per TH consumed in acetonitrile and in liposomes. In hexane, a slightly large r stoichiometric factor of approximately 2.5 was measured. This may, i n part, reflect the generation of more reactive alkoxyl radicals in he xane. The reaction environment thus markedly affects the balance betwe en competing TH oxidation pathways but produces comparatively little e ffect on antioxidant stoichiometry. These results imply that competing reaction pathways contribute similarly to the antioxidant chemistry o f TH.