Structural characterization of triacylglycerols as lithiated adducts by electrospray ionization mass spectrometry using low-energy collisionally activated dissociation on a triple stage quadrupole instrument

Structure Determination of Lipid A with Multiple Glycosylation Sites by Tandem MS of Lithium-Adducted Negative Ions

FLATn is a tandem mass spectrometric technique that can be used to rapidly generate spectral information applicable for structural elucidation of lipids like lipid A from Gram-negative bacterial species from a single bacterial colony. In this study, we extend the scope and capability of FLATn by tandem MS fragmentation of lithium-adducted molecular lipid A anions and fragments (FLATn-Li) that provides additional structural and diagnostic data from FLATn samples allowing for the discrimination of terminal phosphate modifications in a variety of pathogenic and environmental species. Using FLATn-Li, we elucidated the lipid A structure from several bacterial species, including novel structures from arctic bacterioplankton of the Duganella and Massilia genera that favor 4-amino-4-deoxy-l-arabinopyranose (Ara4N) modification at the 1-phosphate position and that demonstrate double glycosylation with Ara4N at the 1 and 4' phosphate positions simultaneously. The structures characterized in this work demonstrate that some environmental psychrophilic species make extensive use of this structural lipid A modification previously characterized as a pathogenic adaptation and the structural basis of resistance to cationic antimicrobial peptides. This observation extends the role of phosphate modification(s) in environmental species adaptation and suggests that Ara4N modification can functionally replace the positive charge of the phosphoethanolamine modification that is more typically found attached to the 1-phosphate position of modified lipid A.

Characterization of Triacylglycerol Estolide Isomers Using High-Resolution Tandem Mass Spectrometry with Nanoelectrospray Ionization

Triacylglycerol estolides (TG-EST) are biologically active lipids extensively studied for their anti-inflammatory and anti-diabetic properties. In this work, eight standards of TG-EST were synthesized and systematically investigated by nanoelectrospray tandem mass spectrometry. Mass spectra of synthetic TG-EST were studied with the purpose of enabling the unambiguous identification of these lipids in biological samples. TG-EST glycerol sn-regioisomers and isomers with the fatty acid ester of hydroxy fatty acid (FAHFA) subunit branched in the ω-, α-, or 10-position were used. Ammonium, lithium, and sodium adducts of TG-EST formed by nanoelectrospray ionization were subjected to collision-induced dissociation (CID) and higher-energy collisional dissociation (HCD). Product ion spectra allowed for identification of fatty acid (FA) and FAHFA subunits originally linked to the glycerol backbone and distinguished the α-branching site of the FAHFA from other estolide-branching isomers. The ω- and 10-branching sites were determined by combining CID with ozone-induced dissociation (OzID). Lithium adducts provided the most informative product ions, enabling characterization of FA, hydroxy fatty acid (HFA), and FAHFA subunits. Glycerol sn-regioisomers were distinguished based on the relative abundance of product ions and unambiguously identified using CID/OzID of lithium and sodium adducts.

Plasma-Droplet Reaction Systems: A Direct Mass Spectrometry Approach for Enhanced Characterization of Lipids at Multiple Isomer Levels

Neutral triacylglyceride (TG) lipids are critical in cellular function, signaling, and energy storage. Multiple molecular pathways control TG structure via nonselective routes making them structurally complex and analytically challenging to characterize. The presence of C=C bond positional isomers exacerbates this challenge as complete structural elucidation is not possible by conventional tandem mass spectrometric methods such as collision-induced dissociation (CID), alone. Herein, we report a custom-made coaxial contained-electrospray ionization (ESI) emitter that allows the fusion of plasma discharge with charged microdroplets during electrospray (ES). Etched capillaries were incorporated into this contained-ES emitter, facilitating the generation of reactive oxygen species (ROS) at low (3 kV) ESI voltages and allowing stable ESI ion signal to be achieved at an unprecedented high (7 kV) spray voltage. The analytical utility of inducing plasma discharge during electrospray was investigated using online ionization of neutral TGs, in situ epoxidation of unsaturation sites, and C=C bond localization via conventional CID mass spectrometry. Collisional activation of the lipid epoxide generated during the online plasma-droplet fusion experiment resulted in a novel fragmentation pattern that showed a quadruplet of diagnostic ions for confident assignment of C=C bond positions and subsequent isomer differentiation. This phenomenon enabled the identification of a novel TG lipid, composed of conjugated linoleic acid, that is isomeric with two other TG lipids naturally found in extra virgin olive oil. To validate our findings, we analyzed various standards of TG lipids, including triolein, trilinolein, and trilinolenin, and isomeric mixtures in the positive-ion mode, each of which produced the expected quadruplet diagnostic fragment ions. Further validation was obtained by analyzing standards of free fatty acids expected from the hydrolysis of the TG lipids in the negative-ion mode, together with isomeric mixtures. The chemistry governing the gas-phase fragmentation of the lipid epoxides was carefully elucidated for each TG lipid analyzed. This comprehensive shotgun lipidomic approach has the potential to impact biomedical research since it can be accomplished on readily available mass spectrometers without the need for instrument modification.

Ammonia Concentration in the Eluent Influences Fragmentation Pattern of Triacylglycerols in Mass Spectrometry Analysis

Correct assessment of the fatty acyl at the glycerol sn-2 position in triacylglycerol (TAG) analysis by liquid chromatography and mass spectrometry (LC-MS) is challenging. Ammonium hydroxide (NH₄OH) is the preferred choice for the solvent additive for the formation of the ammonium adduct ([M + NH₄]⁺). In this study, the influence of different NH₄OH concentrations in the eluents on TAG adduct formation and fragmentation under LC-MS analysis was assessed. Increasing NH₄OH concentrations delayed the chromatographic elution time according to a power function. The [M + NH₄]⁺ and [M + ACN + NH₄]⁺ adducts (where ACN means acetonitrile) were formed at all ammonium concentrations assayed. [M + ACN + NH₄]⁺ predominated above 18.26 mM [NH₄OH], and the intensity of [M + NH₄]⁺ dropped. TAG fragmentation for fatty acyl release in the MS^E was reduced with increasing [M + ACN + NH₄]⁺ adduct, which suggests that ACN stabilizes the adduct in a way that inhibits the rupture of the ester bonds in TAGs. A linear equation (H_sn-I = a × H_[M+NH4]+, where sn-I refers to the sn position of the glycerol (I = 1, 2, or 3) and H is the peak height) was deduced to quantify the dehydroxydiacylglycerol fragment intensity in relation to [M + NH₄]⁺ intensity in the full scan. This equation had a slope mean value of 0.369 ± 0.058 for the sn-1 and sn-3 positions, and of 0.188 ± 0.007 for the sn-2 position.

The impact of the complexing agent on the sensitivity of collision-induced dissociation spectra to fatty acid position for a set of XYZ-type triglycerides

RATIONALE

The development of an automated platform for the positional analysis of triglycerides (TAGs) based on electrospray ionization tandem mass spectrometry (ESI-MS/MS) continues to be pursued. This work evaluates the positional sensitivities of the collision-induced dissociation (CID) spectra of a representative set of XYZ triglycerides using sodium, lithium, and ammonium salts as complexing agents.

