Structural characterization of cardiolipin by tandem quadrupole and multiple-stage quadrupole ion-trap mass spectrometry with electrospray ionization

Analysis of glycosylated cardiolipins from thermophilic bacteria using GC-MS and LC-ESI-MS/MS methods

Unusual glucose-substituted cardiolipins (Glcx-CLs) in three genera of thermophilic bacteria, having more than one glycosidically linked glucose to the hydroxyl of the central glycerol of Glcx-CLs were identified for the first time in thermophilic bacteria of the genera Geobacillus, Meiothermus, and Thermus. The number of glucoses reached up to five units. The structure of glycosidically linked oligosaccharides was determined based on shotgun analysis MS (electrospray high-resolution tandem mass spectrometry), partially methylated alditol acetates were identified by GC-MS, both electron ionization (EI) and positive chemical ionization (PCI), hydrophilic interaction liquid chromatography (HILIC) separation and identification of CLs glycosides by high resolution MS-ESI, and digestion by specific glycosidases.

Charge Inversion of Mono- and Dianions to Cations via Triply Charged Metal Complexes: Application to Lipid Mixtures

Electrospray ionization (ESI) of mixtures can give rise to ions with different masses and charges with overlapping mass-to-charge (m/z) ratios. Such a scenario can be particularly problematic for the detection of low-abundance species in the presence of more highly abundant mixture components. For example, negative mode ESI of polar lipid extracts can result in highly abundant singly charged glyerophospholipids (GPLs), such as phosphatidylethanolamines (PE) and phosphatidylglycerols (PG), that can obscure much less abundant cardiolipins (CLs), which are complex phospholipids with masses roughly double those of GPLs that mostly form doubly charged anions. Despite their low relative abundance, CLs are lipidome components that perform vital biological functions. To facilitate the study of CLs in lipid mixtures without resorting to offline or online separations, we have developed a gas-phase approach employing ion/ion reactions to charge invert anionic lipid species using a trivalent metal-complex. Specifically, ytterbium(III) is shown to readily complex with three neutral ligands, N,N,N',N'-tetra-2-ethylhexyl diglycolamide (TEHDGA), to form [Yb(TEHDGA3)]3+ using ESI. Herein, we describe pilot studies to evaluate [Yb(TEHDGA)3]3+ as an ion/ion reagent to allow for chemical separation of doubly and singly charged anions, using lipid mixtures as examples, without neutralizing ions of either charge state.

Structural characterization of phospholipids and sphingolipids by in-source fragmentation MALDI/TOF mass spectrometry

Phospholipids (PLs) and sphingolipids (SLs) perform critical structural and biological functions in cells. The structure of these lipids, including the stereospecificity and double-bond position of fatty acyl (FA) chains, is critical in decoding lipid biology. In this study, we presented a simple in-source fragmentation (ISF) MALDI/TOF mass spectrometry method that affords complete structural characterization of PL and SL molecules. We analyzed several representative unsaturated lipid species including phosphatidylcholine (PC), plasmalogen PC (pPC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), cardiolipin (CL), sphingomyelin (SM), and ceramide (Cer). Fragment ions reflecting the FA chains at sn-1 and sn-2 position, and those characteristics of the head groups of different PL classes, are readily identified. Specific fragment ions from cleavages of the C-C bond immediately adjacent to the cis C=C double-bond position(s) of FA chains and the trans C=C double bond of the sphingosine constituents allow precise localization of double bonds. The identities of the exemplary product ions from vinylic, allylic, and double-bond cleavages were also verified by LIFT-TOF/TOF. Identification of individual PL species in the lipid mixture was also carried out with ISF-MALDI/TOF. Together, this approach provides a simple yet effective method for structural characterization of PLs and SLs without the additional modification on the instrument hardware, and serves as a simple tool for the identification of lipids.

Enhanced Characterization of Cardiolipins via Hybrid 193 nm Ultraviolet Photodissociation Mass Spectrometry

Cardiolipins (CLs) constitute a structurally complex class of glycerophospholipids with a unique tetraacylated structure accompanied by distinctive functional roles. Aberrations in the composition of this lipid class have been associated with disease states, spurring interest in the development of new approaches to differentiate the structures of diverse CLs in complex mixtures. The structural characterization of these complex lipids using conventional methods, however, suffers from limited resolution and frequently proves unable to discern subtle yet biologically significant features such as unsaturation sites or acyl chain position assignments. Here, we describe the synergistic use of chemical derivatization and hybrid dissociation techniques to characterize CL from complex biological mixtures with both double bond and sn positional isomer resolution in a shotgun mass spectrometry strategy. Utilizing (trimethylsilyl)diazomethane (TMSD), CL phosphate groups were methylated to promote positive-mode ionization by the production of metal-cationized lipids, enabling structural interrogation via hybrid higher-energy collisional activation/ultraviolet photodissociation (HCD/UVPD). This combination of TMSD derivatization and HCD/UVPD fragmentation results in diagnostic product ions that permit distinction and relative quantitation of sn-stereoisomers and the localization of double bonds. Applying this strategy to a total lipid extract from a thyroid carcinoma revealed a previously unreported 18:2/18:1 motif, elucidating a structural feature unique to the lipid class.

