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A sensitive procedure is described for the simul- taneous determination of vitamin E and coenzyme Q homo- logues and a -tocopherol oxidation products using two- isocratic step high pressure liquid chromatography (HPLC) and electrochemical detection in the oxidative mode. Zinc- catalyzed reduction in a post-column reactor allows the de- tection of a -tocopherolquinone, epoxy-tocopherolquinone, and ubiquinones. This technique was used to quantify lipo- philic antioxidants in the liver tissue of rats treated or not with a -tocopherolquinone and in a plant oil. a -Toco- pherolquinone and its epoxide derivatives, formed from a - tocopherol during iron-catalyzed phospholipid peroxida- tion, were also determined in a liposome suspension. The high selectivity and sensitivity of the coulometric detection system enabled use of low oxidation potentials giving little baseline noise, while a fast isolation procedure and quanti- tative recoveries of all oxidized and reduced forms made it possible to measure a high ubiquinol/ubiquinone ratio in liver tissue. Administration of a -tocopherolquinone to rats did not alter the antioxidant status of the liver, despite strong accumulation of both this quinone and its reduced form, a -tocopherolhydroquinone. These results indicate the presence of an efficient reductase and suggest that it could contribute to the protection of cellular membranes from oxidative stress.— Leray, C., M. D. Andriamampandry, M. Freund, C. Gachet, and J-P. Cazenave. Simultaneous de- termination of homologues of vitamin E and coenzyme Q and products of a -tocopherol oxidation. J. Lipid Res. 1998. 39: 2099-2105.