Comparison of diacylglycerol and triacylglycerol lipid oxidation through LC-MS/MS analysis of hydroperoxide isomers

α-Linolenic acid (FA 18:3), an omega-3 fatty acid, has a variety of physiological effects, but is easily oxidized, becoming various FA 18:3 hydroperoxide (FA 18:3;OOH) isomers depending on the oxidation mechanism (e.g., thermal oxidation and photooxidation). This　FA 18:3 is bound to a glyceride molecule and is abundant in edible oils such as flaxseed oil and perilla oil, but it is not clear how differences in the glyceride structure (i.e., triacylglycerol (TG) or diacylglycerol (DG)) affect FA 18:3 oxidation. We have recently developed an LC-MS/MS method that can distinguish between isomers of FA 18:3;OOH. In this study, we further improved our method to simultaneously analyze TG- and DG-FA 18:3;OOH isomers and clarified how differences in the glyceride structure affect these oxidations.
After examining various conditions, we established an LC-MS/MS method that allows simultaneous analysis of TG- and DG-FA 18:3;OOH (e.g., TG 18:3_18:3_18:3;OOH and DG 18:3_18:3;OOH) isomers depending on the oxidation mechanism (thermal oxidation and photooxidation). We then prepared a oil containing both TG and DG, thermally oxidized it, and analyzed it by the developed LC-MS/MS method, and found that certain isomers specific for thermal oxidation were indeed detected, and interestingly, DG 18:3_18:3;OOH isomers appeared to form at a slower rate than TG 18:3_18:3_18:3;OOH isomers. These results suggest that FA 18:3 in DG would be more stable against thermal oxidation, probably due to conformational differences between DG and TG. Although further investigation is needed, such differences in oxidation behavior based on glyceride structure may lead to new possibilities in the edible oil industry, such as in extending shelf life.