Characterization of the Uncommon Lipid Families in Corynebacterium glutamicum by Mass Spectrometry

This book chapter provides readers the step-by-step instruction for cell growth, lipid isolation, and lipid analysis to obtain the lipidome of Corynebacterium glutamicum (C. glutamicum) in the genus Corynebacterium, a biotechnologically important bacterium. We separate the lipid families by preparative HPLC with an analytical C-8 column, followed by linear ion-trap multiple stage mass spectrometry (LIT MSⁿ) with high-resolution mass measurement to define the structures of cytidine diphosphate diacylglycerol (CDP-DAG), glucuronosyl diacylglycerol (GlcA-DAG), α-D-mannopyranosyl-(1 → 4)-α-D-glucuronyl diacylglycerol (Man-GlcA-DAG), 1-mycolyl-2-acyl-phosphatidylglycerol (MA-PG), and acyl trehalose monomycolate (acyl-TMM) whose structures have been previously mis-assigned or not defined by mass spectrometric means. We also define the structures of mycolic acid, phosphatidylglycerol, phosphatidylinositol, cardiolipin, trehalose dimycolate lipids in the cell wall. The similarity of the lipidome to that in the Mycobacterium genera is consistent with the notion that Corynebacterium and Mycobacterium are gram-positive bacteria belonging to the suborder Corynebacterineae.

Lipid composition and molecular species of phospholipid in oyster Crassostrea lugubris (Sowerby, 1871) from Lang Co Beach, Hue Province, Vietnam

Oysters are widely distributed worldwide, but are mainly concentrated in tropics and subtropics. Total lipid (TL), fatty acid (FA) composition of TL and polar lipid (PoL) fractions, phospholipid (PL) class, and molecular species composition in soft tissues of Crassostrea lugubris were investigated for the first time from Vietnam. Phosphatidylglycolic acid (PGA) is the new phospholipid class first identified in marine species in general and Crassostrea lugubris in particular. Main eight classes of PL were determined in PoL fraction: diphosphatidylglycerol (DPG), phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylserine (PS), ceramide aminoethylphosphonate (CAEP), CAEP with hydroxylated FAs (CAEP-OH), and lysophosphatidylcholine. PE and PC accounted for approximately 63% of total known PL. Polyunsaturated FAs accounted for more than 30% of TL. Ninety molecular species of glycerophospholipids, including PGA, PE, PC, PS, PI, DPG, and PG, and sphingophosphonolipids (CAEP) were identified in PoL. Alkenyl-acyl forms of glycerophospholipids were predominated in the molecular species of PGA, PE, and PS. PGA 38:1 (p18:0/20:1), PE 40:6 (p18:0/22:6 and p18:1/22:5), PC 30:0 (14:0/16:0), PS 38:1 (p18:0/20:1), PI 40:5 (20:1/20:4), PG 32:0 (16:0/16:0), DPG 88:24 (22:6/22:6/22:6/22:6), and CAEP 34:2 (d18:2/16:0) were major molecular species.

Detailed structural characterization of cardiolipins from various biological sources using a complex analytical strategy comprising fractionation, hydrolysis and chiral chromatography

Cardiolipins (1,3-bis(sn-3'-phosphatidyl)-sn-glycerol) (CLs) are widespread in many organisms, from bacteria to higher green plants and mammals. CLs were observed in Gram-positive bacterium of the genus Kocuria, brewer's yeast Saccharomyces, the green alga Chlamydomonas, spinach and beef heart. A mixture of molecular species of CLs was obtained from total lipids by hydrophilic interaction liquid chromatography (HILIC), and these were further separated and identified by reversed phase LC/MS with negative tandem electrospray ionization. The majority of CLs molecular species from each organism were cleaved using phospholipase C from Bacillus cereus. This phospholipase cleaves CLs into 1,2-diglycerols and phosphatidylglycerol 3-phosphates, which were then separated. After CLs cleavage, diacylglycerols such as sn-1,2-diacyl-3-acetyl-glycerols (i.e., triacylglycerols) were separated and identified by chiral chromatography/MS-positive tandem ESI. Significant differences in the composition of the molecular species between the 3-(3-sn-phosphatidyl) and 1-(3-sn-phosphatidyl) moieties of CLs were found in all organisms tested. Molecular species of CLs that contained four different fatty acids were identified in all five samples, and CLs containing very long chain fatty acids were identified in yeast. In addition, CLs containing both enantiomers (at the sn-2 carbon) were present in the bacterium tested. These findings were further supported by data already published in GenBank where, in the same family - Micrococcaceae - both enzymes responsible for chirality in the sn-2 position, glycerol-3-phosphate and glycerol-1-phosphate dehydrogenases, were present.

LIPIC: An Automated Workflow to Account for Isotopologue-Related Interferences in Electrospray Ionization High-Resolution Mass Spectra of Phospholipids

In the past decade, hydrophilic interaction liquid chromatography (HILIC) has emerged as an efficient alternative to reversed-phase chromatography (RPC) for the analysis of phospholipid (PL) mixtures based on mass spectrometric detection. Since the separation of PL by HILIC is chiefly based on their headgroup, the mass spectrum of each class can be obtained by spectral averaging under the corresponding HILIC band. Using experimental m/z values resulting from high mass resolution/accuracy instruments, the sum compositions of PL in a specific class can be thus inferred but partial overlapping may occur between signals related to the M + 0 isotopologue of one species and the M + 2/M + 4 isotopologues of species having one/two more C═C bonds in their chemical structures. Here, an automated workflow, named LIPIC (lipid isotopic pattern interference correction), is proposed to account for such interferences. Starting from the experimentally verified assumption that peaks in isotope patterns are Gaussian, LIPIC predicts, as a function of m/z ratio, signal intensities due to M + 2 and M + 4 isotopologues of species with one or two more C = C bonds than the target one and calculates the corrected intensity for the M + 0 isotopologue of the latter. Thanks to an iterative procedure, the suggested algorithm compensates also for slight shifts occurring between experimental and theoretical m/z ratios related to isotopologue peaks. Examples of applications to simulated and experimental mass spectra of two PL classes, i.e., phosphatidylcholines (PC) and cardiolipins (CL), emphasize the increased extent of correction at the increase of molecular masses of involved species.

