Oxidized lipids have been well known as potential markers of oxidative stress or other disorders in physiological/pathological processes, such as cardiovascular diseases, neurodegenerative diseases, and even some cancers. Lipidomic analysis is a modern and advanced approach to conducting informative elucidation for lipid molecules, which benefits biomarker hunting, clinical determination, and biological pathway exploration. In recent years, as mass spectrometry technology advances, lipidomics is extending the investigating range towards oxidized lipids, which is called oxidative lipidomics. This approach combines the complexity of the lipidome and focuses attention on oxidation. Because oxidized lipids are usually at low levels and artificial oxidation is undesired, oxidative lipidomics is more challenging than ordinary lipidomics, particularly in sample preparation, standards, and spectrum analysis. Nowadays, oxidative lipidomic analysis has been applied to biomedical science for various diseases in clinics and animal experiments on disease models. This review aims at the strategies and methods of oxidative lipidomics and the summary of current applications. The development of chromatographic-MS spectrometric technology is hopeful to allow a deeper and broader understanding of lipids and oxidation concerning human health.

Recent advances in lipidomics have allowed for the measurement of precise levels of biologically active lipids in clinical human samples, which can aid us in understanding the involvement of bioactive lipids in the pathogenesis of human diseases. Among the various human diseases, a variety of membrane lipid-derived mediators have been demonstrated to possess important roles in the initiation, maintenance, and modulation of neuropathic pain (NP). Lysophosphatidic acid (LPA) has been identified as being the main initiator of NP based on experimental animal models, as well as clinical studies using a lipidomics approach. Currently, there is no specific medical treatment and no objective laboratory testing for NP. Based on these conditions, we have described the possible implementation of inhibitors of autotaxin, a producing enzyme for LPA, and LPA measurements in the cerebrospinal fluid (CSF) as a therapeutic target and a potential diagnostic marker.

Phosphoinositides (PIPs), which are phosphorylated forms of phosphatidylinositol (PI), are minor but essential components of membrane phospholipids. There are seven PIP classes differing in phosphorylation at the 3, 4, and 5 positions of the inositol ring of PI. Each PIP class plays crucial roles in various cellular events, such as signal transduction and membrane trafficking, while dysregulation of PIP metabolism is associated with the development of diseases such as cancer. Recent studies suggest the importance of the composition of the two fatty acyl chains in PIPs as well as the phosphorylation status of the inositol ring of PIPs. Therefore, there is an increasing demand for analysis of PIPs by mass spectrometry which can provide information on the fatty acyl chains of PIPs. Recently, mass spectrometric methods that can comprehensively analyze molecular species of all PIP regioisomers have been developed. In this short review, we describe past and present mass spectrometric analyses of PIPs as well as potential future improvements in the method.

Peroxisomes are subcellular organelles that are involved in various biological processes, including lipid synthesis and metabolism. Inherited dysfunctions of enzymes in the peroxisome cause a number of peroxisomal disorders associated with unusual lipid metabolites, as exemplified by X-linked adrenoleukodystrophy (X-ALD), the most prevalent peroxisomal disorder. Liquid chromatography linked to electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) is a pivotal technique for analysis of disease-specific metabolites to facilitate development of diagnostic markers and identification of disease-causing substances. Recent advances in MS have enabled intensive metabolomic targeting of very small amounts of lipids. However, it remains challenging to simultaneously analyze complex lipids such as glycosphingolipids. In addition, it is not easy to obtain stable isotopically labeled compounds for use as internal standards. Here, the application of LC-ESI-MS/MS in the analysis of phospholipids, glycosphingolipids (gangliosides, hexosylceramides, lactosylceramides, and globosides), and fatty acyl-coenzyme A is described. As an example, features of lipid species in X-ALD are described on the basis of the authors’ previous studies. A novel method for preparation of deuterated free fatty acids is also introduced, because deuterated compounds are useful as internal standards and probes for enzymes such as lysophospholipid acyltransferases that directly recognize a hydroxyoctadecadienoic acid (HODE) as an oxidized linoleic fatty acid.

The saturation metabolism of polyunsaturated fatty acids represents a mode of detoxification metabolism by the gut microbiota. It generates a series of functional fatty acids, such as hydroxy fatty acids (e.g., 10-hydroxy-cis-12-octadecenoic acid (HYA) from linoleic acid), oxo fatty acids, conjugated fatty acids, and trans fatty acids. In this study, we prepared 45 synthetic standards, including HYA and related fatty acid metabolites, and developed liquid chromatography-tandem mass spectrometry-based targeted lipidomics. This method was successfully applied for the comprehensive analysis of the gut microbiota-dependent production of fatty acid metabolites in mouse feces, plasma, and tissues under different dietary conditions.

Extracellular vesicles (EVs) are secreted from donor cells that bind to receptors on receiving cells to mediate signal transduction. EVs contain nucleic acids, proteins, miRNAs and small molecules, including lipid species and other metabolites. These molecules are likely functional mediators and have potential as candidate disease biomarkers and/or therapeutic targets. However, EV-selective isolation and the optimal extraction of small molecules are essential to observe the variation in EV molecules, and a more detailed classification of lipid species is required for lipidomic profiling in EVs. Here, we examined methanol precipitation to extract a wide polarity range of small molecules from isolated EVs of human plasma by kit-based metabolic and lipidomic profiling using ultrahigh-performance liquid chromatography triple quadrupole tandem mass spectrometry and multivariate analyses. A total of 278 and 379 molecules were identified in EVs and plasma, respectively. The contents of triglycerides consisting of polysaturated fatty acids were significantly higher in EVs than in plasma, whereas the contents of cholesterol esters were lower in EVs. This method of EV lipidomic profiling may be essential to reveal the function of EVs and utilize them for future biomarker discovery in the clinical field.

Lysophosphatidylthreonine (LysoPT) is a potent inducer of mast cell degranulation that was previously identified as an analog of a bioactive lysophospholipid, lysophoshatidylserine (LysoPS). However, it remains to be solved if LysoPT is present in vivo. In the present study, we tried to detect LysoPT in biological samples using liquid chromatography/tandem mass spectrometry (LC-MS/MS) and examined its tissue distribution. In several mouse tissues, we detected LysoPT as a mixture of LysoPT with several different fatty acid chains, i.e., LysoPT species. C18:0-LysoPT was a major LysoPT species in various tissues and plasma with the highest expression in the stomach. The fatty acid species and tissue distributions of LysoPS and LysoPT were similar, suggesting that the two lysophospholipids share a common synthetic pathway. The present study indicates that LysoPT behaves as an endogenous lysophospholipid mediator.