Aroxyl-Radical-Scavenging Rate Increases Remarkably under the Coexistence of α-Tocopherol and Ubiquinol-10 (or Vitamin C): Finding of Synergistic Effect on the Reaction Rate

Abstract

Measurements of aroxyl radical (ArO(•))-scavenging rate constants (ks(AOH)) of antioxidants (AOHs) (α-tocopherol (α-TocH), ubiquinol-10 (UQ10H2), and sodium ascorbate (Na(+)AsH(-))) were performed in 2-propanol/water (2-PrOH/H2O, 5/1, v/v) solution using stopped-flow spectrophotometry. ks(AOH) values were measured not only for each AOH but also for the mixtures of two AOHs ((i) α-TocH and UQ10H2 and (ii) α-TocH and Na(+)AsH(-)). A notable synergistic effect that the ks(AOH) values increase 1.6, 2.5, and 6.8 times for α-TocH, UQ10H2, and Na(+)AsH(-), respectively, was observed for the solutions including two kinds of AOHs. Furthermore, measurements of the regeneration rates of α-tocopheroxyl radical (α-Toc(•)) to α-TocH by UQ10H2 and Na(+)AsH(-) were performed in 2-PrOH/H2O using double-mixing stopped-flow spectrophotometry. Second-order rate constants (kr) obtained for UQ10H2 and Na(+)AsH(-) were 2.01 × 10(5) and 1.19 × 10(6) M(-1) s(-1), respectively. In fact, UV-vis absorption of α-Toc(•) (λmax = 428 nm), which had been produced by reaction of α-TocH with ArO(•), disappeared under the existence of UQ10H2 or Na(+)AsH(-) due to the above fast regeneration reaction. The result indicates that the prooxidant effect of α-Toc(•) is suppressed by the coexistence of UQ10H2 or Na(+)AsH(-). As α-TocH, UQ10H2, and ascorbate monoanion (AsH(-)) coexist in relatively high concentrations in plasma, blood, and various tissues, the above synergistic effect, that is, the increase of the free-radical-scavenging rate and suppression of the prooxidant reaction, may function in biological systems.