METHODS

A set of triglycerides were synthesized and analyzed via ESI-MS/MS using an ion trap mass spectrometer. Using three different complexing agents, the product ion spectra of the corresponding precursor ions for twelve XYZ TAGs were collected, where X, Y, and Z represent C_16:0 , C_18:1(c-9) , C_{18:2(cc-9,12)} , and C_{20:4(cccc-5,8,11,14)} fatty acid chains. These data were then used to prepare ternary plots for four positional isomer systems to evaluate the positional sensitivity differences among the three different complexing agents.

RESULTS

The positional sensitivities for each of the four positional isomer systems were robust for the sodium and lithium adducts. The CID data for the sodium and lithium TAGs demonstrated an unfavorable loss of the fatty acid in the center position and showed a higher sensitivity to fatty acid position, when compared with the CID data for ammonium adducts, especially for the arachidonic acid containing triglycerides.

CONCLUSIONS

The data shows that that the relative abundances of the DAG product ions for the XYZ-type TAGs when using sodium and lithium complexing agent adducts are sensitive to fatty acid position and are consistent for the diverse array of TAGs studied in this work. This suggests that using sodium or lithium as the complexing agent may be advantageous for the development of an automated platform for the positional analysis of complex TAG mixtures based on ESI-MS/MS.

The foundations and development of lipidomics

For over a century, the importance of lipid metabolism in biology was recognized but difficult to mechanistically understand due to the lack of sensitive and robust technologies for identification and quantification of lipid molecular species. The enabling technological breakthroughs emerged in the 1980s with the development of soft ionization methods (Electrospray Ionization and Matrix Assisted Laser Desorption/Ionization) that could identify and quantify intact individual lipid molecular species. These soft ionization technologies laid the foundations for what was to be later named the field of lipidomics. Further innovative advances in multistage fragmentation, dramatic improvements in resolution and mass accuracy, and multiplexed sample analysis fueled the early growth of lipidomics through the early 1990s. The field exponentially grew through the use of a variety of strategic approaches, which included direct infusion, chromatographic separation, and charge-switch derivatization, which facilitated access to the low abundance species of the lipidome. In this Thematic Review, we provide a broad perspective of the foundations, enabling advances, and predicted future directions of growth of the lipidomics field.

Automated data-driven mass spectrometry for improved analysis of lipids with dual dissociation techniques

Lipidomics is an important component of most multi-Omics systems biology studies and is largely driven by mass spectrometry (MS). Because lipids are tight regulators of multiple cellular functions, including energy homeostasis, membrane structures and cell signaling, lipidomics can provide a deeper understanding of variations underlying disease states and can become an even more powerful platform when combined with other omics, including genomics or proteomics. However, data analysis, especially in lipid annotation, poses challenges due to the heterogeneity of functional head groups and fatty acyl chains of varying hydrocarbon lengths and degrees of unsaturation. As there are various MS/MS fragmentation sites in lipids that are class-dependent, obtaining MS/MS data that includes as many fragment ions as possible is critical for structural characterization of lipids in lipidomics workflow. Here, we report an improved lipidomics methodology that resulted in increased coverage of lipidome using: 1) An automated data-driven MS/MS acquisition scheme in which inclusion and exclusion lists were automatically generated from the full scan MS of sample injections, followed by creation of updated lists over iterative analyses; and, 2) Incorporation of dual dissociation techniques of higher-energy collision dissociation and collision-induced dissociation for more accurate characterization of phosphatidylcholine species. Inclusion lists were created automatically based on full scan MS signals from samples and through iterative analyses, ions in the inclusion list that were fragmented were automatically moved to the exclusion list in subsequent runs. We confirmed that analytes with low MS response that did not undergo MS/MS events in conventional data-dependent analysis were successfully fragmented using this approach. Overall, this automated data-driven data acquisition approach resulted in a higher coverage of lipidome and the use of dual dissociation techniques provided additional information that was critical in characterizing the side chains of phosphatidylcholine species.

Enabling High Structural Specificity to Lipidomics by Coupling Photochemical Derivatization with Tandem Mass Spectrometry

Lipids have pivotal roles in many biological processes, including energy storage, signal transduction, and plasma membrane formation. A disruption of lipid homeostasis is found to be associated with a range of diseases, such as cardiovascular diseases, diabetes, and cancer. Fundamental lipid biology and disease diagnostics can benefit from monitoring lipid changes in cells, tissues, organs, or the whole biological system. Therefore, it is important to develop lipid analysis tools to achieve comprehensive lipid characterization and quantitation. Over the past two decades, mass spectrometry (MS) has become the method of choice for qualitative and quantitative analyses of lipids, owing to its high sensitivity, multiplexed analysis, and soft ionization features. With the rapid development and adoption of ultrahigh-resolution MS, isobaric lipids can now be routinely resolved. By contrast, the structural characterization and quantitation of isomeric lipids remain an analytical challenge. Although some lipid C═C location or sn-isomers can be resolved by chromatography, ion mobility, or selective ionization approaches, a detailed structural characterization on the lipidome-wide level needs to be achieved.Over the past six years, we have successfully combined the Paternò-Büchi (PB) reaction, which is a UV-promoted photocycloaddition reaction specific to the C═C, with tandem MS (MS/MS) to locate the C═C in lipids and quantify lipid C═C location isomers. The PB reactions have analytical advantages such as a simple experimental setup, rapid lipid C═C derivatization, and highly specific C═C cleavage during PB-MS/MS to produce abundant diagnostic ions. More importantly, without a need of isomer separation or a comparison to authentic standards, PB-MS/MS can be directly applied to identify and quantify a mixture of lipid C═C location isomers, often coexisting with molar ratios sensitive to the biological state of the system. The PB-MS/MS method is compatible with conventional shotgun lipidomics employing a nanoelectrospray ionization or a large-sale lipid structural analysis via liquid chromatography (LC) coupled to any mass spectrometer with tandem MS capability. The PB-MS/MS method is highly versatile, as a variety of PB reagents can be tailored to a broad range of applications. Besides UV-promoted PB reactions, visible-light PB reactions have also been developed to offer more flexibility for a lipid analysis. By using selected PB reagents, the sn-positions of fatty acyls can be resolved together with C═C locations in phospholipids. This method has been used in lipidomic analyses of tissue, blood, and plasma from animal models and clinical samples, demonstrating the potential of using lipid C═C or sn-location isomer ratios for phenotyping and disease diagnostics. Lipid isomer-resolving MS imagings of tissues and single-cell lipid analysis have also been demonstrated by a proper implementation of PB-MS/MS.

