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

Abstract

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

Kinetic Study of the Prooxidant Effect of α‐Tocopherol. Hydrogen Abstraction from Lipids by α‐Tocopheroxyl Radical

“…However, for instance, if ''antioxidant'' a-TocH in high concentrations exists in edible oils, lipid peroxidations are accelerated, and further, the rate of lipid peroxidation increases by increasing the concentration of a-TocH [7,8,10]. Similarly, a-TocH acts as a prooxidant to oxidize polyunsaturated lipids in LDL in the absence of other endogenous antioxidants such as vitamin C and ubiquinol-10, as reported by Bowry et al [9,29].…”

“…It is well known that a-TocH is the major endogenous lipid-soluble antioxidant in human plasma [27], low-density lipoprotein (LDL) [24,28] and edible oils, and acts as an antioxidant by protecting polyunsaturated fatty acids and lipids from peroxidation. However, for instance, if ''antioxidant'' a-TocH in high concentrations exists in edible oils, lipid peroxidations are accelerated, and further, the rate of lipid peroxidation increases by increasing the concentration of a-TocH [7,8,10]. Similarly, a-TocH acts as a prooxidant to oxidize polyunsaturated lipids in LDL in the absence of other endogenous antioxidants such as vitamin C and ubiquinol-10, as reported by Bowry et al [9,29].…”

“…If a-tocopherols exist in biomembranes and oils, the a-TocÁ radicals may react with unsaturated lipids (LH) (reaction 3) and lipid hydroperoxides (LOOH) (reaction 4). These reactions 3 and 4 are known as prooxidant reactions, which induce the degradation of unsaturated lipids [7][8][9][10]. a-TocÁ radicals may be regenerated by vitamin C (ascorbate anion, AsH -) (reaction 5) [1] and/or ubiquinol-10 [11] in biomembranes, to protect the above prooxidant effects.…”

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

“…Toc nonradical products(NRP) k 2 d (9) As shown in Figure 2d, the maximum absorbance (that is, the concentration) of α-Toc • observed at t max increases with increasing [α-TocH] and approaches a constant value, because at high [α-TocH] α-Toc • appears rapidly and the decay of α-Toc • is very small and negligible. Therefore, we can estimate the ε value of α-Toc • radical at λ max = 428 nm using the relation (absorbance (of α-Toc • at t max ) = ε × [ArO • ] t = 0 ), as reported in a previous work.…”

Comparison of different vehicles to study the effect of tocopherols on gene expression in intestinal cells

“…Due to the phenolic structure, such a phenomenon is highly likely to occur with tocopherols. The stability discrepancy between a-and g-tocopherol in ethanolic solution could be due to the fact that a-tocopherol is consumed by more side reactions than g-tocopherol in an oxidative environment [30]. When associated with BSA, which displays well known antioxidant properties [31], tocopherols were partly protected from oxidative damage.…”