Localization of Carbon-Carbon Double Bond and Cyclopropane Sites in Cardiolipins via Gas-Phase Charge Inversion Reactions

Cardiolipins (CLs) are comprised of two phosphatic acid moieties bound to a central glycerol backbone and are substituted with four acyl chains. Consequently, a vast number of distinct CL structures are possible in different biological contexts, representing a significant analytical challenge. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) has become a widely used approach for the detection, characterization, and quantitation of complex lipids, including CLs. Central to this approach is fragmentation of the [CLs - H]- or [CL - 2H]2- anions by collision-induced dissociation (CID). Product ions in the resulting tandem mass spectra confirm the CL subclass assignment and reveal the numbers of carbons and degrees of unsaturation in each of the acyl chains. Conventional CID, however, affords limited structural elucidation of the fatty acyl chains, failing to discriminate isomers arising from different site(s) of unsaturation or cyclopropanation and potentially obscuring their metabolic origins. Here, we report the application of charge inversion ion/ion chemistry in the gas phase to enhance the structural elucidation of CLs. Briefly, CID of [CL - H]2- anions generated via negative ion ESI allowed for the assignment of individual fatty acyl substituents and phosphatidic acid moieties. Next, gas-phase derivatization of the resulting CL product ions, including fatty acyl carboxylate anions, was effected with gas-phase ion/ion charge inversion reactions with tris-phenanthroline magnesium reagent dications. Subsequent isolation and activation of the charge-inverted fatty acyl complex cations permitted the localization of both carbon-carbon double bond and cyclopropane motifs within each of the four acyl chains of CLs. This approach was applied to the de novo elucidation of unknown CLs in a biological extract revealing distinct isomeric populations and regiochemical relationships between double bonds and carbocyles.

Shotgun Lipidomic Analysis of Leishmania Cells

To understand the interplay of lipids between Leishmania promastigotes, amastigotes, and vertebrate host cells, a robust method for cultivating Leishmania parasites, lipid extraction, and shotgun lipidomic analysis with loop injection is described. This book chapter provides the step-by-step workflow to guide readers from sample preparation to the global lipid analysis by multiple stage mass spectrometry with high resolution and tandem quadrupole mass spectrometric approaches toward studying the metabolomic roles that lipids may play in Leishmania parasite infections.

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.

Enhancing detection and characterization of lipids using charge manipulation in electrospray ionization-tandem mass spectrometry

Heightened awareness regarding the implication of disturbances in lipid metabolism with respect to prevalent human-related pathologies demands analytical techniques that provide unambiguous structural characterization and accurate quantitation of lipids in complex biological samples. The diversity in molecular structures of lipids along with their wide range of concentrations in biological matrices present formidable analytical challenges. Modern mass spectrometry (MS) offers an unprecedented level of analytical power in lipid analysis, as many advancements in the field of lipidomics have been facilitated through novel applications of and developments in electrospray ionization tandem mass spectrometry (ESI-MS/MS). ESI allows for the formation of intact lipid ions with little to no fragmentation and has become widely used in contemporary lipidomics experiments due to its sensitivity, reproducibility, and compatibility with condensed-phase modes of separation, such as liquid chromatography (LC). Owing to variations in lipid functional groups, ESI enables partial chemical separation of the lipidome, yet the preferred ion-type is not always formed, impacting lipid detection, characterization, and quantitation. Moreover, conventional ESI-MS/MS approaches often fail to expose diverse subtle structural features like the sites of unsaturation in fatty acyl constituents or acyl chain regiochemistry along the glycerol backbone, representing a significant challenge for ESI-MS/MS. To overcome these shortcomings, various charge manipulation strategies, including charge-switching, have been developed to transform ion-type and charge state, with aims of increasing sensitivity and selectivity of ESI-MS/MS approaches. Importantly, charge manipulation approaches afford enhanced ionization efficiency, improved mixture analysis performance, and access to informative fragmentation channels. Herein, we present a critical review of the current suite of solution-based and gas-phase strategies for the manipulation of lipid ion charge and type relevant to ESI-MS/MS.

Free-Radical-Mediated Formation of Trans-Cardiolipin Isomers, Analytical Approaches for Lipidomics and Consequences of the Structural Organization of Membranes

Free-radical-mediated processes, such as peroxidation, isomerization and hydrogenation affecting fatty acid integrity and biological functions, have a trans-disciplinary relevance. Cardiolipins (CL, (1,3-diphosphatidyl-sn-glycerol)) and tetra-linoleoyl-CL are complex phospholipids, exclusively present in the Inner Mitochondrial Membrane (IMM) lipids, where they maintain membrane integrity and regulate enzyme functionalities. Peroxidation pathways and fatty acid remodeling are known causes of mitochondrial disfunctions and pathologies, including cancer. Free-radical-mediated isomerization with the change of the cis CL into geometrical trans isomers is an unknown process with possible consequences on the supramolecular membrane lipid organization. Here, the formation of mono-trans CL (MT-CL) and other trans CL isomers (T-CL) is reported using CL from bovine heart mitochondria and thiyl radicals generated by UV-photolysis from 2-mercaptoethanol. Analytical approaches for CL isomer separation and identification via ¹H/¹³C NMR are provided, together with the chemical study of CL derivatization to fatty acid methyl esters (FAME), useful for lipidomics and metabolomics research. Kinetics information of the radical chain isomerization process was obtained using γ-irradiation conditions. The CL isomerization affected the structural organization of membranes, as tested by the reduction in unilamellar liposome diameter, and accompanied the well-known process of oxidative consumption induced by Fenton reagents. These results highlight a potential new molecular modification pathway of mitochondrial lipids with wide applications to membrane functions and biological consequences.

