Lipid oxidation, a process that has been associated with the deterioration of foods and the pathogenesis of various diseases, forms lipid hydroperoxides as the primary product. Lipid oxidation can happen through different oxidation mechanisms (e.g., singlet oxygen oxidation and free radical oxidation), and importantly, lipid hydroperoxides that are formed through different oxidation mechanisms differ in their positional and stereoisomeric structures of the hydroperoxide group. Thus, analysis of the detailed structures of lipid hydroperoxides, especially around the hydroperoxide group, enables the estimation of lipid oxidation mechanisms. Recent advances in analytical techniques including our use of sodium ion during MS/MS analysis, have enabled the structural analysis of various lipid hydroperoxides as well as the estimation of oxidation mechanisms. One example is the oxidation of squalene, which is a lipid present in foods and human skin surface lipids. Oxidation of squalene causes a decrease in the nutritional value of foods as well as adverse skin conditions; thus, we recently made approaches towards estimating squalene oxidation mechanisms via structural analysis of squalene monohydroperoxide, the primary oxidation product of squalene. We herein review our recent efforts towards estimation of lipid oxidation mechanisms by structural analysis of lipid hydroperoxides, with focus on squalene oxidation.
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