The antioxidant effects of αand γ-tocopherols (at 0, 10, 100, 500, and 1000 ppm) were evaluated in a model system based on the autoxidation of methyl linoleate in bulk for 4 d at 40°C. Samples were collected every 24 h and analyzed for the 9 cis,trans, 9 trans,trans, 13 cis,trans, and 13 trans,trans isomers of hydroperoxide, hydroxy, and ketodiene oxidation products by high-performance liquid chromatography. Results showed that both αand γ-tocopherols are effective hydrogen donors as evidenced by their abilities to inhibit the formation of hydroperoxides, hydroxy compounds, and ketodienes and the cis,trans to trans,trans isomerization of hydroperoxides. Compared with γ-tocopherol, α-tocopherol was a more efficient antioxidant at very low concentrations (10 ppm) but a less efficient antioxidant at the high concentrations (100-1000 ppm). This paradoxical behavior is explained on the basis of differences in ease of hydrogen donation between the two tocopherol homologs. Although α-tocopherol shows some loss of efficiency with increasing concentration, it is not a prooxidant when compared to the control void of antioxidants.
α α-, γ γand δ δ-Tocopherols as inhibitors of isomerization and decomposition of cis,trans methyl linoleate hydroperoxidesThe effects of α-, γand δ-tocopherols on the stability and decomposition reactions of lipid hydroperoxides were studied. Isomerization and decomposition of cis,trans methyl linoleate hydroperoxides (cis,trans ML-OOH) in hexadecane at 40 °C were followed by high-performance liquid chromatography. Due to its higher hydrogen donating ability, α-tocopherol was more efficient than γand δ-tocopherols in inhibiting the isomerization of cis,trans ML-OOH to trans,trans ML-OOH. α-Tocopherol stabilized hydroperoxides into the cis,trans configuration, whereas γand δ-tocopherols allowed hydroperoxides to convert into trans,trans isomers. Thus, the biological importance of α-tocopherol as compared to other tocopherols may be partly due to its better efficacy in protecting the cis,trans configuration of hydroperoxides formed, for example, in the enzymatic oxidation of polyunsaturated fatty acids. The isomeric configuration of hydroperoxides has an impact on biological activities of further oxidation products of polyunsaturated fatty acids. Paradoxically, the order of activity of tocopherols with regard to hydroperoxide decomposition was different from that obtained for hydroperoxide isomerization. γand δ-tocopherols were more efficient inhibitors of ML-OOH decomposition when compared to α-tocopherol. A loss of antioxidant efficiency, observed as the tocopherol concentration increased from 2 to 20 mM, was highest for α-tocopherol but was also evident for γand δ-tocopherols. Thus, the differences in the relative effects of tocopherols at differing concentrations seem to result from a compromise between their radical scavenging efficiency and participation in side reactions of peroxidizing nature.
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