Proton Transfer Reactions for the Gas-Phase Separation, Concentration, and Identification of Cardiolipins

Cardiolipin (CL) analysis demands high specificity, due to the extensive diversity of CL structures, and high sensitivity, due to their low relative abundance within the lipidome. While electrospray ionization mass spectrometry (ESI-MS) is the most widely used technology in lipidomics, the potential for multiple charging presents unique challenges for CL identification and quantification. Depending on the conditions, ESI-MS of lipid extracts in negative ion mode can give rise to cardiolipins ionized as both singly and doubly deprotonated anions. This signal degeneracy diminishes the signal-to-noise ratio, while in addition (for direct infusion), the dianion population falls within a m/z range already heavily congested with monoanions from more abundant glycerophospholipid subclasses. Herein, we describe a direct infusion strategy for CL profiling from total lipid extracts utilizing gas-phase proton-transfer ion/ion reactions. In this approach, lipid extracts are ionized by negative ion ESI generating both singly deprotonated phospholipids and doubly deprotonated CL anions. Charge reduction of the negative ion population by ion/ion reactions leads to an enhancement in singly deprotonated [CL - H]- species via proton transfer to the corresponding [CL - 2H]2-̅ dianions. To concentrate the [CL - H]- anion signal, multiple iterations of ion accumulation and proton-transfer ion/ion reaction can be performed prior to subsequent interrogation. Mass selection and collisional activation of the enriched population of [CL - H]- anions facilitates the assignment of individual fatty acyl substituents and phosphatidic acid moieties. Demonstrated advantages of this new approach derive from the improved performance in complex mixture analysis affording detailed characterization of low abundant CLs directly from a total biological extract.

Unveiling the biodiversity of lipid species in Corynebacteria- characterization of the uncommon lipid families in C. glutamicum and pathogen C. striatum by mass spectrometry

Uncommon lipids in biotechnologically important Corynebacterium glutamicum and pathogen Corynebacterium striatum in genus Corynebacterium are isolated and identified by linear ion-trap multiple stage mass spectrometry (LIT MSⁿ) with high resolution mass measurement. We redefined several lipid structures that were previously mis-assigned or not defined, including cytidine diphosphate diacylglycerol (CDP-DAG), glucuronosyl diacylglycerol (GlcA-DAG), (α-d-mannopyranosyl)-(1 → 4)-(α-D-glucuronyl diacyglycerol (Man-GlcA-DAG), 1-mycolyl-2-acyl-phosphatidylglycerol (MA-PG), acyl trehalose monomycolate (acyl-TMM). We also report the structures of mycolic acid, phosphatidylglycerol, phosphatidylinositol, cardiolipin, trehalose dimycolate lipids in which many isomeric structures are present. The LIT MSⁿ approaches afford identification of the functional group, the fatty acid substituents and their regiospecificity in the molecules, revealing the biodiversities of the lipid species in two Corynebacterium strains that have played very different and important roles in human nutrition and health.

Advances in lipidomics

The present article examines recently published literature on lipids, mainly focusing on research involving glycero-, glycerophospho- and sphingo-lipids. The primary aim is identification of distinct profiles in biologic lipidomic systems by ultra-high-performance liquid chromatography (UHPLC) coupled with mass spectrometry (MS, tandem MS) with multivariate data analysis. This review specifically targets lipid biomarkers and disease pathway mechanisms in humans and artificial targets. Different specimen matrices such as primary blood derivatives (plasma, serum, erythrocytes, and blood platelets), faecal matter, urine, as well as biologic tissues (liver, lung and kidney) are highlighted.

Analysis of artificially oxidized cardiolipins and monolyso-cardiolipins via liquid chromatography/high-resolution mass spectrometry and Kendrick mass defect plots after hydrophilic interaction liquid chromatography based sample preparation

RATIONALE

Cardiolipins (CL) are a special lipid class which plays a main role in energy metabolism in mitochondria and is involved in apoptosis. In contrast to other glycerophospholipids, they contain four fatty acyl residues which results in a high structural diversity. Oxidation, for example by reactive oxygen species, or lyso forms such as monolyso-CL (MLCL), increases this diversity. Mass spectrometric analysis and computational identification of CL, MLCL and their oxidation products is therefore a challenging task.

METHODS

In order to distinguish CL, MLCL and their oxidation products, a liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed. A hydrophilic interaction liquid chromatography (HILIC)-based solid-phase extraction (SPE) clean-up approach was developed for CL enrichment. Graphical analysis of CL, MLCL and their oxidation products was carried out by a three-dimensional Kendrick mass defect (3D-KMD) plot module, as well as a refined lipid search module of the open-source metabolomics data mining software MZmine 2.

RESULTS

The HILIC-based SPE clean-up enabled complete separation of polar and nonpolar lipid classes. A yeast (Saccharomyces cerevisiae) lipid extract, which was artificially oxidized by means of the Fenton reaction, was analyzed by the developed LC/MS/MS method. CL species with differences in chain length and degree of unsaturation have been separated by high-performance liquid chromatography (HPLC). In total 66 CL, MLCL and oxidized species have been identified utilizing 3D-KMD plots in combination with database matching using MZmine 2. For further characterization of annotated species, MS/MS experiments have been utilized.

CONCLUSIONS

3D-KMD plots capturing chromatographic and high-resolution mass spectrometry data have been successfully used for graphical identification of CL, MLCL as well as their oxidized species. Therefore, we chose multiple KMD bases such as hydrogen and oxygen to visualize the degree of unsaturation and oxidation capturing chromatographic data by means of a color-coded paint scale as the third dimension. In combination with database matching, the analysis of low concentrated lipid species in complex samples has been significantly improved.

