Stopped-flow kinetic study of the regeneration reaction of tocopheroxyl radical by reduced ubiquinone-10 in solution.

Kinetic study of the reaction between tocopheroxyl (vitamin E radical) and ubiquinol-10 (reduced ubiquinone, n = 10) has been performed. The rates of reaction of ubiquinol with alpha-tocopheroxyl 1 and seven kinds of alkyl substituted tocopheroxyl radicals 2-8 in solution have been determined spectrophotometrically, using a stopped-flow technique. The result shows that the rate constants decrease as the total electron-donating capacity of the alkyl substituents on the aromatic ring of tocopheroxyls increases. For the tocopheroxyls with two alkyl substituents at ortho positions (C-5 and C-7), the second-order rate constants, k1, obtained vary in the order of 10(2), and decrease predominantly, as the size of two ortho-alkyl groups (methyl, ethyl, isopropyl and tert-butyl) in tocopheroxyl increases. On the other hand, the reaction between tocopheroxyl and ubiquinone-10 (oxidized ubiquinone) has not been observed. The result indicates that ubiquinol-10 regenerates tocopherol by donating a hydrogen atom of the 1-OH and/or 4-OH group to the tocopheroxyl radical. For instance, the k1 values obtained for alpha-tocopheroxyl are 3.74 x 10(5) M-1.s-1 and 2.15 x 10(5) M-1.s-1 in benzene and ethanol solution at 25 degrees C, respectively. The above reaction rates, k1, obtained were compared with those of vitamin C with alpha-tocopheroxyl reported by Packer et al. (k2 = 1.55 x 10(6) M-1.s-1) and Scarpa et al. (k2 = 2 x 10(5) M-1.s-1), which is well known as a usual regeneration reaction of tocopheroxyl in biomembrane systems. The result suggests that ubiquinol-10 also regenerates the tocopheroxyl to tocopherol and prevents lipid peroxidation in various tissues and mitochondria.