Overview of Lipidomic Analysis of Triglyceride Molecular Species in Biological Lipid Extracts

Triglyceride (TG) is a class of neutral lipids, which functions as an energy storage depot and is important for cellular growth, metabolism, and function. The composition and content of TG molecular species are crucial factors for nutritional aspects in food chemistry and are directly associated with several diseases, including atherosclerosis, diabetes, obesity, stroke, etc. As a result of the complexities of aliphatic moieties and their different connections/locations to the glycerol backbone in TG molecules, accurate identification of individual TG molecular species and quantitative assessment of TG composition and content are particularly challenging, even at the current stage of lipidomics development. Herein, methods developed for analysis of TG species, such as liquid chromatography-mass spectrometry with a variety of columns and different mass spectrometric techniques, shotgun lipidomics approaches, and ion-mobility-based analysis, are reviewed. Moreover, the potential limitations of the methods are discussed. It is our sincere hope that the overviews and discussions can provide some insights for researchers to select an appropriate approach for TG analysis and can serve as the basis for those who would like to establish a methodology for TG analysis or develop a new method when novel tools become available. Biologically accurate analysis of TG species with an enabling method should lead us toward improving the nutritional quality, revealing the effects of TG on diseases, and uncovering the underlying biochemical mechanisms related to these diseases.

Lipidomics of Environmental Microbial Communities. I: Visualization of Component Distributions Using Untargeted Analysis of High-Resolution Mass Spectrometry Data

Lipids, as one of the main building blocks of cells, can provide valuable information on microorganisms in the environment. Traditionally, gas or liquid chromatography coupled to mass spectrometry (MS) has been used to analyze environmental lipids. The resulting spectra were then processed through individual peak identification and comparison with previously published mass spectra. Here, we present an untargeted analysis of MS¹ spectral data generated by ultra-high-pressure liquid chromatography coupled with high-resolution mass spectrometry of environmental microbial communities. Rather than attempting to relate each mass spectrum to a specific compound, we have treated each mass spectrum as a component, which can be clustered together with other components based on similarity in their abundance depth profiles through the water column. We present this untargeted data visualization method on lipids of suspended particles from the water column of the Black Sea, which included >14,000 components. These components form clusters that correspond with distinct microbial communities driven by the highly stratified water column. The clusters include both known and unknown compounds, predominantly lipids, demonstrating the value of this rapid approach to visualize component distributions and identify novel lipid biomarkers.

A novel sn-1,3 specific lipase from Janibacter sp. as catalysts for the high-yield synthesis of long-medium-long type structured triacylglycerols

Our study offers a novel sn-1,3 specific lipase MAJ1 from marine member Janibacter sp. strain HTCC2649 for preparing long-medium-long (LML) type structured triacylglycerols (TAGs). Firstly, the resin ECR1030 was selected as a suitable support for the immobilization of lipase MAJ1. An efficient synthesis of LML-type structured TAGs by the immobilized lipase MAJ1-catalyzed interesterification of methyl palmitate and tricaprylin was studied in a solvent-free system. The reaction conditions, including substrate molar ratio, temperature and enzyme loading, were optimized. Under the optimum conditions (immobilized lipase MAJ1 of 45 U/g, substrate molar ratio of 4:1, temperature of 35 °C, reaction time of 24 h), the structured TAGs with double long chains (DLCST) were obtained in a yield of 44.3 mol%. Secondly, multi-dimensional mass spectrometry-based shotgun lipidomics (MDMS-SL) was employed to quantify each TAG positional isomer in DLCST. The content of 1,3-dipalmitoyl-2-capryloyl-sn-glycerol in DLCST was 97.6% determined by the MDMS-SL technology.

Perspective on Emerging Mass Spectrometry Technologies for Comprehensive Lipid Structural Elucidation

Lipids and metabolites are of interest in many clinical and research settings because it is the metabolome that is increasingly recognized as a more dynamic and sensitive molecular measure of phenotype. The enormous diversity of lipid structures and the importance of biological structure-function relationships in a wide variety of applications makes accurate identification a challenging yet crucial area of research in the lipid community. Indeed, subtle differences in the chemical structures of lipids can have important implications in cellular metabolism and many disease pathologies. The speed, sensitivity, and molecular specificity afforded by modern mass spectrometry has led to its widespread adoption in the field of lipidomics on many different instrument platforms and experimental workflows. However, unambiguous and complete structural identification of lipids by mass spectrometry remains challenging. Increasingly sophisticated tandem mass spectrometry (MS/MS) approaches are now being developed and seamlessly integrated into lipidomics workflows to meet this challenge. These approaches generally either (i) alter the type of ion that is interrogated or (ii) alter the dissociation method in order to improve the structural information obtained from the MS/MS experiment. In this Perspective, we highlight recent advances in both ion type alteration and ion dissociation methods for lipid identification by mass spectrometry. This discussion is aimed to engage investigators involved in fundamental ion chemistry and technology developments as well as practitioners of lipidomics and its many applications. The rapid rate of technology development in recent years has accelerated and strengthened the ties between these two research communities. We identify the common characteristics and practical figures of merit of these emerging approaches and discuss ways these may catalyze future directions of lipid structural elucidation research.

In Situ Separation and Analysis of Lipids by Paper Spray Ionization Mass Spectrometry

Paper spray ionization (PSI) is an extractive ambient ionization technique for mass spectrometry (MS), whereby a triangular paper tip serves as the sampling base and the electrospray tip. During PSI, analytes are extracted and transported to the edge of the paper tip by the applied spraying solvent. Analytes can be purified from a sample matrix and separated from each other by this transportation process. In this study, we investigated and utilized the analyte transportation process of PSI for the in situ separation and analysis of lipid mixtures. We found that differential transport of phosphatidylcholine (PC) and triacylglycerol (TAG), the two most abundant lipid classes in animals, occurred during PSI. We also found that the order in which these lipids moved strongly depended on how the spraying solvent was applied to the paper base. The more polar PC moved faster than the less polar TAG during PSI, when a polar solvent was slowly fed into a paper tip, whereas TAG was transported faster than PC when excess solvent was applied to the tip at once. In addition, we achieved a complete separation and detection of PC and TAG by slowly supplying a nonpolar solvent to a PSI tip.

Double bond localization in unsaturated rhamnolipid precursors 3-(3-hydroxyalkanoyloxy)alkanoic acids by liquid chromatography-mass spectrometry applying online Paternò-Büchi reaction

Lipids are biomolecules with a broad variety of chemical structures, which renders them essential not only for various biological functions but also interestingly for biotechnological applications. Rhamnolipids are microbial glycolipids with surface-active properties and are widely used biosurfactants. They are composed of one or two L-rhamnoses and up to three hydroxy fatty acids. Their biosynthetic precursors are 3-hydroxy(alkanoyloxy)alkanoic acids (HAAs). The latter are also present in cell supernatants as complex mixtures and are extensively studied for their potential to replace synthetically derived surfactants. The carbon chain lengths of HAAs determine their physical properties, such as their abilities to foam and emulsify, and their critical micelle concentration. Despite growing biotechnological interest, methods for structural elucidation are limited and often rely on hydrolysis and analysis of free hydroxy fatty acids losing the connectivity information. Therefore, a high-performance liquid chromatography-mass spectrometry method was developed for comprehensive structural characterization of intact HAAs. Information is provided on chain length and number of double bonds in each hydroxy fatty acid and their linkage by tandem mass spectrometry (MS/MS). Post-column photochemical derivatization by online Paternὸ-Büchi reaction and MS/MS fragmentation experiments generated diagnostic fragments allowing structural characterization down to the double bond position level. Furthermore, the presented experiments demonstrate a powerful approach for structure elucidation of complex lipids by tailored fragmentation.

