Finding of Remarkable Synergistic Effect on the Aroxyl-Radical-Scavenging Rates (k) under the Coexistence of Vitamin E Homologues (or Vegetable Oils) and Ubiquinol-10: Proposal of A New Mechanism to Explain An Increase of k Value.

Measurements of aroxyl (ArO · )-radical-scavenging rate constants (k) of antioxidants (AOHs) (i.e., α-, β-, γ-, δ-Tocopherol (TocH) and ubiquinol-10 (UQH)) were performed in ethanol/chloroform/HO (50/50/1, v/v) solution, using stopped-flow spectrophotometry. k values were measured not only for each AOH, but also for the mixtures of two AOHs (i.e., TocH and UQH). k values for α-, β-, γ-, δ-TocH increased 1.21, 1.28, 1.55, and 1.19 times, respectively, under the coexistence of constant concentrations of UQH. Similar measurements were performed for eight vegetable oils 1 - 8, containing different concentrations of α-, β-, γ-, δ-tocopherol (TocH) and -tocotrienol (Toc-3H). k values of all eight vegetable oils 1 - 8 also increased 1.24 - 1.54 times under the coexistence of constant concentrations of UQH. A new mechanism to explain the notable increase of k values under the coexistence of two kinds of phenolic AOHs was proposed. UV-vis absorption of α-, β-, γ-Toc · radicals, produced by reaction of α-, β-, γ-TocHs (or vegetable oils 1 - 8) with ArO · , disappeared under the coexistence of TocHs (or oils) and UQH, suggesting that the prooxidant reaction resulting from the presence of Toc · radicals is suppressed in the presence of UQH.