Fast and Quantitative Phospholipidomic Analysis of SH-SY5Y Neuroblastoma Cell Cultures Using Liquid Chromatography-Tandem Mass Spectrometry and 31P Nuclear Magnetic Resonance

Global lipid analysis still lags behind proteomics with respect to the availability of databases, experimental protocols, and specialized software. Determining the lipidome of cellular model systems in common use is of particular importance, especially when research questions involve lipids directly. In Parkinson's disease research, there is a growing awareness for the role of the biological membrane, where individual lipids may contribute to provoking α-synuclein oligomerisation and fibrillation. We present an analysis of the whole cell and plasma membrane lipid isolates of a neuroblastoma cell line, SH-SY5Y, a commonly used model system for research on this and other neurodegenerative diseases. We have used two complementary lipidomics methods. The relative quantities of PC, PE, SMs, CL, PI, PG, and PS were determined by ³¹P NMR. Fatty acid chain composition and their relative abundances within each phospholipid group were evaluated by liquid chromatography-tandem mass spectrometry. For this part of the analysis, we have developed and made available a set of Matlab scripts, LipMat. Our approach allowed us to observe several deviations of lipid abundances when compared to published reports regarding phospholipid analysis of cell cultures or brain matter. The most striking was the high abundance of PC (54.7 ± 1.9%) and low abundance of PE (17.8 ± 4.8%) and SMs (2.7 ± 1.2%). In addition, the observed abundance of PS was smaller than expected (4.7 ± 2.7%), similar to the observed abundance of PG (4.5 ± 1.8%). The observed fatty acid chain distribution was similar to the whole brain content with some notable differences: a higher abundance of 16:1 PC FA (17.4 ± 3.4% in PC whole cell content), lower abundance of 22:6 PE FA (15.9 ± 2.2% in plasma membrane fraction), and a complete lack of 22:6 PS FA.

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

Cardiolipins (CL) are anionic dimeric phospholipids bearing four fatty acids, found in inner mitochondrial membrane as structural components and are involved in several processes as oxidative phosphorylation or apoptotic signalling. As other phospholipids, CL can be modified by reactive oxygen species (ROS) and reactive nitrogen species (RNS), which can modulate various cellular functions. Modifications of CL by RNS remain largely unstudied although other nitrated lipids are emerging as bioactive molecules. In this work, we developed a C30-LC-HRMS/MS methodology to identify the nitrated and nitroxidized tetralinoleoyl-cardiolipin (TLCL), using a biomimetic model of nitration, and to disclose specific fragmentation pathways under HCD MS/MS. Using this lipidomics approach, we were able to separate and identify nitro, nitroso, nitronitroso, and nitroxidized TLCL derivatives, comprising 11 different nitrated compounds. These products were identified using accurate mass measurements and the fragmentation pattern acquired in higher-energy collision dissociation (HCD)-tandem MS/MS experiments. These spectra showed classifying fragmentation pathways, yielding phosphatidic acid (PA^-), lysophosphatidic acid (LPA^-), and carboxylate fragment ions with the modifying moiety. Remarkably, the typical neutral losses associated with the added moieties were not observed. In conclusion, this work has developed a new method for the identification of nitroso, nitrated and nitroxidized cardiolipin products by using a C30LC-MS platform method, potentially allowing their detection in biological samples.

Hybrid 193 nm Ultraviolet Photodissociation Mass Spectrometry Localizes Cardiolipin Unsaturations

Developing alternative MS/MS strategies to distinguish isomeric lipids has become a high impact goal in shotgun lipidomics. Novel approaches have been developed to resolve structural features that are not discernible by traditional shotgun methods and have consequently promoted the discovery of new disease biomarkers. However, these methods have largely been limited to characterizing lipids with low structural complexity. Here, ultraviolet photodissociation (UVPD) strategies for phospholipid characterization are expanded for analysis of cardiolipins (CL), a class of phospholipids that exhibits a higher degree of structural complexity. A hybrid collision induced dissociation/193 nm UVPD (CID/UVPD) approach was implemented to pinpoint the location of both double bond and cyclopropyl unsaturations on the four acyl chains of CLs. This strategy was complemented with CID for the de novo elucidation of unknown CLs in biological extracts.

Identification by mass spectrometry of glucosaminylphosphatidylglycerol, a phosphatidylglycerol derivative, produced by Pseudomonas aeruginosa

RATIONALE

Pseudomonas aeruginosa is an opportunistic pathogen bacterium widely considered to be an excellent research model in several areas of molecular studies, namely genomics and proteomics. However, its lipid metabolism is still not totally decrypted. While it is known that this bacterium has the particularity to produce phosphatidylcholine, a lipid mainly found in eukaryotes, other singularities are still to be discovered.

METHODS

P. aeruginosa was grown as planktonic cultures to the stationary state. Membrane pellets were collected and lipids were extracted using the Bligh and Dyer protocol. Lipid extracts were analyzed by Electrospray Ionization Mass Spectrometry (ESI-MS) using high-resolution mass spectrometer (LTQ Orbitrap Elite, Thermo Scientific) in the negative mode. MSⁿ spectra were recorded both in the Orbitrap and in the ion trap analyzer (collision-induced dissociation (CID) or higher energy collision-induced dissociation (HCD) mode).

RESULTS

We observed by mass spectrometry and thin layer chromatography that P. aeruginosa produced an unreferenced lipid in classical growth conditions. MS² analysis of the unknown ion indicates that it is a phosphatidylglycerol derivative. The exact mass shift corresponds to glucosamine which is largely found in the metabolism of this bacterium. MS³ analysis of secondary ions allowed us to conclude that this lipid is a glucosaminylphosphatidylglycerol, a phosphatidylglycerol derivative containing a glucosamine substituted at C4.

CONCLUSIONS

We show here that P. aeruginosa is able to produce glucosaminylphosphatidylglycerols via a probable esterification of phosphatidylglycerols by glucosamine.

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.