One Pot Use of Combilipases for Full Modification of Oils and Fats: Multifunctional and Heterogeneous Substrates

Lipases are among the most utilized enzymes in biocatalysis. In many instances, the main reason for their use is their high specificity or selectivity. However, when full modification of a multifunctional and heterogeneous substrate is pursued, enzyme selectivity and specificity become a problem. This is the case of hydrolysis of oils and fats to produce free fatty acids or their alcoholysis to produce biodiesel, which can be considered cascade reactions. In these cases, to the original heterogeneity of the substrate, the presence of intermediate products, such as diglycerides or monoglycerides, can be an additional drawback. Using these heterogeneous substrates, enzyme specificity can promote that some substrates (initial substrates or intermediate products) may not be recognized as such (in the worst case scenario they may be acting as inhibitors) by the enzyme, causing yields and reaction rates to drop. To solve this situation, a mixture of lipases with different specificity, selectivity and differently affected by the reaction conditions can offer much better results than the use of a single lipase exhibiting a very high initial activity or even the best global reaction course. This mixture of lipases from different sources has been called “combilipases” and is becoming increasingly popular. They include the use of liquid lipase formulations or immobilized lipases. In some instances, the lipases have been coimmobilized. Some discussion is offered regarding the problems that this coimmobilization may give rise to, and some strategies to solve some of these problems are proposed. The use of combilipases in the future may be extended to other processes and enzymes.

Antidiabetic components from the hexane extract of red kidney beans (Phaseolus vulgaris L.): isolation and structure determination

Red kidney beans (Phaseolus vulgaris L.) contain bioactive compounds that are known to exhibit antidiabetic effects via inhibition of α-glucosidase. However, information on the nonpolar components that exhibit antidiabetic activity is limited. Here, we report the isolation and structure determination of components with α-glucosidase inhibitory activity, which were obtained from the hexane extract of red kidney beans. Triacylglycerols (TAGs) were identified as the major components exhibiting inhibitory activity against α-glucosidase. The chemical structure of TAGs was determined by a combination of GC-MS and UPLC-MS/MS. The primary TAGs identified were LnLnLn (trilinolenin) and LnLLn (1,3-dilinolenoyl-2-linoleoyl glycerol). The major fatty acids present in these TAGs were α-linolenic acid (ω-3) and linoleic acid (ω-6). These TAGs were also found to inhibit the α-glucosidase activity in a similar fashion as acarbose. These results suggest that TAGs have potency as antidiabetics and support the potential suitability of red kidney beans for diabetes treatment.

Fragmentation Pathways of Cationized, Saturated, Short-Chain Triacylglycerols: Lithiated and Sodiated Tripropanoyl- and Trihexanoylglycerol

Many methods, often depending on tandem mass spectrometry, have been developed for analysis of complex mixtures of triacylglycerols (TAGs), especially in clinical diagnostics and food authentication. Understanding the fragmentation mechanisms of cationized TAGs has proved problematic. To obtain a better understanding of viable mechanisms, detailed studies including double- and triple-stage tandem mass spectrometry were made using electrospray ionization on lithiated and sodiated tripropanoyl- and trihexanoylglycerols. Density functional theory computations, including a functional parameterized for van der Waals interactions, were used to correlate computed energies with mass spectra. Losses of both a neutral salt and a neutral acid corresponding to a glycerol side chain were observed as major product ions in MS² experiments. Signal intensities at low collision energies correlated well with computed energies. However, an important difference between the lithiated and sodiated ions was the appearance of the sodium cation as a major fragmentation product. Computations on the product ions resulting from the loss of a neutral acid indicated multiple structures for the lithiated ions but mainly a single structure for the sodiated ions. The lithiated product ions could be fragmented further (pseudo-MS³) to give additional structural information, whereas the sodiated ions gave only m/z 23. The longer chain TAG, while giving a much less intense mass spectrum than the shorter chain TAG, gave comparable MS² and MS³ product ion spectra. Taken together, the spectral and computational work described herein offer a new and detailed pathway for collision-induced fragmentation of lithiated and sodiated saturated TAGs.

Insights into the Physiology and Metabolism of a Mycobacterial Cell in an Energy-Compromised State

Mycobacterium tuberculosis, a bacterium that causes tuberculosis, poses a serious threat, especially due to the emergence of drug-resistant strains. M. tuberculosis and other mycobacterial species, such as M. smegmatis, are known to generate an inadequate amount of energy by substrate-level phosphorylation and mandatorily require oxidative phosphorylation (OXPHOS) for their growth and metabolism. Hence, antibacterial drugs, such as bedaquiline, targeting the multisubunit ATP synthase complex, which is required for OXPHOS, have been developed with the aim of eliminating pathogenic mycobacteria. Here, we explored the influence of suboptimal OXPHOS on the physiology and metabolism of M. smegmatis M. smegmatis harbors two identical copies of atpD, which codes for the β subunit of ATP synthase. We show that upon deletion of one copy of atpD (M. smegmatis ΔatpD), M. smegmatis synthesizes smaller amounts of ATP and enters into an energy-compromised state. The mutant displays remarkable phenotypic and physiological differences from the wild type, such as respiratory slowdown, reduced biofilm formation, lesser amounts of cell envelope polar lipids, and increased antibiotic sensitivity compared to the wild type. Additionally, M. smegmatis ΔatpD overexpresses genes belonging to the dormancy operon, the β-oxidation pathway, and the glyoxylate shunt, suggesting that the mutant adapts to a low energy state by switching to alternative pathways to produce energy. Interestingly, M. smegmatis ΔatpD shows significant phenotypic, metabolic, and physiological similarities with bedaquiline-treated wild-type M. smegmatis We believe that the identification and characterization of key metabolic pathways functioning during an energy-compromised state will enhance our understanding of bacterial adaptation and survival and will open newer avenues in the form of drug targets that may be used in the treatment of mycobacterial infections.IMPORTANCE M. smegmatis generates an inadequate amount of energy by substrate-level phosphorylation and mandatorily requires oxidative phosphorylation (OXPHOS) for its growth and metabolism. Here, we explored the influence of suboptimal OXPHOS on M. smegmatis physiology and metabolism. M. smegmatis harbors two identical copies of the atpD gene, which codes for the ATP synthase β subunit. Here, we carried out the deletion of only one copy of atpD in M. smegmatis to understand the bacterial survival response in an energy-deprived state. M. smegmatis ΔatpD shows remarkable phenotypic, metabolic, and physiological differences from the wild type. Our study thus establishes M. smegmatis ΔatpD as an energy-compromised mycobacterial strain, highlights the importance of ATP synthase in mycobacterial physiology, and further paves the way for the identification of novel antimycobacterial drug targets.

Direct liquid extraction surface analysis mass spectrometry of cell wall lipids from mycobacteria: Salt additives for decreased spectral complexity

RATIONALE

Lipids are important mycobacterium cell wall constituents; changes are linked with drug resistance. Liquid extraction surface analysis (LESA) enables direct sampling in a highly sensitive manner. Here we describe protocols for the analysis of lipids from bacterial colonies. Lipids form various adducts, complicating spectra. Salt additives were investigated to circumvent this problem.

