Shi H, Noguchi N, Niki E
Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Japan.
Free Radic Biol Med. 1999 Aug;27(3-4):334-46. doi: 10.1016/s0891-5849(99)00053-2.
Alpha-tocopheryl quinone is a metabolite of alpha-tocopherol (TOH) in vivo. The antioxidant action of its reduced form, alpha-tocopheryl hydroquinone (TQH2), has received much attention recently. In the present study, the antioxidative activity of TQH2 was studied in various systems in vitro and compared with that of ubiquinol-10 (UQH2) or TOH to obtain the basic information on the dynamics of the antioxidant action of TQH2. First, their hydrogen-donating abilities were investigated in the reaction with galvinoxyl, a stable phenoxyl radical, and TQH2 was found to possess greater second-order rate constant (1.0 x 10(4) M(-1) s(-1)) than UQH2 (6.0 x 10(3) M(-1) s(-1)) and TOH (2.4 x 10(3) M(-1) s(-1)) at 25 degrees C in ethanol. The stoichiometric numbers were obtained as 1.9, 2.0, and 1.0 for TQH2, UQH2, and TOH, respectively, in reducing galvinoxyl. Second, their relative reactivities toward peroxyl radicals were assessed in competition with N,N'-diphenyl-p-phenylenediamine (DPPD) and found to be 6.0 (TQH2), 1.9 (UQH2), and 1.0 (TOH). Third, their antioxidant efficacies were evaluated in the oxidation of methyl linoleate in organic solvents and in aqueous dispersions. The antioxidant potency decreased in the order TOH > UQH2 > TQH2, as assessed by either the extent of the reduction in the rate of oxidation or the duration of inhibition period. The reverse order of their reactivities toward radicals and their antioxidant efficacies was interpreted by the rapid autoxidation of TQH2 and UQH2, carried out by hydroperoxyl radicals. Although neither TQH2 nor UQH2 acted as a potent antioxidant by itself, they acted as potent antioxidants in combination with TOH. TQH2 and UQH2 reduced alpha-tocopheroxyl radical to spare TOH, whereas TOH suppressed the autoxidation of TQH2 and UQH2. In the micelle oxidation, the antioxidant activities of TQH2, UQH2, and TOH were similar, whereas 2,2,5,7,8-pentamethyl-6-chromanol exerted much more potent efficacy than TQH2, UQH2, or TOH. These results clearly show that the antioxidant potencies against lipid peroxidation are determined not only by their chemical reactivities toward radicals, but also by the fate of an antioxidant-derived radical and the mobility of the antioxidant at the microenvironment.
α-生育醌是α-生育酚（TOH）在体内的代谢产物。其还原形式α-生育氢醌（TQH2）的抗氧化作用近来备受关注。在本研究中，我们在多种体外体系中研究了TQH2的抗氧化活性，并将其与泛醇-10（UQH2）或TOH的抗氧化活性进行比较，以获取有关TQH2抗氧化作用动力学的基础信息。首先，我们研究了它们与稳定的苯氧自由基加尔vinoxyl反应时的供氢能力，发现在25℃的乙醇中，TQH2的二级速率常数（1.0×10⁴ M⁻¹ s⁻¹）比UQH2（6.0×10³ M⁻¹ s⁻¹）和TOH（2.4×10³ M⁻¹ s⁻¹）更大。在还原加尔vinoxyl时，TQH2、UQH2和TOH的化学计量数分别为1.9、2.0和1.0。其次，我们通过与N,N'-二苯基对苯二胺（DPPD）竞争，评估了它们对过氧自由基的相对反应活性，结果发现分别为6.0（TQH2）、1.9（UQH2）和1.0（TOH）。第三，我们在有机溶剂和水分散体系中评估了它们在亚油酸甲酯氧化中的抗氧化效果。通过氧化速率降低的程度或抑制期的持续时间评估，抗氧化能力的顺序为TOH＞UQH2＞TQH2。它们对自由基的反应活性顺序与抗氧化效果的相反顺序可以通过氢过氧自由基引发的TQH2和UQH2的快速自动氧化来解释。尽管TQH2和UQH2本身都不是强效抗氧化剂，但它们与TOH联合使用时可作为强效抗氧化剂。TQH2和UQH2将α-生育酚自由基还原以节省TOH，而TOH则抑制TQH2和UQH2的自动氧化。在胶束氧化中，TQH2、UQH2和TOH的抗氧化活性相似，而2,2,5,7,8-五甲基-6-色满醇的抗氧化效果比TQH2、UQH2或TOH强得多。这些结果清楚地表明，对脂质过氧化的抗氧化能力不仅取决于它们对自由基的化学反应活性，还取决于抗氧化剂衍生自由基的命运以及抗氧化剂在微环境中的流动性。
