Stopped - flow Kinetic Study of Vitamin E Regeneration Reaction with Biological Hydroquinones ( Reduced Forms of Ubiquinone , Vitamin K , and Tocopherolquinone ) in

A kinetic study of the regeneration reaction of vitamin E (tocopherol) with eight biological hydroquinones (HQs) (ubiquinol-10 (QloH2 1); ubiquinol-0 (QoH2 2); vitamin K1 HQ (VKIH2 3); vitamin Kg HQ (VKSHZ 4); a-, 8-, and y-tocopherol-HQs (a-, B-, and r-TQH2 B-z); and 2,3,5-trimethyl-1,4-HQ (TMQH2 S)) in solution was performed. The second-order rate constants (k4) for the reaction of HQs 1-S with a-tocopheroxyl and 5,7-diisopropyltocopheroxyl radicals in ethanol, benzene, and isopropyl alcohol/water (5:1, v/v) solutions were measured with a stopped-flow spectrophotometer. The order of magnitude of k4 values obtained for HQs is VKlH2 > VK3H2 > a-TQHz > B-TQHZ r-TQH, TMQH2 > Q10H2 > QoH2, being independent of the kinds of tocopheroxyl radicals and the polarity of the solvents. The log of the k4 values obtained for HQs was found to correlate with their peak oxidation pote tials. Comparing the k, value (2.68 X lo6 M” s-l obtained for the reaction of a-tocopheroxyl with vitamin C (sodium ascorbate) with those (k4 = 2.54 x lo5 and 8.15 X 10‘ M” s-’) obtained for the reaction of a-tocopheroxy1 with Q10H2 and (r-TQH2 in isopropyl alcohol/water mixtures, the former is 11 and 3 times as reactive as the latter, respectively. On the other hand, the kz value obtained for sodium ascorbate is smaller than the k4 values obtained for VKIH, and VK3H2. These results suggest that mixtures of vitamin E and these HQs (as well as those of vitamins E and C) may function synergistically as antioxidants in various tissues and mitochondria.