METHODS

Chloroform:methanol mixtures were studied for lipid extraction and analysis by LESA-MS. The inclusion of (ESI-compatible) acetate salts of sodium, potassium or lithium in the extraction solvent was investigated.

RESULTS

We report the detection of bacterial cell wall lipids from mycobacterial species using LESA for the first time. Sampling protocols were optimised for the use of volatile extraction solvents. The inclusion of acetate salt additives in the sampling solvent significantly reduces spectral complexity in comparison with no additives being used.

CONCLUSIONS

LESA offers a sensitive technique for bacterial lipid phenotyping. The inclusion of an acetate salt in the sampling solvent drives adduct formation towards a specific adduct type and thus significantly reduces spectral complexity.

In-depth lipidomic analysis of tri-, di-, and mono-acylglycerols released from milk fat after in vitro digestion

Milk fat is arguably one of the most complex fats found in nature and varies widely between animal species. Analysis of its digestion products is tremendously challenging, due to the complexity, diversity, and large range of concentrations of triacylglycerols (TAGs) and their digestion products (i.e. diacylglycerols (DAGs), monoacylglycerols (MAGs), and free fatty acids (FFAs)). Therefore, a method combined the solid phase extraction (SPE), high-performance liquid chromatography (HPLC) and multi-dimension mass spectrometry (MDMS) was developed to identify and semi-quantify the TAGs, DAGs and MAGs in milk fat after in vitro digestion. Up to 105, 64, 14 and 30 species of TAGs, DAGs, MAGs, and FFAs were determined with their concentrations of 0.01-22.3, 0.01-39.2, 0.01-47.8, and 0.04-191.0 mg/g fat, respectively, during the in vitro digestion of cow and sheep milk. The validation of the method shows that this method was precise and reliable.

An Organic Chemist's Guide to Electrospray Mass Spectrometric Structure Elucidation

Tandem mass spectrometry is an important tool for structure elucidation of natural and synthetic organic products. Fragmentation of odd electron ions (OE⁺) generated by electron ionization (EI) was extensively studied in the last few decades, however there are only a few systematic reviews available concerning the fragmentation of even-electron ions (EE⁺/EE⁻) produced by the currently most common ionization techniques, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). This review summarizes the most important features of tandem mass spectra generated by collision-induced dissociation fragmentation and presents didactic examples for the unexperienced users.

Mapping the triglyceride distribution in NAFLD human liver by MALDI imaging mass spectrometry reveals molecular differences in micro and macro steatosis

Hepatic lipid accumulation, mainly in the form of triglycerides (TGs), is the hallmark of non-alcoholic fatty liver disease (NAFLD). To date, the spatial distribution of individual lipids in NAFLD-affected livers is not well characterized. This study aims to map the triglyceride distribution in normal human liver samples and livers with NAFLD and cirrhosis with imaging mass spectrometry (MALDI IMS). Specifically, whether individual triglyceride species differing by fatty acid chain length and degree of saturation correlate with the histopathological features of NAFLD as identified with classical H&E. Using a recently reported sodium-doped gold-assisted laser desorption/ionization IMS sample preparation, 20 human liver samples (five normal livers, five samples with simple steatosis, five samples with steatohepatitis, and five samples with cirrhosis) were analyzed at 10-μm lateral resolution. A total of 24 individual lipid species, primarily neutral lipids, were identified (22 TGs and two phospholipids). In samples with a low level of steatosis, TGs accumulated around the pericentral zone. In all samples, TGs with different degrees of side-chain saturation and side-chain length demonstrated differential distribution. Furthermore, hepatocytes containing macro lipid droplets were highly enriched in fully saturated triglycerides. This enrichment was also observed in areas of hepatocyte ballooning in samples with steatohepatitis and cirrhosis. In conclusion, macro lipid droplets in NAFLD are enriched in fully saturated triglycerides, indicating a possible increase in de novo lipogenesis that leads to steatohepatitis and cirrhosis.

Mass spectrometry-based shotgun lipidomics - a critical review from the technical point of view

Over the past decade, mass spectrometry (MS)-based "shotgun lipidomics" has emerged as a powerful tool for quantitative and qualitative analysis of the complex lipids in the biological system. The aim of this critical review is to give the interested reader a concise overview of the current state of the technology, focused on lipidomic analysis by mass spectrometry. The pros and cons, and pitfalls associated with each available "shotgun lipidomics" method are discussed; and the new strategies for improving the current methods are described. A list of important papers and reviews that are sufficient rather than comprehensive, covering all the aspects of lipidomics including the workflow, methodology, and fundamentals is also compiled for readers to follow. Graphical abstract ᅟ.

Challenges in Mass Spectrometry-based Lipidomics of Neutral Lipids

Mass spectrometry has played a critical role in the identification and quantitation of neutral lipids such as cholesteryl esters and triacylglycerols present in biological extracts. Various strategies have emerged in order to carry out such lipidomics studies since a large number of neutral lipid molecular species exist in tissues. These include both shotgun approaches as well as those engaging liquid chromatographic separation of species prior to mass spectrometric analysis. Nonetheless challenges remain at every level of the lipidomics experiment, including extraction of lipids, identification of specific species, and quantitation of the vast array of lipids present in the sample extract. Unambiguous identification of molecular species present (qualitative analysis) as well as precise quantitation remains as significant challenges. The relative quantitation enables quite accurate assessment of fold changes of complex lipid species without exact quantitation. The availability of reference standard material as well as relevant internal standards continue to be limited.

The Impact of the Complexing Cation on the Sensitivity of the Collisional-Induced Dissociation Spectra to Fatty Acid Position for a Set of YXY/YYX-type Triglycerides

RATIONAL

The development of an automated platform for the positional analysis of triglycerides based on electrospray tandem mass spectrometry continues to be pursued. This work compares the positional sensitivities of the collisional-induced dissociation spectra for a representative set of YXY/YYX triglycerides using ammonium, silver, sodium and lithium as complexing agents.

METHODS

A set of triglycerides were synthesized and analyzed by electrospray tandem mass spectrometry using an ion trap mass spectrometer. Using different salt additives, the product ion spectra of the corresponding parent ions for twelve systems of the form YXY/YYX, where Y and X represent C16:0 , C18:1(c-9), C18:2(cc-9,12) and C20:4(cccc-5,8,11,14) , were collected. The data was used to prepare two-point calibration plots for each of the twelve positional isomer systems using each of the four complexing agents.

RESULTS

The positional sensitivities for all twelve positional isomer systems were robust for both the sodium and lithium TAG adducts. The CID data for both the sodium and lithium TAG adducts are much less sensitive to the degree of unsaturation and double bond position of the fatty acids constituents than the CID data for the ammonium adducts.

CONCLUSION

Using sodium or lithium TAG adducts may be advantageous for the development of an accurate predictive model for performing positional analysis of complex TAG mixtures based on electrospray tandem mass spectrometry. Ammonium adducts are likely complicated by the ability of the ammonium ion to provide extra stability to some parent ions through hydrogen bond-like interactions.

