Oxidation of α-tocopherol model compound by superoxide anion

Abstract

6-Hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid, a model for α-tocopherol, was found to be oxidized by a xanthine-xanthine oxidase system. This reaction was found to be completely inhibited by the addition of superoxide dismutase, while catalase had no effect, indicating that the oxidation is caused by superoxide anion. The product of the oxidation was identified as 2-hydroxy-2-methyl-4-(3,4,6-trimethylbenzoquinone-2-yl) butanoic acid. An intermediate, which has properties of 8a-hydroxy-2,5,7,8-tetramethyl-chroman-6-one-2-carboxylic acid, was observed spectrophotometrically prior to the formation of the quinone. From an analysis of the inhibition of the rate of reaction by superoxide dismutase, the second-order rate constant for the reaction between 6-hydroxy-2, 5,7,8-tetramethylchroman-2-carboxylic acid and superoxide anion was estimated to be 1.7 × 10 4 m −1 s −1. This value is fairly small compared to the second-order rate constant for the reaction between superoxide dismutase and superoxide anion. However, in view of the localization of α-tocopherol in subcellular membranes, it seems possible that this vitamin scavenges the superoxide anion which may be generated by membrane-bound enzymes participating in the biological oxidation.