alpha-, gamma- and delta-Tocopherols as inhibitors of isomerization and decomposition of cis,trans methyl linoleate hydroperoxides

The effects of alpha-, gamma- and delta -tocopherols on the stability and d ecomposition reactions of lipid hydroperoxides were studied. Isomerization and decomposition of cis,trans methyl linoleate hydroperoxides (cis,trans M L-OOH) in hexadecane at 40 degreesC were followed by high-performance liqui d chromatography. Due to its higher hydrogen donating ability, alpha -tocop herol was more efficient than gamma- and delta -tocopherols in inhibiting t he isomerization of cis,trans ML-OOH to trans,trans ML-OOH. alpha -Tocopher ol stabilized hydroperoxides into the cis,trans configuration, whereas gamm a- and delta -tocopherols allowed hydroperoxides to convert into trans, tra ns isomers. Thus, the biological importance of a-tocopherol as compared to other tocopherols may be partly due to its better efficacy in protecting th e cis,trans configuration of hydroperoxides formed, for example, in the enz ymatic oxidation of polyunsaturated fatty acids. The isomeric configuration of hydroperoxides has an impact on biological activities of further oxidat ion products of polyunsaturated fatty acids. Paradoxically, the order of ac tivity of tocopherols with regard to hydroperoxide decomposition was differ ent from that obtained for hydroperoxide isomerization. gamma- and delta -t ocopherols were more efficient inhibitors of ML-OOH decomposition when comp ared to a-tocopherol. A loss of antioxidant efficiency, observed as the toc opherol concentration increased from 2 to 20 mM, was highest for a-tocopher ol but was also evident for gamma- and delta -tocopherols. Thus, the differ ences in the relative effects of tocopherols at differing concentrations se em to result from a compromise between their radical scavenging efficiency and participation in side reactions of peroxidizing nature.