Mass spectrometry analysis of oxidized phospholipids

The non-enzymatic oxidation of phosphatidylethanolamine and phosphatidylserine and their intriguing roles in inflammation dynamics and diseases

Phosphatidylethanolamine (PE) and phosphatidylserine (PS), along with phosphatidylcholine (PC), are key phospholipids (PL) in cell membranes and lipoproteins, prone to oxidative modifications. Their oxidized forms, OxPE and OxPS, play significant roles in inflammation and immune response. This review explores their structural oxidative changes under non-enzymatic conditions and their roles in physiological and pathological contexts, influencing inflammation, and immunity. Specific oxidations of PE and PS significantly alter their physicochemical properties, leading to enhanced biological functions, reduced activity, or inactivation. OxPE may show pro-inflammatory actions, similar to well-documented OxPC, while the OxPS pro-inflammatory effects are less noted. However, OxPS and OxPE have also shown an antagonistic effect against lipopolysaccharides (LPS), suggesting a protective role against exacerbated immune responses, similar to OxPC. Further research is needed to deepen our understanding of these less-studied OxPL classes. The role of OxPE and OxPS in disease pathogenesis remains largely unexplored, with limited studies linking them to Alzheimer's disease, diabetes, rheumatoid arthritis, traumatic brain injury, and skin inflammation. These findings highlight the potential of OxPE and OxPS as biomarkers for disease diagnosis, monitoring, and therapeutic targeting.

In Silico-Predicted Dynamic Oxlipidomics MS/MS Library: High-Throughput Discovery and Characterization of Unknown Oxidized Lipids

Nontargeted lipidomics using liquid chromatography-tandem mass spectrometry can detect thousands of molecules in biological samples. However, the annotation of unknown oxidized lipids is limited to the structures present in libraries, restricting the analysis and interpretation of experimental data. Here, we describe Doxlipid, a computational tool for oxidized lipid annotation that predicts a dynamic MS/MS library for every experiment. Doxlipid integrates three key simulation algorithms to predict libraries and covers 32 subclasses of oxidized lipids from the three main classes. In the evaluation, Doxlipid achieves very high prediction and characterization performance and outperforms the current oxidized lipid annotation methods. Doxlipid, combined with a molecular network, further annotates unknown chemical analogs in the same reaction or pathway. We demonstrate the broad utility of Doxlipid by analyzing oxidized lipids in ferroptosis hepatocellular carcinoma, tissue samples, and other biological samples, substantially advancing the discovery of biological pathways at the trace oxidized lipid level.

An overview of food lipids toward food lipidomics

Increasing evidence regarding lipids' beneficial effects on human health has changed the common perception of consumers and dietary officials about the role(s) of food lipids in a healthy diet. However, lipids are a wide group of molecules with specific nutritional and bioactive properties. To understand their true nutritional and functional value, robust methods are needed for accurate identification and quantification. Specific analytical strategies are crucial to target specific classes, especially the ones present in trace amounts. Finding a unique and comprehensive methodology to cover the full lipidome of each foodstuff is still a challenge. This review presents an overview of the lipids nutritionally relevant in foods and new trends in food lipid analysis for each type/class of lipids. Food lipid classes are described following the LipidMaps classification, fatty acids, endocannabinoids, waxes, C8 compounds, glycerophospholipids, glycerolipids (i.e., glycolipids, betaine lipids, and triglycerides), sphingolipids, sterols, sercosterols (vitamin D), isoprenoids (i.e., carotenoids and retinoids (vitamin A)), quinones (i.e., coenzyme Q, vitamin K, and vitamin E), terpenes, oxidized lipids, and oxylipin are highlighted. The uniqueness of each food group: oil-, protein-, and starch-rich, as well as marine foods, fruits, and vegetables (water-rich) regarding its lipid composition, is included. The effect of cooking, food processing, and storage, in addition to the importance of lipidomics in food quality and authenticity, are also discussed. A critical review of challenges and future trends of the analytical approaches and computational methods in global food lipidomics as the basis to increase consumer awareness of the significant role of lipids in food quality and food security worldwide is presented.

Utilization of LC-MS/MS and Drift Tube Ion Mobility for Characterizing Intact Oxidized Arachidonate-Containing Glycerophosphatidylethanolamine

Lipid peroxidation is a key component in the pathogenesis of numerous disease states, where the oxidative damage of lipids frequently leads to membrane dysfunction and subsequent cellular death. Glycerophosphoethanolamine (PE) is the second most abundant phospholipid found in cellular membranes and, when oxidized, has been identified as an executor of ferroptotic cell death. PE commonly exists in the plasmalogen form, where the presence of the vinyl ether bond and its enrichment in polyunsaturated fatty acids make it especially susceptible to oxidative degradation. This results in a multitude of oxidized products complicating identification and often requiring several analytical techniques for interpretation. In the present study, we outline an analytical approach for the structural characterization of intact oxidized products of arachidonate-containing diacyl and plasmalogen PE. Intact oxidized PE structures, including structural and positional isomers, were identified using complementary liquid chromatography techniques, drift tube ion mobility, and high-resolution tandem mass spectrometry. This work establishes a comprehensive method for the analysis of intact lipid peroxidation products and provides an important pathway to investigate how lipid peroxidation initially impacts glycerophospholipids and their role in redox biology.

Phyto-oxylipin mediated plant immune response to colonization and infection in the soybean-Phytophthora sojae pathosystem

Introduction

Food security is a major challenge to sustainably supply food to meet the demands of the ever-growing global population. Crop loss due to pathogens is a major concern to overcoming this global food security challenge. Soybean root and stem rot caused by Phytophthora sojae results in approximately 20B $US crop loss annually. Phyto-oxylipins are metabolites biosynthesized in the plants by oxidative transformation of polyunsaturated fatty acids through an array of diverging metabolic pathways and play an important role in plant development and defense against pathogen colonization and infection. Lipid mediated plant immunity is a very attractive target for developing long term resistance in many plants' disease pathosystem. However, little is known about the phyto-oxylipin's role in the successful strategies used by tolerant soybean cultivar to mitigate Phytophthora sojae infection.

Methods

We used scanning electron microscopy to observe the alterations in root morphology and a targeted lipidomics approach using high resolution accurate mass tandem mass spectrometry to assess phyto-oxylipin anabolism at 48 h, 72 h and 96 h post infection.

