Determination of alpha-tocopherolquinone (vitamin E quinone) in human serum, platelets, and red cell membrane samples

A high-performance liquid chromatographic method for the determination of alpha-tocopherolquinone in a few selected biological samples is reported. Samples of human serum, blood platelets, or red cell membranes were saponified and extracted with hexane. A measured aliquot of the extract was evaporated under a stream of nitrogen, and the residue was reconstituted with mobile phase (methanol:water, 98:2) and used directly for liquid chromatography. alpha-Tocopherolquinone was separated on Zorbax C-18 columns (25 cm X 4.6 mm, 5-microns particles) and detected by its absorption at 265 nm. The addition of high levels of base during saponification as well as exposure to fluorescent light results in loss of the quinone. Concentrations of alpha-tocopherolquinone in normal human serum are exceedingly small constituting only 0.02-0.05% of the alpha-tocopherol concentration. The technique is particularly useful in the quantitation of the oxidation of alpha-tocopherol in biological samples under in vitro conditions. For example, incubation of human platelets with diamide or arachidonate resulted in oxidation of alpha-tocopherol and the alpha-tocopherolquinone produced accounted for 11.8 and 30.6%, respectively of the alpha-tocopherol lost.