Kinetic study of free-radical-scavenging action of biological hydroquinones (reduced forms of ubiquinone, vitamin K and tocopherol quinone) in solution

Abstract

Kinetic study of free-radical-scavenging (FRS) action of eight kinds of biologically important hydroquinones (HQ's) (ubiquinol-10 (UQ 10H 2 1), ubiquinol-0 (UQ 0H 2 2), vitamin K 1 HQ (VK 1H 2 3, vitamin K 3 HQ (VK 3H 2 4), α-, β-, γ-tocopherol HQ's (α-, β-, γ-TQH 2 5, 6, 7), and 2,3,5-trimethyl-1,4-HQ (TMQH 2 8)) has been performed. The second-order rate constants, k 3, for the reaction of HQ's 1 ∼ 8 with substituted phenoxyl radical (PhO·) in ethanol, diethyl ether, benzene, and n-hexane have been measured with a stopped-flow spectrophotometer, as a model reaction of HQ's with unstable free radicals (LOO·, LO·, and HO·) in biological systems. The rate constant of UQ 10H 2 1 is similar to that of α-tocopherol in ethanol. The HQ's 3–8 showed higher reactivity than α-tocopherol in ethanol. Especially, the rate constants of VK 1H 2 3 and VK 3H 2 4 were founf to be 31- and 21-fold larger than that of α-tocopherol, respectively, which has the highest reactivity among natural tocopherols. The rate constant of these HQ's increased by decreasing the polarity of solvents. The approximate order of magnitude of k 3 value was (i) VK 1H 2 and VK 3 PrmH 2 > (ii) α-, β- and γ-TQH 2's and TMQH 2 > (iii) α-tocopherol > (iv) UQ 10H 2 and UQ 0H 2 in solution. The result suggests that these biological HQ's also scavenge the active oxygen free radicals and prevent lipid peroxidation in various tissues and membranes. On the other hand, the reaction between substituted phenoxyl and biological quinones has not been observed.