Results and discussion

We observed the presence of biogenic crystals and reinforced epidermal walls in the tolerant cultivar suggesting a mechanism for disease tolerance when compared with susceptible cultivar. Similarly, the unequivocally unique biomarkers implicated in oxylipin mediated plant immunity [10(E),12(Z)-13S-hydroxy-9(Z),11(E),15(Z)-octadecatrienoic acid, (Z)-12,13-dihydroxyoctadec-9-enoic acid, (9Z,11E)-13-Oxo-9,11-octadecadienoic acid, 15(Z)-9-oxo-octadecatrienoic acid, 10(E),12(E)-9-hydroperoxyoctadeca-10,12-dienoic acid, 12-oxophytodienoic acid and (12Z,15Z)-9, 10-dihydroxyoctadeca-12,15-dienoic acid] generated from intact oxidized lipid precursors were upregulated in tolerant soybean cultivar while downregulated in infected susceptible cultivar relative to non-inoculated controls at 48 h, 72 h and 96 h post infection by Phytophthora sojae, suggesting that these molecules may be a critical component of the defense strategies used in tolerant cultivar against Phytophthora sojae infection. Interestingly, microbial originated oxylipins, 12S-hydroperoxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid and (4Z,7Z,10Z,13Z)-15-[3-[(Z)-pent-2-enyl]oxiran-2-yl]pentadeca-4,7,10,13-tetraenoic acid were upregulated only in infected susceptible cultivar but downregulated in infected tolerant cultivar. These microbial originated oxylipins are capable of modulating plant immune response to enhance virulence. This study demonstrated novel evidence for phyto-oxylipin metabolism in soybean cultivars during pathogen colonization and infection using the Phytophthora sojae-soybean pathosystem. This evidence may have potential applications in further elucidation and resolution of the role of phyto-oxylipin anabolism in soybean tolerance to Phytophthora sojae colonization and infection.

Looking in Depth at Oxidized Cholesteryl Esters by LC-MS/MS: Reporting Specific Fragmentation Fingerprints and Isomer Discrimination

Cholesteryl esters (CE) are prone to oxidation under increased oxidative stress conditions, but little is known about oxidized CE species (oxCE). To date, only a few oxCE have been identified, however, mainly based on the detection of molecular ions by mass spectrometry (MS) or target approaches for specific oxCE. The study of oxCE occurring from radical oxidation is still scarcely addressed. In this work, we made a comprehensive assessment of oxCE derivatives and their specific fragmentation patterns to identify detailed structural features and isomer differentiation using high-resolution C18 HPLC-MS- and MS/MS-based lipidomic approaches. The LC-MS/MS analysis allowed us to pinpoint oxCE structural isomers of long-chain and short-chain species, eluting at different retention times (t_R). Data analysis revealed that oxCE can be modified either in the fatty acyl moiety or in the cholesterol ring. The location of the hydroxy/hydroperoxy group originates characteristic fragment ions, namely the unmodified cholestenyl cation (m/z 369) for the isomer with oxidation in the fatty acyl chain or ions at m/z 367 and m/z 385 (369 + 16) when oxygenation occurs in the cholesterol ring. Additionally, we identified CE 18:2 and 20:4 aldehydic and carboxylic short-chain products that showed a clear fragmentation pattern that confirmed the modification in the fatty acyl chain. Specific fragmentation fingerprinting allowed discrimination of the isobaric short-chain species, namely carboxylic short-chain products, from hydroxy aldehyde short-chain products, with a hydroxycholesterol moiety. This new information is important to identify different oxCE in biological samples and will contribute to unraveling their role in biological conditions and diseases such as cardiovascular disease.

Assessing Photosensitized Membrane Damage: Available Tools and Comprehensive Mechanisms

Lipids are important targets of the photosensitized oxidation reactions, forming important signaling molecules, disorganizing and permeabilizing membranes, and consequently inducing a variety of biological responses. Although the initial steps of the photosensitized oxidative damage in lipids are known to occur by both Type I and Type II mechanisms, the progression of the peroxidation reaction, which leads to important end-point biological responses, is poorly known. There are many experimental tools used to study the products of lipid oxidation, but neither the methods nor their resulting observations were critically compared. In this article, we will review the tools most frequently used and the key concepts raised by them in order to rationalize a comprehensive model for the initiation and the progression steps of the photoinduced lipid oxidation.

Structure-specific, accurate quantitation of plasmalogen glycerophosphoethanolamine

Changes in plasmalogen glycerophosphoethanolamine (PE-P) composition (structure and abundance) are a key indicator of altered lipid metabolism. Differential changes in the levels of PE-P have been reported in different disease states, including neurodegenerative diseases. Of particular interest, traumatic brain injury (TBI) has resulted in altered expression of glycerophospholipid profiles, including PE-P. To date, most analytical assays assessing PE-P have focused on general lipidomic workflows to evaluate the relative, semi-quantitative abundance of PE-P during disease progression. This approach provides a broad evaluation of PE-P, yet often lacks specificity and sensitivity for individual PE-P structures which is a necessity for robust quantitative data. The present study highlights the development of a targeted, quantitative method using a HILIC separation and selective reaction monitoring mass spectrometry for the confident identification and accurate quantitation of PE-P. Our innovative method incorporates both the sn-1 alkyl vinyl ether and sn-2 acyl chain as product ion transitions, for specific and sensitive quantitation of 100 PE-P structures. Our method also uniquely allowed for the unambiguous assignment and quantitation of di-unsaturated sn-1 PE-P structures, which to date have not been conclusively quantified. Application of this assay to a TBI mouse model resulted in distinct temporal profiles for plasma PE-P up to 28 days post injury. Plasma PE-P were significantly increased 24 h after induced TBI, followed by a gradual reduction to sham concentrations by day 28. Overall, we established a structure-specific, quantitative assay for identification and quantitation of a comprehensive set of PE-P structures with demonstrated relevance to brain injury.

