Comparison between the free-radical-scavenging activities with vitamin E and ubiquinol in biological systems based on their reaction rates: A research account

Detailed kinetic studies have been performed for the reaction of aroxyl (ArO•) radical with vitamin E (α-, β-, γ-, δ-tocopherol, TocH), ubiquinol-10, and related antioxidants in micellar solution, using a stopped-flow spectrophotometer. The second-order reaction rates (κ_s) obtained increased in the order of hydroquinone<tocol<δ-TocH<ubiquinol-0<γ-TocH˜ β- TocH<ubiquinol-10< α-TocH at pH 4 ˜ 8. The antioxidants which have lower oxidation potentials (E_p) showed higher reactivities. The reaction rates obtained in micellar solution were pH dependent because of the dissociation of OH groups in the antioxidants. For instance, by comparing the κ_s1 values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate κ_s1 (1.21 ×10⁵ M^-1s^-1) for undissociated form, κ_s2 (1.04 × 10⁶ M^-1s^-1) for monoanion and κ_s3 (0 M^-1s^-1) for dianion, and the pK_α1 and pK_α2 values (11.4 and 12.7) were determined. It was found that the relative ratio of κ_s values (100:21:20:2.9) of α-, β-, γ-, δ-tocopherols in micellar dispersion has good correlation with the relative biological activities for rat fetal resorption, rat haemolysis, and chicken muscle dystrophy. The relative antioxidant activities of α-tocopherol and ubiquinol-10 have been discussed on the basis of the products of the κ_s values and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).