Lipid hydroperoxides: Effects of tocopherols and ascorbic acid on their formation and decomposition

In this thesis the effects of tocopherols and ascorbic acid on the formation and decomposition of methyl linoleate hydroperoxides (ML-OOH) were evaluated. The results contribute to the understanding of the antioxidant action of tocopherols and ascorbic acid during lipid oxidation reaction sequences. α-and γ-Tocopherols at 10-1000 ppm levels efficiently inhibited the formation of ML-OOH during the autoxidation of methyl linoleate at 40°C. In addition, α-, γ-and δ-tocopherols (0.2, 2 and 20 mM levels) inhibited the decomposition of ML-OOH in hexadecane. The more detailed analysis of the isomeric distribution of ML-OOH as well as the decomposition products of ML-OOH by HPLC provided information of the mechanisms of action of tocopherols on lipid hydroperoxides. The results suggest that all tocopherols act as hydrogen atom donors to both peroxyl and alkoxyl radicals. α-Tocopherol directed the isomeric distribution of ML-OOH to the cis,trans configuration during the formation and decomposition of ML-OOH. α-Tocopherol inhibited the isomerization reaction by acting as a fast hydrogen atom donor to peroxyl radicals. In the presence of γ-and δ-tocopherols more trans,trans hydroperoxides were formed. The biologically active hydroperoxides, such as those formed in enzymatically catalyzed reactions, have the cis,trans configuration. Thus, it is assumed that this ability of α-tocopherol to stabilize hydroperoxides to the cis,trans configuration might be one of the factors which explains its importance as an antioxidant in biological systems. During the formation of ML-OOH, α-tocopherol showed some loss of efficiency with increasing concentrations but it still strongly inhibited the formation of hydroperoxides when compared to the control sample. Similarly, all tocopherols at higher concentrations experienced a loss of efficiency during the decomposition of ML-OOH. The loss of efficiency was highest for α-tocopherol but was also evident for γ-and δ-tocopherols. The differences in the relative effects of tocopherols at differing concentrations seems to result from a balance between their radical scavenging efficiency and participation in side reactions of an oxidizing nature. All effects of ascorbic acid and ascorbyl palmitate on ML-OOH were relatively small. More detailed analysis of the mechanisms of action showed that they were multifunctional compounds which act as hydrogen atom donors to peroxyl radicals, reducers of hydroperoxides to more stable hydroxy compounds as well as pro-oxidative compounds due to the interactions with metal ions. When α-tocopherol was added to methyl linoleate with appreciable levels of preformed hydroperoxides, its efficiency to inhibit hydroperoxide formation decreased. This result suggests that an optimal concentration of α-tocopherol is needed …