Integrated sample desalting, enrichment, and ionization on an omniphobic glass slide for direct mass spectrometry analysis

RATIONALE

Direct and rapid mass spectrometry (MS) analysis is desired for many applications including environmental monitoring, forensic analysis, chemical and biological defense, and point-of-care testing. However, sample pretreatment is often necessary for analyzing targets from complex matrices using MS due to ion suppression. To achieve rapid MS analysis calls for simple and efficient solutions for sample processing and ionization. Here, a simple sample pretreatment and ionization workflow is reported, which achieves sample desalting, enrichment, and ionization on a single glass slide.

METHODS

Desalting is achieved based on crystallization and re-dissolution-induced spontaneous separation of analytes and salt. Efficient sample enrichment is achieved during the crystallization process by modifying the glass surface with an omniphobic coating. Finally, vibrating sharp-edge spray ionization is employed to ionize the target molecules directly on the glass slide. Thus, all the necessary sample operations prior to MS analysis are completed on the sample glass slide.

RESULTS

Efficient sample enrichment on the omniphobic glass slide is first visualized using food dyes. The benefits of the desalting and enrichment steps for detecting macrolide antibiotics in 1× phosphate buffered saline (PBS) solutions are demonstrated by comparing samples with different treatment procedures. Finally, quantification of macrolide antibiotics from PBS and serum samples is demonstrated. A linear range between 2 nM and 10 μM has been achieved for the serum sample with a limit of detection of 1 nM.

CONCLUSIONS

A simple, flexible, low-cost, and highly integrated workflow for detecting target molecules from complex matrices using MS is demonstrated. This method will be valuable to many applications that require rapid and efficient MS analysis of complex samples.

Does membrane curvature elastic energy play a role in mediating oxidative stress in lipid membranes?

The effects of oxidative stress on cells are associated with a wide range of pathologies. Oxidative stress is predominantly initiated by the action of reactive oxygen species and/or lipoxygenases on polyunsaturated fatty acid containing lipids. The downstream products are oxidised phospholipids, bioactive aldehydes and a range of Schiff base by-products between aldehydes and lipids, or other biomacromolecules. In this review we assess the impact of oxidative stress on lipid membranes, focusing on the changes that occur to the curvature preference (lipid spontaneous curvature) and elastic properties of membranes, since these biophysical properties modulate phospholipid homeostasis. Studies show that the lipid products of oxidative stress reduce stored curvature elastic energy in membranes. Based upon this observation, we hypothesize that the effects of oxidative stress on lipid membranes will be reduced by compounds that increase stored curvature elastic energy. We find a strong correlation appears across literature studies that we have reviewed, such that many compounds like vitamin E, Curcumin, Coenzyme Q10 and vitamin A show behaviour consistent with this hypothesis. Finally, we consider whether age-related changes in lipid composition represent the homeostatic response of cells to compensate for the accumulation of in vivo lipid oxidation products.

Insights in the Role of Lipids, Oxidative Stress and Inflammation in Rheumatoid Arthritis Unveiled by New Trends in Lipidomic Investigations

Rheumatoid arthritis (RA) is a highly debilitating chronic inflammatory autoimmune disease most prevalent in women. The true etiology of this disease is complex, multifactorial, and is yet to be completely elucidated. However, oxidative stress and lipid peroxidation are associated with the development and pathogenesis of RA. In this case, oxidative damage biomarkers have been found to be significantly higher in RA patients, associated with the oxidation of biomolecules and the stimulation of inflammatory responses. Lipid peroxidation is one of the major consequences of oxidative stress, with the formation of deleterious lipid hydroperoxides and electrophilic reactive lipid species. Additionally, changes in the lipoprotein profile seem to be common in RA, contributing to cardiovascular diseases and a chronic inflammatory environment. Nevertheless, changes in the lipid profile at a molecular level in RA are still poorly understood. Therefore, the goal of this review was to gather all the information regarding lipid alterations in RA analyzed by mass spectrometry. Studies on the variation of lipid profile in RA using lipidomics showed that fatty acid and phospholipid metabolisms, especially in phosphatidylcholine and phosphatidylethanolamine, are affected in this disease. These promising results could lead to the discovery of new diagnostic lipid biomarkers for early diagnosis of RA and targets for personalized medicine.

Lipid Peroxidation in Atherosclerotic Cardiovascular Diseases

Significance: Atherosclerotic cardiovascular diseases (ACVDs) continue to be a primary cause of mortality worldwide in adults aged 35-70 years, occurring more often in countries with lower economic development, and they constitute an ever-growing global burden that has a considerable socioeconomic impact on society. The ACVDs encompass diverse pathologies such as coronary artery disease and heart failure (HF), among others. Recent Advances: It is known that oxidative stress plays a relevant role in ACVDs and some of its effects are mediated by lipid oxidation. In particular, lipid peroxidation (LPO) is a process under which oxidants such as reactive oxygen species attack unsaturated lipids, generating a wide array of oxidation products. These molecules can interact with circulating lipoproteins, to diffuse inside the cell and even to cross biological membranes, modifying target nucleophilic sites within biomolecules such as DNA, lipids, and proteins, and resulting in a plethora of biological effects. Critical Issues: This review summarizes the evidence of the effect of LPO in the development and progression of atherosclerosis-based diseases, HF, and other cardiovascular diseases, highlighting the role of protein adduct formation. Moreover, potential therapeutic strategies targeted at lipoxidation in ACVDs are also discussed. Future Directions: The identification of valid biomarkers for the detection of lipoxidation products and adducts may provide insights into the improvement of the cardiovascular risk stratification of patients and the development of therapeutic strategies against the oxidative effects that can then be applied within a clinical setting.

Lipid Peroxidation Assay Using BODIPY-Phenylbutadiene Probes: A Methodological Overview

The assessment of reactive oxygen species has increasing importance in biomedical sciences, due to their biological role in signaling pathways and induction of cell damage at low and high concentrations, respectively. Detection of lipid peroxidation with sensing probes such as some BODIPY dyes has now wide application in studies using fluorescent microplate readers, flow cytometry, and fluorescence microscopy. Two phenylbutadiene derivatives of BODIPY are commonly used as peroxidation probes, non-oxidized probes and oxidized products giving red and green fluorescence, respectively. Peculiar features of lipoperoxidation and BODIPY dye properties make this assessment a rather complex process, not exempt of doubts and troubles. Color changes and fluorescence fading that are not due to lipid peroxidation must be taken into account to avoid misleading results. As a characteristic feature of lipoperoxidation is the propagation of peroxyl radicals, pitfalls and advantages of a delayed detection by BODIPY probes should be considered.