A simple and economical strategy for obtaining calibration plots for relative quantification of positional isomers of YYX/YXY triglycerides using high-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Positional analysis of intact triglycerides could provide greater insights into the link between fatty acid position and lipotoxic diseases. However, this methodology has been impeded by lack of commercial availability of positionally pure triglycerides. This work reports on a strategy for defining calibration plots for YXY/YYX triglyceride systems based on the product ion intensities in the collision-induced dissociation spectra of ammoniated precursor ions.

METHODS

A set of triglycerides were synthesized and analyzed by electrospray ionization tandem mass spectrometry using an ion trap mass spectrometer. The product ion spectra of the ammoniated precursor ions were collected for 42 triglyceride systems of the form YXY/YYX, where Y represents C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} . Three-point calibration plots were prepared by plotting the relative abundance of the YY⁺ product ion vs. the relative abundance of the YYX positional isomer.

RESULTS

The calibration plots were shown to give relative abundances of positional isomers accurate to within ±0.02 for most systems. Using an ion trap, under a controlled set of collision parameters, the slopes of the calibration plots can be used to compare the sensitivities of the product ion intensities to fatty acid position for various triglyceride systems. The average slopes of the calibration plots for the C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} systems were 0.29 ± 0.05, 0.21 ± 0.05 and 0.045 ± 0.005, respectively.

CONCLUSIONS

While the presence of multiple unsaturated fatty acids tends to slightly decrease the slopes of the calibration plots, the data suggest that the sensitivities are sufficient for performing positional analysis of most triglyceride systems. However, the presence of unsaturated fatty acids that contain double bonds close to the carbonyl group, such as arachidonic acid, tends to dramatically decrease positional sensitivity.

Ozone-induced dissociation on a traveling wave high-resolution mass spectrometer for determination of double-bond position in lipids

RATIONALE

The position of C=C within fatty acyl chains affects the biological function of lipids. Ozone-induced dissociation mass spectrometry (OzID-MS) has great potential in determination of lipid double-bond position, but has generally been implemented on low-resolution ion trap mass spectrometers. In addition, most of the OzID-MS experiments carried out so far were focused on the sodiated adducts of lipids; fragmentation of the most commonly observed protonated ions generated in LC/MS-based lipidomics workflow has been less explored.

METHODS

Ozone generated in line from an ozone generator was connected to the trap and transfer gas supply line of a Synapt G2 high-resolution mass spectrometer. Protonated ions of different phosphatidylcholines (PC) were generated by electrospray ionization through direct infusion. Different parameters, including traveling wave height and velocity, trap entrance and DC potential, were adjusted to maximize the OzID efficiency. sn-positional isomers and cis/trans isomers of lipids were compared for their reactivity with ozone.

RESULTS

Traveling wave height and velocity were tuned to prolong the encounter time between lipid ions and ozone, and resulted in improved OzID efficiency, as did increasing trapping region DC and entrance potential. Under optimized settings, at least 1000 times enhancement in OzID efficiency was achieved compared to that under default settings for monounsaturated PC standards. Monounsaturated C=C in the sn-2 PC isomer reacted faster with ozone than the sn-1 isomer. Similarly, the C=C in trans PC reacted faster than in cis PC.

CONCLUSIONS

This is the first implementation of OzID in the trap and transfer region of a traveling wave enabled high-resolution mass spectrometer. The OzID reaction efficiency is significantly improved by slowing down ions in the trap region for their prolonged interaction with ozone. This will facilitate application of high-resolution OzID-MS in lipidomics.

Sphingolipids in marine microalgae: Development and application of a mass spectrometric method for global structural characterization of ceramides and glycosphingolipids in three major phyla

Sphingolipid compositions are crucial for the structural and physiological properties of microalgae membranes. In the present study, we developed a quadrupole time-of-flight (Q-TOF) mass spectrometric method based on MS^E data collection for the identification of sphingolipids with high efficiency, selectivity, sensitivity and mass accuracy and applied this method for precise structural identification and quantitative profiling of ceramides and glycosphingolipids in total lipid extracts from 17 strains of microalgae, including 11 strains of diatom, 3 strains of dinoflagellate and 3 strains of haptophyta. Using this method, four species of sphingolipids including 27 ceramides, 13 monosaccharide ceramides, 18 disaccharide ceramides and 18 trisaccharide ceramides were identified. The compositions of sphingolipid-included glycosyl moieties, long chain bases and N-acyl chains showed a significant difference among different microalgae categories. Some long chain bases including d19:2, d19:3 and d19:4, glycosyl moieties including disaccharide and trisaccharide, and N-acyl chains such as 14:0, 14:1, 24:0, 24:1, h18:1, h19:1 and h22:0-2 can be chosen as the molecular signature for microalgae from three major phyla. This methodology will be useful for a wide range of physiological and pathological studies of sphingolipids. Furthermore, the diversity of sphingolipid structure could provide a new criterion for microalgae chemotaxonomy.

Lipid accumulation and metabolic analysis based on transcriptome sequencing of filamentous oleaginous microalgae Tribonema minus at different growth phases

Filamentous oleaginous microalgae specie Tribonema minus is a promising feedstock for biodiesel production. However, the metabolic mechanism of lipid production in this filamentous microalgal specie remains unclear. Here, we compared the lipid accumulation of T. minus at different growth phases, and described the de novo transcriptome sequencing and assembly and identified important pathways and genes involved in TAG production. Total lipid increased by 2.5-fold and its TAG level in total lipid reached 81.1% at stationary phase. Using the genes involved in the lipid metabolism, the TAG biosynthesis pathways were generated. Moreover, results also demonstrated that, in addition to the observed overexpression of the fatty acid synthesis pathway, TAG production at stationary growth phase was bolstered by repression of the β-oxidation pathway, up-regulation of genes that funnels acetyl-CoA to lipid biosynthesis, especially gene encoding for phospholipid:diacylglycerol acyltransferase (PDAT) which funnels DAG to TAG biosynthesis.

Simultaneous structural identification of diacylglyceryl-N-trimethylhomoserine (DGTS) and diacylglycerylhydroxymethyl-N,N,N-trimethyl-β-alanine (DGTA) in microalgae using dual Li+ /H+ adduct ion mode by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry

RATIONALE

Diacylgycerol-N-trimethylhomoserine (DGTS) and diacylglycerylhydroxymethyl-N,N,N-trimethyl-β-alanine (DGTA) are structural isomers that are the most commonly described betaine lipids in microalgae. The structural differentiation and precise identification of DGTS and DGTA in microalgae need to be established during mass spectrometry analysis.

METHODS

Total lipid was extracted from Amphora spp. with CHCl₃ /CH₃ OH (1:1, v/v). The qualitative analysis of DGTS and DGTA in Amphora spp. was carried out using Li⁺ /H⁺ dual mode by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry operating in MS^E mode (UPLC/QTOF MS^E ).

