Oxidation of squalene by singlet oxygen and free radicals results in different compositions of squalene monohydroperoxide isomers

Naoki Shimizu,Takahiro Eitsuka,Kiyotaka Nakagawa,Masashi Goto,Shunji Kato,Junya Ito,Teruo Miyazawa,Yurika Otoki

doi:10.1038/s41598-018-27455-5

Naoki Shimizu, Takahiro Eitsuka + Show 6 more

Open Access

https://doi.org/10.1038/s41598-018-27455-5

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Abstract

Oxidation of squalene (SQ) causes a decline in the nutritional value of SQ in foods, as well as an accumulation of SQ oxidation products in skin lipids which lead to adverse skin conditions. However, mechanistic insights as to how SQ is oxidized by different oxidation mechanisms have been limited, and thus effective measures towards the prevention of SQ oxidation have not been identified. In this study, we oxidized SQ by either singlet oxygen oxidation or free radical oxidation, and monitored the formation of the six SQ monohydroperoxide (SQOOH) isomers, the primary oxidation products of SQ, at the isomeric level. While singlet oxygen oxidation of SQ resulted in the formation of similar amounts of the six SQOOH isomers, free radical oxidation of SQ mainly formed two types of isomers, 2-OOH-SQ and 3-OOH-SQ. The addition of β-carotene during singlet oxygen oxidation, and the addition of α-tocopherol during free radical oxidation lead to a dose-dependent decrease in the formation of SQOOH isomers. Such results suggest that the analysis of SQOOH at the isomeric level allows for the determination of the cause of SQ oxidation in various samples, and provides a foothold for future studies concerning the prevention of SQ oxidation.

Highlights

Squalene (SQ; Fig. 1) is a linear polyunsaturated triterpene that is a key intermediate metabolite during the biosynthesis of cholesterol and other sterols
In this study, we investigated how SQ is oxidized to SQ monohydroperoxide (SQOOH) by two oxidation mechanisms, singlet oxygen oxidation and free radical oxidation
With the use of our normal phase LC-MS/MS method that enables the individual analysis of the six SQOOH isomers[26], it was identified that all six SQOOH isomers were formed in similar amounts (Fig. 2B)

Summary

Introduction

Squalene (SQ; Fig. 1) is a linear polyunsaturated triterpene that is a key intermediate metabolite during the biosynthesis of cholesterol and other sterols. SQ is suggested to be the initial target of skin lipid oxidation, forming SQ monohydroperoxide (SQOOH) as the primary oxidation product[12,13,14,15]. The identification of the oxidation mechanism that induces the oxidation of SQ (e.g. singlet oxygen oxidation and free radical oxidation) is necessary in order to select an antioxidant that is effective towards the prevention of SQ oxidation. One way to evaluate oxidation mechanisms of lipids is to analyze lipid hydroperoxides, the initial oxidation products of lipid oxidation, at the isomeric level[19,20,21,22,23,24] This is based on the fact that certain lipids (e.g. linoleic acid) form characteristic lipid hydroperoxide isomers depending on the oxidation mechanism that the lipid was oxidized by[25]. We recently established a method to individually detect the six SQOOH isomers[26], based on evidence that the use of sodium ions during LC-MS/MS analysis enables the selective analysis of other lipid hydroperoxide isomers[19,20,21,22,23,24,27,28]

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Results

Conclusion