A Reliable HPLC-ELSD Method for Determination of Cholesterol, Phosphatidylcholine, Lysophosphatidylcholine Content and the Stability of a Liposomal Formulation

Background:

Liposomes continue to play an important role in drug delivery research due to their ability to improve transport and targeting of a wide range of active molecules. Analysis of liposomal components is a key point in the characterization and evaluation of formulation stability. The aim of this work was to develop and validate an HPLC-ELSD method for the characterization and quality control of liposomes.

Methods:

HPLC-ELSD method was validated by assessing selectivity, linearity, precision, accuracy, limit of detection and quantitation. The mobile phase consisted of a 0.1% (v/v) of trifluoroacetic acid (TFA) and methanol in gradient elution. Initial rate was 20:80 (0.1% TFA: methanol), with a ramp reaching 100% methanol. HPLC-MS/MS was used to confirm the presence of the fatty acid mixture in the analyzed lipids, as well as sub-products generated under pre-determined conditions in the stability study.

Results:

A HPLC-ELSD method has been developed to detect and measure cholesterol, phosphatidylcholine and lysophosphatidylcholine. High specificity, sensitivity and linearity within the predetermined range for all the compounds analyzed (R2>0.99) were obtained. Accuracy and precision results for all the compounds were within the acceptance limit of ≤5% and 90-110%, respectively. Mass spectrometry results showed complementary information about the phospholipid composition to evaluate the degree of degradation of liposomes over different storage conditions.

Conclusion:

The method was successfully applied as a quality control tool for the analysis of a wide range of lipids, present in liposomal formulations. HPLC-MS/MS was used to ensure complete elucidation of the lipid components and the detected lyso-forms.

Analysis of oxidised and glycated aminophospholipids: Complete structural characterisation by C30 liquid chromatography-high resolution tandem mass spectrometry

The aminophospholipids (APL), phosphatidylethanolamine (PE) and phosphatidylserine (PS) are widely present in cell membranes and lipoproteins. Glucose and reactive oxygen species (ROS), such as the hydroxyl radical (^•OH), can react with APL leading to an array of oxidised, glycated and glycoxidised derivatives. Modified APL have been implicated in inflammatory diseases and diabetes, and were identified as signalling molecules regulating cell death. However, the biological relevance of these molecules has not been completely established, since they are present in very low amounts, and new sensitive methodologies are needed to detect them in biological systems. Few studies have focused on the characterisation of APL modifications using liquid chromatography-tandem mass spectrometry (LC-MS/MS), mainly using C5 or C18 reversed phase (RP) columns. In the present study, we propose a new analytical approach for the characterisation of complex mixtures of oxidised, glycated and glycoxidised PE and PS. This LC approach was based on a reversed-phase C30 column combined with high-resolution MS, and higher energy C-trap dissociation (HCD) MS/MS. C30 RP-LC separated short and long fatty acyl oxidation products, along with glycoxidised APL bearing oxidative modifications on the glucose moiety and the fatty acyl chains. Functional isomers (e.g. hydroxy-hydroperoxy-APL and tri-hydroxy-APL) and positional isomers (e.g. 9-hydroxy-APL and 13-hydroxy-APL) were also discriminated by the method. HCD fragmentation patterns allowed unequivocal structural characterisation of the modified APL, and are translatable into targeted MS/MS fingerprinting of the modified derivatives in biological samples.

Qualitative analysis of phospholipids and their oxidised derivatives - used techniques and examples of their applications related to lipidomic research and food analysis

Phospholipids (PLs) are important biomolecules that not only constitute structural building blocks and scaffolds of cell and organelle membranes but also play a vital role in cell biochemistry and physiology. Moreover, dietary exogenous PLs are characterised by high nutritional value and other beneficial health effects, which are confirmed by numerous epidemiological studies. For this reason, PLs are of high interest in lipidomics that targets both the analysis of membrane lipid distribution as well as correlates composition of lipids with their effects on functioning of cells, tissues and organs. Lipidomic assessments follow-up the changes occurring in living organisms, such as free radical attack and oxidative modifications of the polyunsaturated fatty acids (PUFAs) build in PL structures. Oxidised PLs (oxPLs) can be generated exogenously and supplied to organisms with processed food or formed endogenously as a result of oxidative stress. Cellular and tissue oxPLs can be a biomarker predictive of the development of numerous diseases such as atherosclerosis or neuroinflammation. Therefore, suitable high-throughput analytical techniques, which enable comprehensive analysis of PL molecules in terms of the structure of hydrophilic group, fatty acid (FA) composition and oxidative modifications of FAs, have been currently developed. This review addresses all aspects of PL analysis, including lipid isolation, chromatographic separation of PL classes and species, as well as their detection. The bioinformatic tools that enable handling of a large amount of data generated during lipidomic analysis are also discussed. In addition, imaging techniques such as confocal microscopy and mass spectrometry imaging for analysis of cellular lipid maps, including membrane PLs, are presented.

Mass spectrometry strategies to unveil modified aminophospholipids of biological interest

The biological functions of modified aminophospholipids (APL) have become a topic of interest during the last two decades, and distinct roles have been found for these biomolecules in both physiological and pathological contexts. Modifications of APL include oxidation, glycation, and adduction to electrophilic aldehydes, altogether contributing to a high structural variability of modified APL. An outstanding technique used in this challenging field is mass spectrometry (MS). MS has been widely used to unveil modified APL of biological interest, mainly when associated with soft ionization methods (electrospray and matrix-assisted laser desorption ionization) and coupled with separation techniques as liquid chromatography. This review summarizes the biological roles and the chemical mechanisms underlying APL modifications, and comprehensively reviews the current MS-based knowledge that has been gathered until now for their analysis. The interpretation of the MS data obtained by in vitro-identification studies is explained in detail. The perspective of an analytical detection of modified APL in clinical samples is explored, highlighting the fundamental role of MS in unveiling APL modifications and their relevance in pathophysiology.

Normal-phase versus reversed-phase thin-layer chromatography (TLC) to monitor oxidized phosphatidylcholines by TLC/mass spectrometry

RATIONALE

Normal-phase thin-layer chromatography (NP-TLC) is an established method for the separation of all major phospholipid classes according to the different polarities of the head groups. In contrast, reversed-phase (RP)-TLC is much less frequently used for this purpose. This study aimed to compare the NP and the RP approach regarding their separation potential of phospholipid oxidation products.