RESULTS

Characteristic fragment ions [C₁₀ H₂₂ O₅ N]⁺ at m/z 236.15 and [C₇ H₁₄ O₂ N]⁺ at m/z 144.10 from the [M + H]⁺ precursor ion can be used for the qualitative analysis of both DGTA and DGTS, whereas the loss of m/z 87 and 74 from the [M + Li]⁺ precursor ion are specific for DGTS, and the loss of m/z 103 from the [M + Li]⁺ precursor ion is only for DGTA. As a result, 9 DGTSs and 16 DGTAs with different fatty acids were identified simultaneously in Amphora spp. Semi-quantitative analysis of DGTS and DGTA in Amphora spp. showed that the contents of DGTS ranged from 0.003 to 0.438 nmol mg^-1 dw, and that of DGTA from 0.004 to 0.414 nmol mg^-1 dw.

CONCLUSIONS

This is the first report to achieve the ambiguous structural identification of DGTS and DGTA by UPLC/QTOF MS^E using dual Li⁺ /H⁺ adduct ion mode, which has remained a challenge in the past. It could provide new insights into their phylogeny and be helpful to characterize the natural phytoplankton communities as intact polar lipid biomarkers. Copyright © 2017 John Wiley & Sons, Ltd.

Systematic evaluation of serum and plasma collection on the endogenous metabolome

AIM

In metabolomics research, the use of different blood collection methods may influence endogenous metabolites.

MATERIALS & METHODS

Ultra HPLC coupled with MS/MS was applied together with multivariate statistics to investigate metabolomics differences in serum and plasma samples handled by different anticoagulants. A total of 135 known representative metabolites were assessed for comprehensive evaluation of the effects of anticoagulants.

RESULTS

Exogenous factors, including separation gel ingredients from the serum collection tubes and the anticoagulants, affected mass spectrometer detection. Heparin plasma yielded the best detection of different functional groups and is therefore the optimal blood specimen for metabolomics research, followed by potassium oxalate plasma.

Rapid Quantification of Low-Viscosity Acetyl-Triacylglycerols Using Electrospray Ionization Mass Spectrometry

Acetyl-triacylglycerols (acetyl-TAG) possess an sn-3 acetate group, which confers useful chemical and physical properties to these unusual triacylglycerols (TAG). Current methods for quantification of acetyl-TAG are time consuming and do not provide any information on the molecular species profile. Electrospray ionization mass spectrometry (ESI-MS)-based methods can overcome these drawbacks. However, the ESI-MS signal intensity for TAG depends on the aliphatic chain length and unsaturation index of the molecule. Therefore response factors for different molecular species need to be determined before any quantification. The effects of the chain length and the number of double-bonds of the sn-1/2 acyl groups on the signal intensity for the neutral loss of short chain length sn-3 groups were quantified using a series of synthesized sn-3 specific structured TAG. The signal intensity for the neutral loss of the sn-3 acyl group was found to negatively correlated with the aliphatic chain length and unsaturation index of the sn-1/2 acyl groups. The signal intensity of the neutral loss of the sn-3 acyl group was also negatively correlated with the size of that chain. Further, the position of the group undergoing neutral loss was also important, with the signal from an sn-2 acyl group much lower than that from one located at sn-3. Response factors obtained from these analyses were used to develop a method for the absolute quantification of acetyl-TAG. The increased sensitivity of this ESI-MS-based approach allowed successful quantification of acetyl-TAG in various biological settings, including the products of in vitro enzyme activity assays.

Analysis of the lithiated leucosceptroids from Leucosceptrum canum to facilitate their identification and differentiation by electrospray ionization tandem mass spectrometry

RATIONALE

Leucosceptroids, isolated from Leucosceptrum canum (L. canum), are sesterterpenoids with novel molecular scaffolds which possess potent antifeedant activities. Their molecular scaffolds comprise a 5,6,5-framework with a great diversity due to different hydroxylation and substituent positions. The biological activities of leucosceptroids are affected by the subtle structural differences. The structural characterization and differentiation of the leucosceptroid isomers are of great importance.

METHODS

Firstly, different kinds of cation adducts of leucosceptroids were examined by adding alkali metal ions to the solution, and the lithiated adducts of leucosceptroid were found to be readily formed and yielded characteristic fragment ions under collision-induced dissociation (CID). Then, a systematic mass spectrometric investigation of the [M + Li](+) ions was carried out to clarify their characteristic fragment pathways by electrospray ionization quadrupole time-of-flight-type tandem mass spectrometry (ESI-QTOF-MS/MS). The proposed fragmentation pathways were confirmed through ion trap ESI-MS(n) (n ≥ 3) spectra. Finally, the proposed MS/MS method was applied to investigate the extracts of L. canum.

RESULTS

A specific fragmentation pathway of the lithiated adduct, which leads to the production of diagnostic ions of leucosceptroids, was observed. This fragmentation is initiated by cleavage of the C-ring and leads to formation of two types of ions by further dissociation. Both pathways could yield characteristic fragment ions, which could be used to define the substituents at the skeletal structure and at the C-ring. The representative characteristic fragmentations of [M + Li](+) ions and the proposed fragmentation pathways were successfully adopted to investigate the L. canum extracts, and a total of eleven leucosceptroids were identified or tentatively characterized.

CONCLUSIONS

The characteristic fragmentation pathways of [M + Li](+) species of leucosceptroid isomers were proposed. Three types of leucosceptroid isomers were successfully differentiated. Eleven leucosceptroids were characterized from L. canum extracts. The fragmentation knowledge will facilitate the analysis of leucosceptroids and other sesterterpenoids in future research. Copyright © 2016 John Wiley & Sons, Ltd.

Improved Butanol-Methanol (BUME) Method by Replacing Acetic Acid for Lipid Extraction of Biological Samples

Extraction of lipids from biological samples is a critical step in lipidomics, especially for shotgun lipidomics where lipid extracts are directly infused into a mass spectrometer. The butanol-methanol (BUME) extraction method was originally developed to extract lipids from plasma samples with 1 % acetic acid. Considering some lipids are sensitive to acidic environments, we modified this protocol by replacing acetic acid with lithium chloride solution and extended the modified extraction to tissue samples. Although no significant reduction of plasmalogen levels in the acidic BUME extracts of rat heart samples was found, the modified method was established to extract various tissue samples, including rat liver, heart, and plasma. Essentially identical profiles of the majority of lipid classes were obtained from the extracts of the modified BUME and traditional Bligh-Dyer methods. However, it was found that neither the original, nor the modified BUME method was suitable for 4-hydroxyalkenal species measurement in biological samples.

Sodium-Doped Gold-Assisted Laser Desorption Ionization for Enhanced Imaging Mass Spectrometry of Triacylglycerols from Thin Tissue Sections

The deposition of sodium salts followed by a sputtered layer of gold has been demonstrated to be a power combination for the analysis of triacylglycerols (TAGs) from tissue sections by laser desorption ionization (LDI) imaging mass spectrometry (IMS). Various sodium salts were tested for their capability to ionize TAGs and their ability to produce fast drying, small crystals (≤3 μm). The spray deposition of a sodium acetate and carbonate buffer mixture at pH 10.3 on which a 28 ± 3 nm sputtered layer of gold (Au-CBS) is subsequently deposited was found to provide the most effective combination for TAG analysis by high imaging resolution IMS. Under these conditions, a 30-fold increase in TAG signal intensity was observed when compared to matrix-assisted LDI (MALDI) methods using 2,5-dihydrobenzoic acid as matrix. Furthermore, Au-CBS led to an increase in the number of detected TAG species from ∼7 with DHB to more than 25 with the novel method, while few phospholipid signals were observed. These results were derived from the IMS investigation of fresh frozen mouse liver and rabbit adrenal gland tissue sections with a range of higher spatial resolutions between 35 and 10 μm. Au-CBS-LDI MS presents a highly sensitive and specific alternative to MALDI MS for imaging of TAGs from tissue sections. This novel approach has the potential to provide new biological insights on the role of TAGs in both health and disease.

