Electrochemical oxidation of phosphatidylethanolamines studied by mass spectrometry

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.

Oxidized phospholipid-protein adducts: The future targets of interest

Phospholipids are key biomolecules with important roles as components of membranes, lipoproteins and as signalling molecules. However, phospholipids are quite prone to oxidation. Upon oxidation they generate several types of oxidation products including long chain oxidation products, as hydroperoxyl and hydroxy derivatives, and highly reactive oxidation products, like small aldehydes and truncated oxidized phospholipids. The formation of protein adducts with small electrophilic aldehydes (like malondialdehyde) is now well studied, however, the aggregation of proteins with truncated oxidized phospholipids lacks research. This paper provides a short overview of the formation of protein adducts with truncated oxidized phospholipids as well as a gathering of the research on this topic. The literature found reports the synthesis, detection and fragmentation of this type of adducts, mainly focusing on truncated oxidized phospholipid' products from phosphatidylcholine class and few peptides and proteins, as human serum albumin and Apo B100, leaving unattended the screening in vivo and in disease correlation, thus lacking possible association with their biological role. These adducts are a consequence of oxidative modifications to important biomolecules and their involvement in the organism is still unclear, revealing the urgent need for more investigation in this area.

Analytical Toolbox to Unlock the Diversity of Oxidized Lipids

ConspectusLipids are diverse class of small biomolecules represented by a large variety of chemical structures. In addition to the classical biosynthetic routes, lipids can undergo numerous modifications via introduction of small chemical moieties forming hydroxyl, phospho, and nitro derivatives, among others. Such modifications change the physicochemical properties of a parent lipid and usually result in new functionalities either by mediating signaling events or by changing the biophysical properties of lipid membranes. Over the last decades, a large body of evidence indicated the involvement of lipid modifications in a variety of physiological and pathological events. For instance, lipid (per)oxidation for a long time was considered as a hallmark of oxidative stress and related proinflammatory signaling. Recently, however, with the burst in the development of the redox biology field, oxidative modifications of lipids are also recognized as a part of regulatory and adaptive events that are highly specific for particular cell types, tissues, and conditions.The initial diversity of lipid species and the variety of possible lipid modifications result in an extremely large chemical space of the epilipidome, the subset of the natural lipidome formed by enzymatic and non-enzymatic lipid modifications occurring in biological systems. Together with their low natural abundance, structural annotation of modified lipids represents a major analytical challenge limiting the discovery of their natural variety and functions. Furthermore, the number of available chemically characterized standards representing various modified lipid species remains limited, making analytical and functional studies very challenging. Over the past decade we have developed and implemented numerous analytical methods to study lipid modifications and applied them in the context of different biological conditions. In this Account, we outline the development and evolution of modern mass-spectrometry-based techniques for the structural elucidation of modified/oxidized lipids and corresponding applications. Research of our group is mostly focused on redox biology, and thus, our primary interest was always the analysis of lipid modifications introduced by redox disbalance, including lipid peroxidation (LPO), oxygenation, nitration, and glycation. To this end, we developed an array of analytical solutions to measure carbonyls derived from LPO, oxidized and nitrated fatty acid derivatives, and oxidized and glycated complex lipids. We will briefly describe the main analytical challenges along with corresponding solutions developed by our group toward deciphering the complexity of natural epilipdomes, starting from in vitro-oxidized lipid mixtures, artificial membranes, and lipid droplets, to illustrate the diversity of lipid modifications in the context of metabolic diseases and ferroptotic cell death.

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.

Characterization of Non-volatile Oxidation Products Formed from Triolein in a Model Study at Frying Temperature

Frying allows cooking food while promoting their organoleptic properties, imparting crunchiness and flavor. The drawback is the oxidation of triacylglycerides (TAGs), leading to the formation of primary oxidized TAGs. Although they have been associated with chronic and degenerative diseases, they are precursors of pleasant flavors in fried foods. Nevertheless, there is a lack of knowledge about the oxidation species present in foods and their involvement in positive/negative health effects. In this work, high-resolution (HR) C₃₀ reversed-phase (RP)-liquid chromatography (LC)-tandem HR mass spectrometry (MS/MS) was used to identify primary oxidation TAGs resulting from heating triolein (160 °C, 5 min). This allows simulating the initial heating process of frying oils usually used to prepare fried foods, such as chips, crisps, and snacks. Beyond hydroxy, dihydroxy, hydroperoxy, and hydroxy-hydroperoxy derivatives, already reported in phospholipids oxidized by Fenton reaction, new compounds were identified, such as dihydroxy-hydroperoxy-triolein derivatives and positional isomers (9/10- and 9/12-dihydroxy-triolein derivatives). These compounds should be considered when proposing flavor formation pathways and/or mitigating lipid-derived reactive oxygen species occurring during food frying.

A high-throughput targeted metabolomics method for the quantification of 104 non-polar metabolites in cholesterol, eicosanoid, and phospholipid metabolism: application in the study of a CCl4-induced liver injury mouse model

Experimental workflow of 104 non-polar metabolites in cholesterol, eicosanoid, and phospholipid metabolisms analysis using UPLC-QqQ-MS.

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.

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.

Modulation of the inflammatory response of immune cells in human peripheral blood by oxidized arachidonoyl aminophospholipids

Aminophospholipids (APL), phosphatidylethanolamine (PE) and phosphatidylserine (PS), can be oxidized upon oxidative stress. Oxidized PE and PS have been detected in clinical samples of different pathologies and may act as modulators of the inflammatory response. However, few studies have focused on the effects of oxidized APL (ox-APL) esterified with arachidonic acid, even though a considerable number of studies have assessed the modulation of the immune system by oxidized 1-palmitoyl-2-arachidonoyl-sn-3-glycerophosphocholine (OxPAPC). In the present study, we have used flow cytometry to evaluate the ability of oxidized PAPE (OxPAPE) and PAPS (OxPAPS) to promote or suppress an inflammatory phenotype on monocytes subsets and myeloid dendritic cells (mDCs). The results indicate that OxPAPE increases the frequency of all monocyte subpopulations expressing TNF-α, which promotes an inflammatory response. However, immune cell stimulation with OxPAPE in the presence of LPS results in a decrease of TNF-α expressed by classical monocytes. Incubation with OxPAPS and LPS induces a decrease in TNF-α produced by monocytes, and a significant decrease in IL-1β expressed by monocytes and mDCs, indicating that OxPAPS reduces the LPS-induced pro-inflammatory expression in these populations. These results show the importance of OxPAPE and OxPAPS as modulators of the inflammatory response and demonstrate their possible contribution to the onset and resolution of human diseases related to oxidative stress and inflammation.