METHODS

Commercially available 1-palmitoyl-2-oleoyl-sn-phosphatidylcholine (POPC) (PC 16:0/18:1) and 1-palmitoyl-2-linoleoyl-sn-phosphatidylcholine (PLPC) (PC 16:0/18:2) were oxidized by NaMnO₄ . Oxidation products were subsequently separated by NP- and RP-TLC and analyzed by electrospray ionization mass spectrometry.

RESULTS

In comparison with NP-TLC, RP-TLC was clearly superior regarding the separation of oxidation products of phospholipids. RP-TLC enabled the separation not only of primary oxidation products of POPC such as alcohols and ketones but also of secondary oxidation products. Furthermore some oxidation products, such as aldehydes, were only detectable by ESI after RP-TLC but not after NP-TLC.

CONCLUSIONS

RP-TLC is the method of choice to characterize oxidized PL such as oxidized phosphatidylcholines.

Lipid Unsaturation Properties Govern the Sensitivity of Membranes to Photoinduced Oxidative Stress

Unsaturated lipid oxidation is a fundamental process involved in different aspects of cellular bioenergetics; dysregulation of lipid oxidation is often associated with cell aging and death. To study how lipid oxidation affects membrane biophysics, we used a chlorin photosensitizer to oxidize vesicles of various lipid compositions and degrees of unsaturation in a controlled manner. We observed different shape transitions that can be interpreted as an increase in the area of the targeted membrane followed by a decrease. These area modifications induced by the chemical modification of the membrane upon oxidation were followed in situ by Raman tweezers microspectroscopy. We found that the membrane area increase corresponds to the lipids' peroxidation and is initiated by the delocalization of the targeted double bonds in the tails of the lipids. The subsequent decrease of membrane area can be explained by the formation of cleaved secondary products. As a result of these area changes, we observe vesicle permeabilization after a time lag that is characterized in relation with the level of unsaturation. The evolution of photosensitized vesicle radius was measured and yields an estimation of the mechanical changes of the membrane over oxidation time. The membrane is both weakened and permeabilized by the oxidation. Interestingly, the effect of unsaturation level on the dynamics of vesicles undergoing photooxidation is not trivial and thus carefully discussed. Our findings shed light on the fundamental dynamic mechanisms underlying the oxidation of lipid membranes and highlight the role of unsaturations on their physical and chemical properties.

Microbes vs. chemistry in the origin of the anaerobic gut lumen

The succession from aerobic and facultative anaerobic bacteria to obligate anaerobes in the infant gut along with the differences between the compositions of the mucosally adherent vs. luminal microbiota suggests that the gut microbes consume oxygen, which diffuses into the lumen from the intestinal tissue, maintaining the lumen in a deeply anaerobic state. Remarkably, measurements of luminal oxygen levels show nearly identical pO₂ (partial pressure of oxygen) profiles in conventional and germ-free mice, pointing to the existence of oxygen consumption mechanisms other than microbial respiration. In vitro experiments confirmed that the luminal contents of germ-free mice are able to chemically consume oxygen (e.g., via lipid oxidation reactions), although at rates significantly lower than those observed in the case of conventionally housed mice. For conventional mice, we also show that the taxonomic composition of the gut microbiota adherent to the gut mucosa and in the lumen throughout the length of the gut correlates with oxygen levels. At the same time, an increase in the biomass of the gut microbiota provides an explanation for the reduction of luminal oxygen in the distal vs. proximal gut. These results demonstrate how oxygen from the mammalian host is used by the gut microbiota, while both the microbes and the oxidative chemical reactions regulate luminal oxygen levels, shaping the composition of the microbial community throughout different regions of the gut.

Oxidation-Induced Increase In Photoreactivity of Bovine Retinal Lipid Extract

The mammalian retina contains a high level of polyunsaturated fatty acids, including docosahexaenoic acid (22:6) (DHA), which are highly susceptible to oxidation. It has been shown that one of the products of DHA oxidation-carboxyethylpyrrole (CEP), generated in situ, causes modifications of retinal proteins and induces inflammation response in the outer retina. These contributing factors may play a role in the development of age-related macular degeneration (AMD). It is also possible that some of the lipid oxidation products are photoreactive, and upon irradiation with blue light may generate reactive oxygen species. Therefore, in this work we analysed oxidation-induced changes in photoreactivity of lipids extracted from bovine neural retinas. Lipid composition of bovine neural retinas closely resembles that of human retinas making the bovine tissue a convenient model for studying the photoreactivity and potential phototoxicity of oxidized human retinal lipids. Lipid composition of bovine neural retinas Folch' extracts (BRex) was determined by gas chromatography (GC) and liquid chromatography coupled to an electrospray ionization source-mass spectrometer (LC-ESI-MS) analysis. Liposomes prepared from BRex, equilibrated with air, were oxidized in the dark at 37 °C for up to 400 h. The photoreactivity of BRex at different stages of oxidation was studied by EPR-oximetry and EPR-spin trapping. Photogeneration of singlet oxygen (¹O₂, ¹Δ_g) by BRex was measured using time-resolved detection of the characteristic phosphorescence at 1270 nm. To establish contribution of lipid components to the analysed photoreactivity of Folch' extract of bovine retinas, a mixture of selected synthetic lipids in percent by weight (w/w %) ratio resembling that of the BRex has been also studied. Folch's extraction of bovine neural retinas was very susceptible to oxidation despite the presence of powerful endogenous antioxidants such as α-tocopherol and zeaxanthin. Non-oxidized and oxidized BRex photogenerated singlet oxygen with moderate quantum yield. Blue-light induced generation of superoxide anion by Folch' extract of bovine neural retinas strongly depended on the oxidation time. The observed photoreactivity of the studied extract gradually increased during its in vitro oxidation.

