Analysis of vitamin E and its oxidation products by HPLC with electrochemical detection.

A sensitive HPLC procedure with postcolumn reduction and electrochemical detection was developed for the analysis of vitamin E and its oxidation products, alpha-tocopherylquinone, epoxy-alpha-tocopherylquinones, and 8a-(lipid-dioxy)-alpha-tocopherones. After the separation on a reversed-phase column, on-line zinc-catalyzed reduction allowed the detection of alpha-tocopherylquinone and epoxy-alpha-tocopheryl-quinones, whereas platinum-catalyzed reduction allowed the detection of 8a-(lipid-dioxy)-alpha-tocopherones. The lowest detectable level of each compound was about 0.2 pmol at the signal-to-noise ratio of 3. This method was applied to the detection of alpha-tocopherol products in peroxidized human plasma. When the plasma was peroxidized by the addition of a free radical initiator, peaks corresponding to alpha-tocopherylquinone, epoxy-alpha-tocopherylquinones, and the addition products of alpha-tocopherol with peroxyl radicals derived from cholesteryl ester hydroperoxides and PC hydroperoxides were observed. The amount of these oxidation products in the plasma increased with the depletion of endogenous alpha-tocopherol. The results indicate that the method is useful to detect the oxidation products formed by the peroxyl radical-trapping reactions of alpha-tocopherol in biological systems.