Kinetic study of the tocopherol regeneration reaction by biological hydroquinones in micellar solution

The rate constants (kr) of the regeneration reaction of 7-t-butyl-5-isopropyltocopheroxyl with ubiquinol-10 (UQ10H2), ubiquinol-0 (UQ0H2), alpha-, beta-, and gamma-tocopherolhydroquinones (alpha-, beta-, gamma-TQH2), 2,3,5-trimethyl-1,4-hydroquinone (TMQH2), vitamin K3 hydroquinone (VK3H2), and vitamin C (Vit C) have been measured in 2-propanol/water and micellar solutions by a stopped-flow spectrophotometer. The kr values of these hydroquinones (HQs) in micellar solution remained constant at pHs of 6-9 and increased rapidly by increasing the pH value. The kr values decreased in the order of VK3H2 > gamma-TQH2 > or = alpha-TQH2 > beta-TQH2 > or = UQ10H2 > or = TMQH2 > UQ0H2 >> Vit C at pHs of 6-9. These HQs are dibasic acids and can exist in three different molecular forms, depending on pH. By comparing the kr values with the mole fraction of each molecular form of the HQs, the reaction rate kr1 for the undissociated form, kr2 for the monoanion, and kr3 for the dianion and the pKa1 and pKa2 values were determined. It has been found that the kr values of UQ10H2, alpha-TQH2, beta-TQH2, and gamma-TQH2 (plastoquinol model) are 460, 1430, 494, and 1530 times larger than that of Vit C at pH 7.0, respectively, although the values are similar to that of Vit C in 2-propanol/water. The biological HQs and Vit C coexist in many tissues of animals and plants, and thus, the relative antioxidant activities of HQs and Vit C have been tentatively discussed based on the products of kr values by concentrations in several tissues. The results suggest that these HQs show high activity for the tocopherol regeneration in biological systems.