LipidPioneer : A Comprehensive User-Generated Exact Mass Template for Lipidomics

Lipidomics, the comprehensive measurement of lipid species in a biological system, has promising potential in biomarker discovery and disease etiology elucidation. Advances in chromatographic separation, mass spectrometric techniques, and novel substrate applications continue to expand the number of lipid species observed. The total number and type of lipid species detected in a given sample are generally indicative of the sample matrix examined (e.g., serum, plasma, cells, bacteria, tissue, etc.). Current exact mass lipid libraries are static and represent the most commonly analyzed matrices. It is common practice for users to manually curate their own lists of lipid species and adduct masses; however, this process is time-consuming. LipidPioneer, an interactive template, can be used to generate exact masses and molecular formulas of lipid species that may be encountered in the mass spectrometric analysis of lipid profiles. Over 60 lipid classes are present in the LipidPioneer template and include several unique lipid species, such as ether-linked lipids and lipid oxidation products. In the template, users can add any fatty acyl constituents without limitation in the number of carbons or degrees of unsaturation. LipidPioneer accepts naming using the lipid class level (sum composition) and the LIPID MAPS notation for fatty acyl structure level. In addition to lipid identification, user-generated lipid m/z values can be used to develop inclusion lists for targeted fragmentation experiments. Resulting lipid names and m/z values can be imported into software such as MZmine or Compound Discoverer to automate exact mass searching and isotopic pattern matching across experimental data. Graphical Abstract ᅟ.

Lipoprotein profiling methodology based on determination of apolipoprotein concentration

Aim: Abnormal lipid metabolism results in the alteration of lipid compositions in lipoproteins; therefore an accurate and quantitative analytical approach is required for the detailed structural characterization of lipoproteins. However, the specific lipid composition of each lipoprotein particle is poorly understood. Materials & methods: Lipid composition of very-low-density lipoprotein and low-density lipoprotein particles derived from myocardial infarction-prone rabbits was determined by normalization of lipidomics data using apoB-100 levels. Results: The ratio of lipid levels between very-low-density lipoprotein and low-density lipoprotein particles was different according to not only lipid classes, but also phosphatidylethanolamine subclasses by applying our developed methodology to myocardial infarction-prone rabbits. Conclusion: Our novel analytical approach represents to be a potentially useful tool to obtain particle-specific lipid components of lipoproteins.

Chlorinated Phospholipids and Fatty Acids: (Patho)physiological Relevance, Potential Toxicity, and Analysis of Lipid Chlorohydrins

Chlorinated phospholipids are formed by the reaction of hypochlorous acid (HOCl), generated by the enzyme myeloperoxidase under inflammatory conditions, and the unsaturated fatty acyl residues or the head group. In the first case the generated chlorohydrins are both proinflammatory and cytotoxic, thus having a significant impact on the structures of biomembranes. The latter case leads to chloramines, the properties of which are by far less well understood. Since HOCl is also widely used as a disinfecting and antibacterial agent in medicinal, industrial, and domestic applications, it may represent an additional source of danger in the case of abuse or mishandling. This review discusses the reaction behavior of in vivo generated HOCl and biomolecules like DNA, proteins, and carbohydrates but will focus on phospholipids. Not only the beneficial and pathological (toxic) effects of chlorinated lipids but also the importance of these chlorinated species is discussed. Some selected cleavage products of (chlorinated) phospholipids and plasmalogens such as lysophospholipids, (chlorinated) free fatty acids and α-chloro fatty aldehydes, which are all well known to massively contribute to inflammatory diseases associated with oxidative stress, will be also discussed. Finally, common analytical methods to study these compounds will be reviewed with focus on mass spectrometric techniques.

New Insights on Non-Enzymatic Oxidation of Ganglioside GM1 Using Mass Spectrometry

Gangliosides are acidic glycosphingolipids that are present in cell membranes and lipid raft domains, being particularly abundant in central nervous systems. They participate in modulating cell membrane properties, cell-cell recognition, cell regulation, and signaling. Disturbance in ganglioside metabolism has been correlated with the development of diseases, such as neurodegenerative diseases, and in inflammation. Both conditions are associated with an increased production of reactive oxidation species (ROS) that can induce changes in the structure of biomolecules, including lipids, leading to the loss or modification of their function. Oxidized phospholipids are usually involved in chronic diseases and inflammation. However, knowledge regarding oxidation of gangliosides is scarce. In order to evaluate the effect of ROS in gangliosides, an in vitro biomimetic model system was used to study the susceptibility of GM1 (Neu5Acα2-3(Galβ1-3GalNAcβ1-4)Galβ1-4Glcβ1Cer) to undergo oxidative modifications. Oxidation of GM1 under Fenton reaction conditions was monitored using high resolution electrospray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS). Upon oxidation, GM1 underwent oxidative cleavages in the carbohydrate chain, leading to the formation of other gangliosides GM2 (GalNAcβ1-4Gal(Neu5Acα2-3)1-4Glcβ1Cer), GM3 (Neu5Acα2-3Galβ1-4Glcβ1Cer), asialo-GM1 (Galβ1-3GalNAcβ1-4Galβ1-4Glcβ1Cer), asialo-GM2 (GalNAcβ1-4Galβ1-4Glcβ1Cer), of the small glycolipids lactosylceramide (LacCer), glucosylceramide (GlcCer), and of ceramide (Cer). In addition, oxygenated GM1 and GM2 (as keto and hydroxy derivatives), glycans, oxidized glycans, and oxidized ceramides were also identified. Nonenzymatic oxidation of GM1 under oxidative stress contributes to the generation of other gangliosides that may participate in the imbalance of gangliosides metabolism in vivo, through uncontrolled enzymatic pathways and, consequently, play some role in neurodegenerative processes. Graphical Abstract ᅟ.

Do cinnamylideneacetophenones have antioxidant properties and a protective effect toward the oxidation of phosphatidylcholines?