Molecular structural diversity of mitochondrial cardiolipins

Current strategies used to quantitatively describe the biological diversity of lipids by mass spectrometry are often limited in assessing the exact structural variability of individual molecular species in detail. A major challenge is represented by the extensive isobaric overlap present among lipids, hampering their accurate identification. This is especially true for cardiolipins, a mitochondria-specific class of phospholipids, which are functionally involved in many cellular functions, including energy metabolism, cristae structure, and apoptosis. Substituted with four fatty acyl side chains, cardiolipins offer a particularly high potential to achieve complex mixtures of molecular species. Here, we demonstrate how systematically generated high-performance liquid chromatography-mass spectral data can be utilized in a mathematical structural modeling approach, to comprehensively analyze and characterize the molecular diversity of mitochondrial cardiolipin compositions in cell culture and disease models, cardiolipin modulation experiments, and a broad variety of frequently studied model organisms.

Evaluation of cardiolipin nanodisks as lipid replacement therapy for Barth syndrome

Barth syndrome (BTHS) is a mitochondrial disorder characterized by cardiomyopathy and skeletal muscle weakness. Disease results from mutations in the tafazzin (TAZ) gene, encoding a phospholipid transacylase. Defective tafazzin activity results in an aberrant cardiolipin (CL) profile. The feasibility of restoring the intracellular CL profile was tested by in vivo administration of exogenous CL in nanodisk (ND) delivery particles. Ninety mg/kg CL (as ND) was administered to doxycycline-inducible taz shRNA knockdown (KD) mice once a week. After 10 weeks of CL-ND treatment, the mice were sacrificed and tissues harvested. Liquid chromatography-mass spectrometry of extracted lipids revealed that CL-ND administration failed to alter the CL profile of taz KD or WT mice. Thus, although CL-ND were previously shown to be an effective means of delivering CL to cultured cells, this effect does not extend to an in vivo setting. We conclude that CL-ND administration is not a suitable therapy option for BTHS.

LC-MS/MS versus TLC plus GC methods: Consistency of glycerolipid and fatty acid profiles in microalgae and higher plant cells and effect of a nitrogen starvation

Methods to analyze lipidomes have considerably evolved, more and more based on mass spectrometry technics (LC-MS/MS). However, accurate quantifications using these methods require ¹³C-labeled standards for each lipid, which is not feasible because of the very large number of molecules. Thus, quantifications rely on standard molecules representative of a whole class of lipids, which might lead to false estimations of some molecular species. Here, we determined and compared glycerolipid distributions from three different types of cells, two microalgae (Phaeodactylum tricornutum, Nannochloropsis gaditana) and one higher plant (Arabidopsis thaliana), using either LC-MS/MS or Thin Layer Chromatography coupled with Gas Chromatography (TLC-GC), this last approach relying on the precise quantification of the fatty acids present in each glycerolipid class. Our results showed that the glycerolipid distribution was significantly different depending on the method used. How can one reconcile these two analytical methods? Here we propose that the possible bias with MS data can be circumvented by systematically running in tandem with the sample to be analyzed a lipid extract from a qualified control (QC) of each type of cells, previously analyzed by TLC-GC, and used as an external standard to quantify the MS results. As a case study, we applied this method to compare the impact of a nitrogen deficiency on the three types of cells.

The phospholipase iPLA2γ is a major mediator releasing oxidized aliphatic chains from cardiolipin, integrating mitochondrial bioenergetics and signaling

Cardiolipin (CL) is a dimeric phospholipid with critical roles in mitochondrial bioenergetics and signaling. Recently, inhibition of the release of oxidized fatty acyl chains from CL by the calcium-independent phospholipase A₂γ (iPLA₂γ)-selective inhibitor (R)-BEL suggested that iPLA₂γ is responsible for the hydrolysis of oxidized CL and subsequent signaling mediated by the released oxidized fatty acids. However, chemical inhibition by BEL is subject to off-target pharmacologic effects. Accordingly, to unambiguously determine the role of iPLA₂γ in the hydrolysis of oxidized CL, we compared alterations in oxidized CLs and the release of oxidized aliphatic chains from CL in experiments with purified recombinant iPLA₂γ, germ-line iPLA₂γ^-/- mice, cardiac myocyte-specific iPLA₂γ transgenic mice, and wild-type mice. Using charge-switch high mass accuracy LC-MS/MS with selected reaction monitoring and product ion accurate masses, we demonstrated that iPLA₂γ is the major enzyme responsible for the release of oxidized aliphatic chains from CL. Our results also indicated that iPLA₂γ selectively hydrolyzes 9-hydroxy-octadecenoic acid in comparison to 13-hydroxy-octadecenoic acid from oxidized CLs. Moreover, oxidative stress (ADP, NADPH, and Fe³⁺) resulted in the robust production of oxidized CLs in intact mitochondria from iPLA₂γ^-/- mice. In sharp contrast, oxidized CLs were readily hydrolyzed in mitochondria from wild-type mice during oxidative stress. Finally, we demonstrated that CL activates the iPLA₂γ-mediated hydrolysis of arachidonic acid from phosphatidylcholine, thereby integrating the production of lipid messengers from different lipid classes in mitochondria. Collectively, these results demonstrate the integrated roles of CL and iPLA₂γ in lipid second-messenger production and mitochondrial bioenergetics during oxidative stress.

