α-, γ- and δ-tocopherols as inhibitors of isomerization and decomposition of cis, trans methyl linoleate hydroperoxides

The effects of α-, γ- and 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 γ- and 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 γ- and 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. γ- and 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 γ- and 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.