Stopped-Flow Investigation of Antioxidant Activity of Tocopherols

The orders of relative tocopherol biopotency, scavenging activity of lipid peroxyl radicals, and quenching activity of singlet oxygen are as follows: α-tocopherol > β-tocopherol ≥ γ-tocopherol > δ-tocopherol. However, the reverse is the case for the activity to protect fats and oils from oxidation: α-tocopherol < β-tocopherol < γ-tocopherol < δ-tocopherol. The reason for this reverse has been studied by measuring weight gains of solutions including methyl linoleate and various antioxidants at daily intervals. Antioxidant activity is expressed as an induction period in days. Induction periods are compared with the second-order rate constants for scavenging of an aroxyl radical by the antioxidants. A plot of the logarithm of the secondorder rate constant vs. the induction period is found to give a good linear fit, and the slope is negative. From these results, it is considered that the larger the peroxyl-radical-scavenging activity of a tocopherol, the more favorable the production of the tocopheroxyl radical, and thus the more efficient the oxidation of fats and oils by the tocopheroxyl radical. This is not the case for tocopherols in vivo, because vitamin C and ubiquinol reduce tocopheroxyl radicals formed from tocopherols. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: [605][606][607][608][609][610] 2005

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“…k s is a measure of scavenging activity of lipid peroxyl radicals by the antioxidants [10,31]. The longest τ was obtained for KME-4 (25).…”

Section: Loomentioning

confidence: 99%

Why is the order reversed? peroxyl‐scavenging activity and fats‐and‐oils protecting activity of vitamin E

Mukai

Noborio

Nagaoka

2005

Int J of Chemical Kinetics

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“…Because each tocopherol molecule can trap two peroxyl radicals, the stoichiometric factor (n) for the four tocopherol isomers is theoretically considered to be equal to 2. The rate at which tocopherol isomers react with peroxyl radicals is a direct measure of their antioxidant efficiency (Burton & Ingold, 1981;Niki et al, 1984;Mukai et al, 1986). It has been determined that c!-tocopherol is the most efficient chain-breaking antioxidant among the four isomers.…”

Section: A-tocopherol As the Free Radical Scavengermentioning

confidence: 99%

Vitamin E: Mechanism of Its Antioxidant Activity.

Yamauchi

1997

FSTI

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The antioxidant activity of vitamin E (a-tocopherol) during the peroxidation of unsaturated lipids has been reviewed based on its reaction products. Free-radical scavenging reactions of a-tocopherol take place via the a-tocopheroxyl radical as an intermediate. If a suitable free radical is present, a non-radical product can be formed from the coupling of the free radical with the a-tocopheroxyl radical. The reaction products of a-tocopherol with lipid-peroxyl radicals are 8a-(lipid-dioxy)-a-tocopherones which are hydrolyzed to a-tocopherylquinone. If the supply of oxygen is insufficient, a-tocopherol can trap the carbon-centered radicals of lipids to form 6-0-(lipidalkyl)-a-tocopherols. On the other hand, the dimer and trimer of a-tocopherol is formed by the bimolecular self-reaction of the a-tocopheroxyl radical in a reaction mixture containing a large amount of a-tocopherol. The other product-forming pathway yields isomeric epoxy-a-tocopherylquinones and their precursors, epoxyhydroperoxy-a-tocopherones, but the mechanism of this pathway remains unknown. The reaction products of other vitamin E compounds (7-and 6-tocopherols) during lipid peroxidation are almost the same as those formed from the a-tocopherol. The tocopheroxyl radicals of 7-and 6-tocopherols prefer to react with each other to form dimeric products that are still effective as antioxidants.

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“…In a previous work (Mukai et al, 1986), we measured the reaction rates (k s ) of vitamin E analogues with 2,6-di-t-butyl-4- * Corresponding author. Tel.…”

Section: Introductionmentioning

confidence: 99%