Analysis of diacylglycerols by ultra performance liquid chromatography-quadrupole time-of-flight mass spectrometry: Double bond location and isomers separation

Diacylglycerols (DAGs) are important lipid intermediates and have been implicated in human diseases. Isomerism complicates their mass spectrometric analysis; in particular, it is difficult to identify fatty acid substituents and locate the double bond positions in unsaturated DAGs. We have developed an analytical strategy using ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF MS) in conjunction with dimethyl disulfide (DMDS) derivatization and collision cross-section (CCS) measurement to characterize DAGs in biological samples. The method employs non-aqueous reversed-phase chromatographic separation and profile collision energy (CE) mode for MS(E) and MS/MS analyses. Three types of fragment ions were produced simultaneously. Hydrocarbon ions (m/z 50-200) obtained at high CE helped to distinguish unsaturated and saturated DAGs rapidly. Neutral loss ions and acylium ions (m/z 300-400) produced at low CE were used to identify fatty acid substituents. Informative methyl thioalkane fragment ions were used to locate the double bonds of unsaturated DAGs. Mono-methylthio derivatives were formed mainly by the reaction of DAGs with DMDS, where methyl thiol underwent addition to the first double bond farthest from the ester terminus of unsaturated fatty acid chains. The addition of CCS values maximized the separation of isomeric DAG species and improved the confidence of DAG identification. Fourteen DAGs were identified in mouse myotube cells based on accurate masses, characteristic fragment ions, DMDS derivatization, and CCS values.

Sequential Collision- and Ozone-Induced Dissociation Enables Assignment of Relative Acyl Chain Position in Triacylglycerols

Unambiguous identification of isomeric lipids by mass spectrometry represents a significant analytical challenge in contemporary lipidomics. Herein, the combination of collision-induced dissociation (CID) with ozone-induced dissociation (OzID) on an ion-trap mass spectrometer is applied to the identification of triacylglycerol (TG) isomers that vary only by the substitution pattern of fatty acyl (FA) chains esterified to the glycerol backbone. Isolated product ions attributed to loss of a single FA arising from CID of [TG + Na](+) ions react rapidly with ozone within the ion trap. The resulting CID/OzID spectra exhibit abundant ions that unequivocally reveal the relative position of FAs along the backbone. Isomeric TGs containing two or three different FA substituents are readily differentiated by diagnostic ions present in their CID/OzID spectra. Compatibility of this method with chromatographic separations enables the characterization of unusual TGs containing multiple short-chain FAs present in Drosophila.

Identification of genes associated with resilience/vulnerability to sleep deprivation and starvation in Drosophila

BACKGROUND AND STUDY OBJECTIVES

Flies mutant for the canonical clock protein cycle (cyc(01)) exhibit a sleep rebound that is ∼10 times larger than wild-type flies and die after only 10 h of sleep deprivation. Surprisingly, when starved, cyc(01) mutants can remain awake for 28 h without demonstrating negative outcomes. Thus, we hypothesized that identifying transcripts that are differentially regulated between waking induced by sleep deprivation and waking induced by starvation would identify genes that underlie the deleterious effects of sleep deprivation and/or protect flies from the negative consequences of waking.

DESIGN

We used partial complementary DNA microarrays to identify transcripts that are differentially expressed between cyc(01) mutants that had been sleep deprived or starved for 7 h. We then used genetics to determine whether disrupting genes involved in lipid metabolism would exhibit alterations in their response to sleep deprivation.

SETTING

Laboratory.

PATIENTS OR PARTICIPANTS

Drosophila melanogaster.

INTERVENTIONS

Sleep deprivation and starvation.

MEASUREMENTS AND RESULTS

We identified 84 genes with transcript levels that were differentially modulated by 7 h of sleep deprivation and starvation in cyc(01) mutants and were confirmed in independent samples using quantitative polymerase chain reaction. Several of these genes were predicted to be lipid metabolism genes, including bubblegum, cueball, and CG4500, which based on our data we have renamed heimdall (hll). Using lipidomics we confirmed that knockdown of hll using RNA interference significantly decreased lipid stores. Importantly, genetically modifying bubblegum, cueball, or hll resulted in sleep rebound alterations following sleep deprivation compared to genetic background controls.

CONCLUSIONS

We have identified a set of genes that may confer resilience/vulnerability to sleep deprivation and demonstrate that genes involved in lipid metabolism modulate sleep homeostasis.

Localization of double bonds in triacylglycerols using high-performance liquid chromatography/atmospheric pressure chemical ionization ion-trap mass spectrometry

A method for localizing double bonds in triacylglycerols using high-performance liquid chromatography-tandem mass spectrometry with atmospheric pressure chemical ionization (APCI) was developed. The technique was based on collision-induced dissociation or pulsed Q collision-induced dissociation of the C3H5N(+•) adducts ([M + 55](+•)) formed in the presence of acetonitrile in the APCI source. The spectra were investigated using a large series of standards obtained from commercial sources and prepared by randomization. The fragmentation spectra made it possible to determine (i) the total number of carbons and double bonds in the molecule, (ii) the number of carbons and double bonds in acyls, (iii) the acyl in the sn-2 position on the glycerol backbone, and (iv) the double-bond positions in acyls. The double-bond positions were determined based on two types of fragments (alpha and omega ions) formed by cleavages of C-C bonds vinylic to the original double bond. The composition of the acyls and their positions on glycerol were established from the masses and intensities of the ions formed by the elimination of fatty acids from the [M + 55](+•) precursor. The method was applied for the analysis of triacylglycerols in olive oil and vernix caseosa.

Lipidomic analysis of cerebrospinal fluid by mass spectrometry-based methods

Lipids are natural substances found in all living organisms. Essential to the integrity of cell membranes, they also have many biological functions linked to energy storage and cell signaling, and are involved in a large number of heterogeneous diseases such as cancer, diabetes, neurological disorders, and inherited metabolic diseases. Lipids are challenging to analyze because of their huge structural diversity and numerous species. Up to now, lipid analysis has been achieved by targeted approaches focusing on selected families and relying on extraction protocols and chromatographic methods coupled to various detectors including mass spectrometry. Thanks to the technological improvements achieved in the fields of chromatography, high-resolution mass spectrometry and bioinformatics, it is possible to perform global lipidomic analyses enabling the concomitant detection, identification and relative quantification of many lipid species belonging to different families. The aim of this review is to focus on mass spectrometry-based methods to perform lipid and lipidomic analyses and on their application to the analysis of cerebrospinal fluid.