Cardiolipins Are Biomarkers of Mitochondria-Rich Thyroid Oncocytic Tumors

Oncocytic tumors are characterized by an excessive eosinophilic, granular cytoplasm due to aberrant accumulation of mitochondria. Mutations in mitochondrial DNA occur in oncocytic thyroid tumors, but there is no information about their lipid composition, which might reveal candidate theranostic molecules. Here, we used desorption electrospray ionization mass spectrometry (DESI-MS) to image and chemically characterize the lipid composition of oncocytic thyroid tumors, as compared with nononcocytic thyroid tumors and normal thyroid samples. We identified a novel molecular signature of oncocytic tumors characterized by an abnormally high abundance and chemical diversity of cardiolipins (CL), including many oxidized species. DESI-MS imaging and IHC experiments confirmed that the spatial distribution of CLs overlapped with regions of accumulation of mitochondria-rich oncocytic cells. Fluorescent imaging and mitochondrial isolation showed that both mitochondrial accumulation and alteration in CL composition of mitochondria occurred in oncocytic tumors cells, thus contributing the aberrant molecular signatures detected. A total of 219 molecular ions, including CLs, other glycerophospholipids, fatty acids, and metabolites, were found at increased or decreased abundance in oncocytic, nononcocytic, or normal thyroid tissues. Our findings suggest new candidate targets for clinical and therapeutic use against oncocytic tumors. Cancer Res; 76(22); 6588-97. ©2016 AACR.

Known unknowns of cardiolipin signaling: The best is yet to come

Since its discovery 75years ago, a wealth of knowledge has accumulated on the role of cardiolipin, the hallmark phospholipid of mitochondria, in bioenergetics and particularly on the structural organization of the inner mitochondrial membrane. A surge of interest in this anionic doubly-charged tetra-acylated lipid found in both prokaryotes and mitochondria has emerged based on its newly discovered signaling functions. Cardiolipin displays organ, tissue, cellular and transmembrane distribution asymmetries. A collapse of the membrane asymmetry represents a pro-mitophageal mechanism whereby externalized cardiolipin acts as an "eat-me" signal. Oxidation of cardiolipin's polyunsaturated acyl chains - catalyzed by cardiolipin complexes with cytochrome c. - is a pro-apoptotic signal. The messaging functions of myriads of cardiolipin species and their oxidation products are now being recognized as important intracellular and extracellular signals for innate and adaptive immune systems. This newly developing field of research exploring cardiolipin signaling is the main subject of this review. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Greazy: Open-Source Software for Automated Phospholipid Tandem Mass Spectrometry Identification

Lipid identification from data produced with high-throughput technologies is essential to the elucidation of the roles played by lipids in cellular function and disease. Software tools for identifying lipids from tandem mass (MS/MS) spectra have been developed, but they are often costly or lack the sophistication of their proteomics counterparts. We have developed Greazy, an open source tool for the automated identification of phospholipids from MS/MS spectra, that utilizes methods similar to those developed for proteomics. From user-supplied parameters, Greazy builds a phospholipid search space and associated theoretical MS/MS spectra. Experimental spectra are scored against search space lipids with similar precursor masses using a peak score based on the hypergeometric distribution and an intensity score utilizing the percentage of total ion intensity residing in matching peaks. The LipidLama component filters the results via mixture modeling and density estimation. We assess Greazy's performance against the NIST 2014 metabolomics library, observing high accuracy in a search of multiple lipid classes. We compare Greazy/LipidLama against the commercial lipid identification software LipidSearch and show that the two platforms differ considerably in the sets of identified spectra while showing good agreement on those spectra identified by both. Lastly, we demonstrate the utility of Greazy/LipidLama with different instruments. We searched data from replicates of alveolar type 2 epithelial cells obtained with an Orbitrap and from human serum replicates generated on a quadrupole-time-of-flight (Q-TOF). These findings substantiate the application of proteomics derived methods to the identification of lipids. The software is available from the ProteoWizard repository: http://tiny.cc/bumbershoot-vc12-bin64 .

High-resolution mass spectrometry of small molecules bound to membrane proteins

Small molecules are known to stabilize membrane proteins and to modulate their function and oligomeric state, but such interactions are often hard to precisely define. Here we develop and apply a high-resolution, Orbitrap mass spectrometry-based method for analyzing intact membrane protein-ligand complexes. Using this platform, we resolve the complexity of multiple binding events, quantify small molecule binding and reveal selectivity for endogenous lipids that differ only in acyl chain length.

Functional diversity of complex I subunits in Candida albicans mitochondria

Our interest in the mitochondria of Candida albicans has progressed to the identification of several proteins that are critical to complex I (CI) activity. We speculated that there should be major functional differences at the protein level between mammalian and fungal mitochondria CI. In our pursuit of this idea, we were helped by published data of CI subunit proteins from a broad diversity of species that included two subunit proteins that are not found in mammals. These subunit proteins have been designated as Nuo1p and Nuo2p (NADH-ubiquinone oxidoreductases). Since functional assignments of both C. albicans proteins were unknown, other than having a putative NADH-oxidoreductase activity, we constructed knock-out strains that could be compared to parental cells. The relevance of our research relates to the critical roles of both proteins in cell biology and pathogenesis and their absence in mammals. These features suggest they may be exploited in antifungal drug discovery. Initially, we characterized Goa1p that apparently regulates CI activity but is not a CI subunit protein. We have used the goa1∆ for comparisons to Nuo1p and Nuo2p. We have demonstrated the critical role of these proteins in maintaining CI activities, virulence, and prolonging life span. More recently, transcriptional profiling of the three mutants and an ndh51∆ (protein is a highly conserved CI subunit) has revealed that there are overlapping yet also different functional assignments that suggest subunit specificity. The differences and similarities of each are described below along with our hypotheses to explain these data. Our conclusion and perspective is that the C. albicans CI subunit proteins are highly conserved except for two that define non-mammalian functions.

Cardiolipin signaling mechanisms: collapse of asymmetry and oxidation

SIGNIFICANCE

An ancient anionic phospholipid, cardiolipin (CL), ubiquitously present in prokaryotic and eukaryotic membranes, is essential for several structural and functional purposes.

RECENT ADVANCES

The emerging role of CLs in signaling has become the focus of many studies.