Cinnamylideneacetophenones (CA) are an important group of α,β,γ,δ-diunsaturated ketones that have been widely used in a variety of synthetic transformations. Biological studies concerning these compounds are scarce and refer mainly to antiviral and antibacterial evaluations. Curcumin (CR), a natural polyphenol, is a yellow pigment extracted from the plant Curcuma longa, which is one of the major spices used in the Indian culinary. It has been reported that CR has cancer chemopreventive properties in a range of animal models of chemical carcinogenesis, along with antioxidative and anti-inflammatory properties. Inspired by the biological activity shown by CR and their structural resemblance with CA, it was considered to study the ability of the latter molecules to inhibit lipid oxidation induced by the hydroxyl radical (Fenton reaction) by electrospray ionization (ESI) mass spectrometry (MS) using phosphatidylcholine (PC) liposomes as a model of cell membrane. Compound 4, holding a methylated hydroxy group in the position R(2), and CR showed similar effects in inhibiting lipid peroxidation. In the presence of 7, the extension of oxidation was higher than the one verified in all other compounds. Other methodologies, namely DPPH radical scavenging and oxygen radical absorption capacity (ORAC) assays, were performed to complement and clarify the results attained by oxidation of PC monitored by ESI-MS and to evaluate the antioxidant profile of compounds. For both assays, compound 7 showed to be rather efficient due to its specific structure. This derivative can form a quite stable allylic radical by abstraction of a hydrogen atom which accounts for these results.

Acyl Chain Disorder and Azelaoyl Orientation in Lipid Membranes Containing Oxidized Lipids

Oxidized phospholipids occur naturally in conditions of oxidative stress and have been suggested to play an important role in a number of pathological conditions due to their effects on a lipid membrane acyl chain orientation, ordering, and permeability. Here we investigate the effect of the oxidized phospholipid 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC) on a model membrane of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using a combination of (13)C-(1)H dipolar-recoupling nuclear magnetic resonance (NMR) experiments and united-atom molecular dynamics (MD) simulations. The obtained experimental order parameter SCH profiles show that the presence of 30 mol % PazePC in the bilayer significantly increases the gauche content of the POPC acyl chains, therefore decreasing the thickness of the bilayer, although with no stable bilayer pore formation. The MD simulations reproduce the disordering effect and indicate that the orientation of the azelaoyl chain is highly dependent on its protonation state with acyl chain reversal for fully deprotonated states and a parallel orientation along the interfacial plane for fully protonated states, deprotonated and protonated azelaoyl chains having negative and positive SCH profiles, respectively. Only fully or nearly fully protonated azelaoyl chain are observed in the (13)C-(1)H dipolar-recoupling NMR experiments. The experiments show positive SCH values for the azelaoyl segments confirming for the first time that oxidized chains with polar termini adopt a parallel orientation to the bilayer plane as predicted in MD simulations.

Unique plasma metabolomic signatures of individuals with inherited disorders of long-chain fatty acid oxidation

Blood and urine acylcarnitine profiles are commonly used to diagnose long-chain fatty acid oxidation disorders (FAOD: i.e., long-chain hydroxy-acyl-CoA dehydrogenase [LCHAD] and carnitine palmitoyltransferase 2 [CPT2] deficiency), but the global metabolic impact of long-chain FAOD has not been reported. We utilized untargeted metabolomics to characterize plasma metabolites in 12 overnight-fasted individuals with FAOD (10 LCHAD, two CPT2) and 11 healthy age-, sex-, and body mass index (BMI)-matched controls, with the caveat that individuals with FAOD consume a low-fat diet supplemented with medium-chain triglycerides (MCT) while matched controls consume a typical American diet. In plasma 832 metabolites were identified, and partial least squared-discriminant analysis (PLS-DA) identified 114 non-acylcarnitine variables that discriminated FAOD subjects and controls. FAOD individuals had significantly higher triglycerides and lower specific phosphatidylethanolamines, ceramides, and sphingomyelins. Differences in phosphatidylcholines were also found but the directionality differed by metabolite species. Further, there were few differences in non-lipid metabolites, indicating the metabolic impact of FAOD specifically on lipid pathways. This analysis provides evidence that LCHAD/CPT2 deficiency significantly alters complex lipid pathway flux. This metabolic signature may provide new clinical tools capable of confirming or diagnosing FAOD, even in subjects with a mild phenotype, and may provide clues regarding the biochemical and metabolic impact of FAOD that is relevant to the etiology of FAOD symptoms.

Bioprospecting of Marine Macrophytes Using MS-Based Lipidomics as a New Approach

The marine environment supports a remarkable diversity of organisms which are a potential source of natural products with biological activities. These organisms include a wide variety of marine plants (from micro- to macrophytes), which have been used in the food and pharmaceutical industry. However, the biochemistry and biological activities of many of these macrophytes (namely macroalgae and halophytes, including seagrasses) are still far from being fully explored. Most popular bioactive components include polysaccharides, peptides, phenolics and fatty acids (FAs). Polar lipids (glycolipids, phospholipids and betaine lipids) are emerging as novel value-added bioactive phytochemicals, rich in n-3 FA, with high nutritional value and health beneficial effects for the prevention of chronic diseases. Polar lipids account various combinations of polar groups, fatty acyl chains and backbone structures. The polar lipidome of macrophytes is remarkably diverse, and its screening represents a significant analytical challenge. Modern research platforms, particularly mass spectrometry (MS)-based lipidomic approaches, have been recently used to address this challenge and are here reviewed. The application of lipidomics to address lipid composition of marine macrophytes will contribute to the stimulation of further research on this group and foster the exploration of novel applications.

Detection and confirmation of serum lipid biomarkers for preeclampsia using direct infusion mass spectrometry

Despite substantial research, the early diagnosis of preeclampsia remains elusive. Lipids are now recognized to be involved in regulation and pathophysiology of some disease. Shotgun lipidomic studies were undertaken to determine whether serum lipid biomarkers exist that predict preeclampsia later in the same in pregnancy. A discovery study was performed using sera collected at 12-14 weeks pregnancy from 27 controls with uncomplicated pregnancies and 29 cases that later developed preeclampsia. Lipids were extracted and analyzed by direct infusion into a TOF mass spectrometer. MS signals, demonstrating apparent differences were selected, their abundances determined, and statistical differences tested. Statistically significant lipid markers were reevaluated in a second confirmatory study having 43 controls and 37 preeclampsia cases. Multi-marker combinations were developed using those lipid biomarkers confirmed in the second study. The initial study detected 45 potential preeclampsia markers. Of these, 23 markers continued to be statistically significant in the second confirmatory set. Most of these markers, representing several lipid classes, were chemically characterized, typically providing lipid class and potential molecular components using MS(2) Several multi-marker panels with areas under the curve >0.85 and high predictive values were developed. Developed panels of serum lipidomic biomarkers appear to be able to identify most women at risk for preeclampsia in a given pregnancy at 12-14 weeks gestation.

