Effects of alpha-tocopherol and ascorbyl palmitate on the isomerization and decomposition of methyl linoleate hydroperoxides

in order to study antioxidant action on lipid hydroperoxide decomposition, the effects of alpha -tocopherol (TOH) and ascorbyl palmitate on the decomp osition rate and reaction sequences of 9- and 13-cis, trans methyl linoleat e hydroperoxide (cis,trans ML-OOH) decomposition in hexadecane were studied at 40 degreesC. Decomposition of cis,trans ML-OOH as well as the formation and isomeric configuration of methyl linoleate hydroxy and ketodiene compo unds were followed by high-performance liquid chromatographic analysis. TOH effectively inhibited the decomposition of ML-OOH. The decomposition rate was two times slower at 0.2 mM and more than 10 times slower at 2 and 20 mM of TOH. Ascorbyl palmitate (0.2, 2, and 20 mM) slightly accelerated the de composition of ML-OOH. Both compounds had an effect an the reaction sequenc es of ML-OOH decomposition. At high levels TOH inhibited the isomerization of cis, trans ML-OOH to Irans,trans ML-OOH through peroxyl radicals and inc reased the formation of hydroxy compounds. Further, the majority of the hyd roxy and ketodiene compounds formed had a cis,trans configuration, indicati ng that cis, trans ML-OOH decomposed through alkoxyl radicals without isome rization. These results suggest that when inhibiting the decomposition of h ydroperoxides, TOH can act as a hydrogen atom donor to both peroxyl and alk oxyl radicals. In the presence of ascorbyl palmitate, cis, trans ML-OOH dec omposed rapidly but without isomerization. In contrast to TOH, the majority of hydroxy compounds were cis,trans, but the ketodiene compounds were tran s, trans isomers. This indicates that ascorbyl palmitate reduced cis,trans ML-OOH to the corresponding hydroxy compounds. However, the simultaneous fo rmation of trans,trans ketodiene compounds suggests that ML-OOH decompositi on, similar to the control sample, also occurred in these samples. Thus, un der these experimental conditions, the reduction of ML-OOH to more stable h ydroxy compounds did not occur to an extent significant enough to inhibit t he radical chain reactions of ML-OOH decomposition.