Liquid chromatography/tandem mass spectrometry characterization of nitroso, nitrated and nitroxidized cardiolipin products

Advances in methods to analyse cardiolipin and their clinical applications

Cardiolipin (CL) is a mitochondria-exclusive phospholipid, primarily localised within the inner mitochondrial membrane, that plays an essential role in mitochondrial architecture and function. Aberrant CL content, structure, and localisation have all been linked to impaired mitochondrial activity and are observed in the pathophysiology of cancer and neurological, cardiovascular, and metabolic disorders. The detection, quantification, and localisation of CL species is a valuable tool to investigate mitochondrial dysfunction and the pathophysiological mechanisms underpinning several human disorders. CL is measured using liquid chromatography, usually combined with mass spectrometry, mass spectrometry imaging, shotgun lipidomics, ion mobility spectrometry, fluorometry, and radiolabelling. This review summarises available methods to analyse CL, with a particular focus on modern mass spectrometry, and evaluates their advantages and limitations. We provide guidance aimed at selecting the most appropriate technique, or combination of techniques, when analysing CL in different model systems, and highlight the clinical contexts in which measuring CL is relevant.

In-silico-library-based method enables rapid and comprehensive annotation of cardiolipins and cardiolipin oxidation products using high resolution tandem mass spectrometer

Accumulated evidences suggest that cardiolipins (CLs) and cardiolipin oxidation products (oxCLs) are a class of essential molecules that play critical roles in many physiological functions. Diversity of four acyl chains leads to high structure complexity for cardiolipin species including CLs, monolysocardiolipins (MLCLs) and their oxCLs. The ability to rapidly identify CL species can be implemented by the match of mass spectrometry (MS)-based in-silico spectral database. In this study, after optimizing the chromatography conditions and MS detection, an in-silico library containing 377,754 simulated tandem mass spectra deducing from 31,578 CLs to 52,160 of MLCLs was successfully augmented based on LipidBlast templates. For the construction of the oxCLs' library, twenty-five fatty acyls oxidation products relating to nine oxidation types were permuted and combined. A total of 42,180 oxCL spectra were predicted based on the experimental measurements of oxCLs forming by artificially oxidation. Applying the in-silico database to murine mitochondria and cell samples enabled the sensitive and comprehensive annotation of 86 MLCLs, 307 CLs and 112 oxCLs with high annotation confidence. Compared to the conventional method, our proposed in-silico database provides a more comprehensive interpretation for CL species' characterization with high throughput and sensitivity in nontarget lipidomic study.

Mass spectrometry-based study defines the human urine nitrolipidome

Nitrated fatty acids (NO2-FA) are an endogenous class of signaling mediators formed mainly during digestion and inflammation. The signaling actions of NO2-FA have been extensively studied, but their detection and characterization lagged. Several different nitrated fatty acid species have been reported in animals and humans, but their formation remains controversial, and a systemic approach to define the endogenous pool of NO2-FA is needed. Herein, we screened for endogenous NO2-FA in urine from healthy human volunteers as this is the main excretion route for NO2-FA and its metabolites, and it provides an excellent matrix for evaluation. Only isomers of two fatty acids, conjugated linoleic and linolenic acid were found to be nitrated. Several, previously unknown, nitrated species were identified and confirmed using high-resolution mass spectrometry, fragmentation analysis, and compared to synthetic nitrated standards, the main group corresponding to nitrated conjugated linolenic acid (NO2-CLnA). In contrast, we were unable to confirm the presence of previously reported nitrated omega-3's, oleic acid, arachidonic acid and α- and γ-linolenic acid, suggesting that their biological formation and presence in humans should be re-evaluated. Metabolite analysis of NO2-CLnA in human urine identified cysteine adducts and β-oxidation products, which were compared to the metabolic products of nitrated standards obtained using primary mouse hepatocytes. Importantly, NO2-CLnA isomers belong to two defined groups, are electrophilic, participate in Michael addition reactions and account for 39% of total urinary NO2-FA, highlighting their relative abundance and possible role in cell signaling.