Soybean phosphatidylcholine liposomes as model membranes to study lipid peroxidation photoinduced by pterin

Oxidized pterins, efficient photosensitizers under UVA irradiation, accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder. Soybean phosphatidylcholine (SoyPC) liposomes were employed as model membranes to investigate if pterin (Ptr), the parent compound of oxidized pterins, is able to photoinduced lipid peroxidation. Size exclusion chromatography and dialysis experiments showed that Ptr is not encapsulated inside the liposomes and the lipid membrane is permeable to this compound. The formation of conjugated dienes and trienes, upon UVA irradiation, was followed by absorption at 234 and 270 nm, respectively. The photoproducts were characterized by mass spectrometry and oxygenation of SoyPC was demonstrated. In addition, analysis of MS/MS spectra suggested the formation hydroperoxides. Finally, the biological implications of the findings are discussed.

Hydrophilic interaction liquid chromatography-electrospray ionization-tandem mass spectrometry of a complex mixture of native and oxidized phospholipids

A mixture of native and oxidized phospholipids (PLs), generated by the soybean lipoxygenase type V-catalyzed partial oxidation of a lipid extract obtained from human platelets, was analyzed by Hydrophilic Interaction Liquid Chromatography-ElectroSpray Ionization-Tandem Mass Spectrometry (HILIC-ESI-MS/MS). The complexity of the resulting mixture was remarkable, considering that the starting lipid extract, containing (as demonstrated in a previous study) about 130 native PLs, was enriched with enzymatically generated hydroperoxylated derivatives and chemically generated hydroxylated forms of PLs bearing polyunsaturated side chains. Nonetheless, the described analytical approach proved to be very powerful; indeed, focusing on phosphatidylcolines (PCs), the most abundant PL class in human platelets, about fifty different native/oxidized species could be identified in a single HILIC-ESI-MS/MS run. Low-energy collision induced dissociation tandem MS (CID-MS/MS) experiments on chromatographically separated species showed single neutral losses of H2O2 and H2O to be typical fragmentation pathways of hydroperoxylated PCs, whereas a single H2O loss was observed for hydroxylated ones. Moreover, diagnostic losses of n-hexanal or n-pentanol were exploited to recognize PCs hydroperoxylated on the last but five carbon atom of a ɷ-6 polyunsaturated side chain. Despite the low resolution of the 3D ion trap mass analyzer used, the described HILIC-ESI-MS/MS approach appears very promising for the identification of oxidized lipids in oxidatively stressed complex biological systems.

Oxidative Damage to Biomimetic Membrane Systems: In Situ Fe(II)/Ascorbate Initiated Oxidation and Incorporation of Synthetic Oxidized Phospholipids

Damage to cellular membranes from oxidative stress has been implicated in aging related diseases. We report the effects of oxidative damage on the structure and properties of biomimetic phospholipid membrane systems. Two oxidation methods were used, in situ oxidation initiated using Fe(II) and ascorbate, and the incorporation of a synthetic "oxidized" phospholipid, PoxnoPC, into biomimetic membranes. The biomimetic systems employed included multibilayer stacks, tethered bilayers, and phospholipid monolayers studied using a combination of reflectometry, attenuated total reflection infrared spectroscopy, electrochemical impedance spectroscopy, and neutron diffraction. We show that oxidation with Fe(II) and ascorbate caused an increase in the order of the membrane, attributed to cross-linking of the phospholipids, and a change in the electrical permeability of the membrane, but no significant impact on the thickness or completeness of the membrane. Incorporation of PoxnoPC, on the other hand, had a larger impact on the structure of the membrane. Inversion of the aldehyde-terminated truncated sn-2 chain of PoxnoPC into the head group region was observed, along with a slight decrease in the thickness and order of the membrane.

Dysfunctional High-Density Lipoprotein: An Innovative Target for Proteomics and Lipidomics

High-Density Lipoprotein-Cholesterol (HDL-C) is regarded as an important protective factor against cardiovascular disease, with abundant evidence of an inverse relationship between its serum levels and risk of cardiovascular disease, as well as various antiatherogenic, antioxidant, and anti-inflammatory properties. Nevertheless, observations of hereditary syndromes featuring scant HDL-C concentration in absence of premature atherosclerotic disease suggest HDL-C levels may not be the best predictor of cardiovascular disease. Indeed, the beneficial effects of HDL may not depend solely on their concentration, but also on their quality. Distinct subfractions of this lipoprotein appear to be constituted by specific protein-lipid conglomerates necessary for different physiologic and pathophysiologic functions. However, in a chronic inflammatory microenvironment, diverse components of the HDL proteome and lipid core suffer alterations, which propel a shift towards a dysfunctional state, where HDL-C becomes proatherogenic, prooxidant, and proinflammatory. This heterogeneity highlights the need for further specialized molecular studies in this aspect, in order to achieve a better understanding of this dysfunctional state; with an emphasis on the potential role for proteomics and lipidomics as valuable methods in the search of novel therapeutic approaches for cardiovascular disease.

Identification of Oxidation Compounds of 1-Stearoyl-2-linoleoyl-sn-glycero-3-phosphoethanolamine during Thermal Oxidation

Heat-induced oxidative modification of phosphatidylethanolamine molecular species as potential functional food components was investigated. 1-Stearoyl-2-linoleoyl-sn-glycero-3-phosphoethanolamine (SLPE) was chosen as a model. The optimal temperature for hydroperoxide formation was determined by mass spectrometry. The maximal level of formation of this compound was obtained at 125 °C. The structures of nonvolatile organic compounds (non-VOCs) were identified using liquid chromatography-electrospray ionization mass spectrometry combined with an acid treatment. Kinetics of formation of non-VOCs was monitored over time. Results showed that the level of the SLPE precursor rapidly decreased during thermal oxidation and oxygenated products, such as hydroxyl, oxo, or epoxy groups, were formed. The VOCs formed from oxidized SLPE were determined by headspace solid-phase microextraction followed by gas chromatography-mass spectrometry analysis. The result showed that a saturated methyl ketone (2-heptanone) was the most predominant VOC of SLPE. Kinetics indicated that the formation of VOCs was related not only to the decomposition of hydroperoxides but also to the further decomposition of non-VOCs.