Honglian Shi

Comparative study on dynamics of antioxidative action of α-tocopheryl hydroquinone, ubiquinol, and α-tocopherol against lipid peroxidation

Abstract α-Tocopheryl quinone is a metabolite of α-tocopherol (TOH) in vivo. The antioxidant action of its reduced form, α-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 × 104 M−1 s−1) than UQH2 (6.0 × 103 M−1 s−1) and TOH (2.4 × 103 M−1 s−1) at 25°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 α-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.

Stoichiometric and kinetic studies on Ginkgo biloba extract and related antioxidants.

Owing to increasing evidence showing the importance of lipid peroxidation in oxidative stress in vivo, the role and evaluation of antioxidants have received much attention. Ginkgo biloba extract (GBE), well-known as an efficient drug against diseases induced by free radicals, has been suggested to exert its effect by antioxidant action. A method was established to determine the activity of GBE as a hydrogen donor by stoichiometric and kinetic studies, and GBE was compared with several other antioxidants such as α-tocopherol, propyl gallate, and two kinds of flavonoids which are found in GBE, quercetin, and kaempferol. It was found that there were 6.62×1019 active hydrogens in 1g of GBE. Stoichiometric studies showed that one molecule of α-tocopherol reacted with one molecule of galvinoxyl radical. For quercetin, kaempferol and propyl gallate, the experimental stoichiometric numbers were 4.0, 1.9, and 3.1, respectively. The rates of reaction of antioxidants with galvinoxyl in ethanol were determined spectrophotometrically, using a stopped-flow technique. The second-order rate constant, k2, obtained at 25°C was 0.13 (g/l)−1s−1 for GBE and 5.9×103, 2.1×103, 1.2×104, and 2.4×103 M−1s−1 for quercetin, kaempferol, propyl gallate, and α-tocopherol, respectively. The second-order rate constant, k2′, on the molar basis of active hydroxyl groups in the tested substances obtained at 25°C decreased in the order of propyl gallate > α-tocopherol> quercetin>GBF∼kaempferol. This is the first study on GBE as an antioxidant which reports both stoichiometric and kinetic results.

Diverse functions of antioxidants.

All biological organisms have developed a defense system against oxidative stress, which is comprised of many kinds of antioxidants. Antioxidants are classified by function into four categories; preventive antioxidants; radical scavenging antioxidants; repair and de novo antioxidants; and adaptation. Radical scavenging antioxidants have the greatest advantage. Although the activities of radical scavenging antioxidant are determined by several factors, their chemical structure is of key importance. Furthermore, radical scavenging antioxidants have been explored to have a novel function by which they regulate gene expression of cell.

1‐Methyl‐4‐phenyl‐2,3‐dihydropyridinium is transformed by ubiquinone to the selective nigrostriatal toxin 1‐methyl‐4‐phenylpyridinium

We have studied the interaction of coenzyme Q with 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) and its metabolites, 1‐methyl‐4‐phenyl‐2,3‐dihydropyridinium (MPDP+) and 1‐methyl‐4‐phenylpyridinium (MPP+), the real neurotoxin to cause Parkinsons disease. Incubation of MPTP or MPDP+ with rat brain synaptosomes induced complete reduction of endogenous ubiquinone‐9 and ubiquinone‐10 to corresponding ubiquinols. The reduction occurred in a time‐ and MPTP/MPDP+ concentration‐dependent manner. The reduction of ubiquinone induced by MPDP+ went much faster than that by MPTP. MPTP did not reduce liposome‐trapped ubiquinone‐10, but MPDP+ did. The real toxin MPP+ did not reduce ubiquinone in either of the systems. The reduction by MPTP but not MPDP+ was completely prevented by pargyline, a type B monoamine oxidase (MAO‐B) inhibitor, in the synaptosomes. The results indicate that involvement of MAO‐B is critical for the reduction of ubiquinone by MPTP but that MPDP+ is a reductant of ubiquinone per se. It is suggested that ubiquinone could be an electron acceptor from MPDP+ and promote the conversion from MPDP+ to MPP+ in vivo, thus accelerating the neurotoxicity of MPTP.