Investigation of cardiolipin oxidation products as a new endpoint for oxidative stress in C. elegans by means of online two-dimensional liquid chromatography and high-resolution mass spectrometry

The investigation of neurodegenerative and age-related diseases is a highly relevant topic in current research. Especially oxidative stress is thought to be the common underlying mechanism in diseases such as Parkinson's or Alzheimer's disease. The nematode Caenorhabditis elegans (C. elegans) is a prominent model organism, which is often used for such investigations and has gained extensive recognition in research regarding the linkage of reactive oxygen species (ROS) and neurodegeneration. Not only studies regarding genomics and proteomics have been increasingly conducted, also the number of studies based on the lipidome is rising. The phospholipid class of cardiolipin (CL) is a unique lipid class, which is exclusively located in mitochondria and is therefore of great relevance regarding oxidative stress and associated diseases. CL oxidation products have become a prominent marker for oxidative stress in various organisms. However, the CL distribution in the nematode C. elegans is still scarcely known on the molecular level and oxidation products have not yet been identified. In this work, we demonstrate the importance of CL distribution and the applicability of CL oxidation products as a sensitive marker for oxidative stress in C. elegans. For this reason, the CL distribution was determined by means of online two-dimensional liquid chromatography hyphenated with high-resolution mass spectrometry (2D-LC/HRMS). Subsequently, worms were treated with tert-butyl hydroperoxide (tBOOH) in order to provoke oxidative stress and induce the artificial formation of oxidized CL. We were able to detect increasing amounts of CL oxidation products of highly unsaturated CL species in a concentration-dependent manner. This finding emphasizes the great potential of CL oxidation products as a sensitive marker substance of oxidative stress in C. elegans, which is not only directly linked to mitochondria function but also favourable to other oxidative stress markers in terms of the needed sample material, relative substance stability and specificity of the oxidation site.

Advancing Target Identification of Nitrated Phospholipids in Biological Systems by HCD Specific Fragmentation Fingerprinting in Orbitrap Platforms

Nitrated phospholipids have recently been detected in vitro and in vivo and associated with beneficial health effects. They were identified and quantified in biological samples by lipidomics methodologies using liquid chromatography-collision-induced dissociation (CID) tandem mass spectrometry (MS/MS) acquired with the linear ion trap mass spectrometer. Only a few studies have used higher-energy collision dissociation (HCD)-MS/MS in high-resolution Orbitraps to characterize nitrated phosphatidylserines and nitrated cardiolipins, highlighting the marked differences in the fragmentation patterns when using CID or HCD fragmentation methods. In this study, we aimed to evaluate the fragmentation of nitrated phosphatidylcholine and nitrated phosphatidylethanolamine species under HCD-MS/MS. We studied the effect of normalized collision energy (NCE) in the fragmentation pattern to identify the best acquisition conditions and reporter ions to detect nitrated phospholipids. The results showed that the intensity of the typical neutral loss of nitrous acid (HNO₂) diminishes with increasing NCE, becoming non-detectable for a higher NCE. Thus, the loss of HNO₂ could not be the most suitable ion/fragment for the characterization of nitrated phospholipids under HCD. In HCD-MS/MS new fragment ions were identified, corresponding to the nitrated fatty acyl chains, NO₂-RCOO⁻, (NO₂-RCOOH-H₂O + H)⁺, and (NO₂-RCOOH + H)⁺, suggested as potential reporter ions to detect nitrated phospholipids when using the HCD-MS/MS lipidomics analysis.

Formation of Oxidatively Modified Lipids as the Basis for a Cellular Epilipidome

While often regarded as a subset of metabolomics, lipidomics can better be considered as a field in its own right. While the total number of lipid species in biology may not exceed the number of metabolites, they can be modified chemically and biochemically leading to an enormous diversity of derivatives, many of which retain the lipophilic properties of lipids and thus expand the lipidome greatly. Oxidative modification by radical oxygen species, either enzymatically or chemically, is one of the major mechanisms involved, although attack by non-radical oxidants also occurs. The modified lipids typically contain more oxygens in the form of hydroxyl, epoxide, carbonyl and carboxylic acid groups, and nitration, nitrosylation, halogenation or sulfation can also occur. This article provides a succinct overview of the types of species formed, the reactive compounds involved and the specific molecular sites that they react with, and the biochemical or chemical mechanisms involved. In many cases, these modifications reduce the stability of the lipid, and breakdown products are formed, which themselves have interesting properties such as the ability to react with other biomolecules. Publications on the biological effects of modified lipids are growing rapidly, supporting the concept that some of these biomolecules have potential signaling and regulatory effects. The question therefore arises whether modified lipids represent an "epilipidome", analogous to the epigenetic modifications that can control gene expression.