CRITICAL ISSUES

In this work, we describe two major pathways through which mitochondrial CLs may fulfill the signaling functions via utilization of their (i) asymmetric distribution across membranes and translocations, leading to the surface externalization and (ii) ability to undergo oxidation reactions to yield the signature products recognizable by the executionary machinery of cells.

FUTURE DIRECTIONS

We present a concept that CLs and their oxidation/hydrolysis products constitute a rich communication language utilized by mitochondria of eukaryotic cells for diversified regulation of cell physiology and metabolism as well as for inter-cellular interactions.

Characterization of polar lipids of Listeria monocytogenes by HCD and low-energy CAD linear ion-trap mass spectrometry with electrospray ionization

Listeria monocytogenes (L. monocytogenes) is a facultative, Gram-positive, food-borne bacterium, which causes serious infections. Although it is known that lipids play important roles in the survival of Listeria, the detailed structures of these lipids have not been established. In this contribution, we described linear ion-trap multiple-stage mass spectrometric approaches with high-resolution mass spectrometry toward complete structural analysis including the identities of the fatty acid substituents and their position on the glycerol backbone of the polar lipids, mainly phosphatidylglycerol, cardiolipin (CL), and lysyl-CL from L. monocytogenes. The location of the methyl side group along the fatty acid chain in each lipid family was characterized by a charge-switch strategy. This is achieved by first alkaline hydrolysis to release the fatty acid substituents, followed by tandem mass spectrometry on their N-(4-aminomethylphenyl) pyridinium (AMPP) derivatives as the M+ ions. Several findings in this study are unique: (1) we confirm the presence of a plasmalogen PG family that has not been previous reported; (2) an ion arising from a rare internal loss of lysylglycerol residue was observed in the MS(2) spectrum of lysyl-CL, permitting its distinction from other CL subfamilies.

Glycerophospholipids

Glycerophospholipids are phosphodiesters of glycerol and an alcohol referred to as the polar headgroup. The two glycerol carbinol groups are typically esterified with long chain fatty acyl groups which impart hydrophobic character to these otherwise polar molecules. The simplest glycerophospholipid is phosphatidic acid which is a monophosphoester of diacylglycerol. The common polar headgroups are choline, ethanolamine, serine, inositol, and glycerol which divide the phospholipids into different classes. This chapter discusses glycerophosphocholine lipids (PC), glycerophosphoethanolamine lipids (PE), phosphatidylserine (PS), phosphatidic acid (PA), phosphatidylinositol (PI), phosphatidylglycerol (PG), bis(acyl-lysophosphatidyl)glycerol (BMP) and cardiolipin.

A comprehensive insight into the lipid composition of Myxococcus xanthus by UPLC-ESI-MS

Analysis of whole cell lipid extracts of bacteria by means of ultra-performance (UP)LC-MS allows a comprehensive determination of the lipid molecular species present in the respective organism. The data allow conclusions on its metabolic potential as well as the creation of lipid profiles, which visualize the organism's response to changes in internal and external conditions. Herein, we describe: i) a fast reversed phase UPLC-ESI-MS method suitable for detection and determination of individual lipids from whole cell lipid extracts of all polarities ranging from monoacylglycerophosphoethanolamines to TGs; ii) the first overview of a wide range of lipid molecular species in vegetative Myxococcus xanthus DK1622 cells; iii) changes in their relative composition in selected mutants impaired in the biosynthesis of α-hydroxylated FAs, sphingolipids, and ether lipids; and iv) the first report of ceramide phosphoinositols in M. xanthus, a lipid species previously found only in eukaryotes.

Cardiolipin interaction with subunit c of ATP synthase: solid-state NMR characterization

The interaction of lipids with subunit c from F1F0 ATP synthase is studied by biophysical methods. Subunit c from both Escherichia coli and Streptococcus pneumoniae interacts and copurifies with cardiolipin. Solid state NMR data on oligomeric rings of F0 show that the cardiolipin interacts with the c subunit in membrane bilayers. These studies offer strong support for the hypothesis that F0 has specific interactions with cardiolipin.

Imaging mass spectrometry of diversified cardiolipin molecular species in the brain

MALDI imaging mass spectrometry (MALDI-IMS) has been used successfully in mapping different lipids in tissue sections, yet existing protocols fail to detect the diverse species of mitochondria-unique cardiolipins (CLs) in the brain which are essential for cellular and mitochondrial physiology. We have developed methods enabling the imaging of individual CLs in brain tissue. This was achieved by eliminating ion suppressive effects by (i) cross-linking carboxyl/amino containing molecules on tissue with 1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide hydrochloride and (ii) removing highly abundant phosphatidylcholine head groups via phospholipase C treatment. These treatments allowed the detection of CL species at 100 μm resolution and did not affect the amount or molecular species distribution of brain tissue CLs. When combined with augmented matrix application, these modifications allowed the visualization and mapping of multiple CL species in various regions of the brain including the thalamus, hippocampus, and cortex. Areas such as the dentate and stratum radiatum exhibited higher CL signals than other areas within the hippocampal formation. The habenular nuclear (Hb)/dorsal third ventricle (D3 V) and lateral ventricle (LV) areas were identified as CL "hot spots". Our method also allowed structural MS/MS fragmentation and mapping of CLs with identified fatty acid residues and demonstrated a nonrandom distribution of individual oxidizable (polyunsaturated fatty acid containing) and nonoxidizable (nonpolyunsaturated containing) CLs in different anatomical areas of the brain. To our knowledge, this method is the first label-free approach for molecular mapping of diversified CLs in brain tissue.

Group VIA phospholipase A2 mitigates palmitate-induced β-cell mitochondrial injury and apoptosis

Palmitate (C16:0) induces apoptosis of insulin-secreting β-cells by processes that involve generation of reactive oxygen species, and chronically elevated blood long chain free fatty acid levels are thought to contribute to β-cell lipotoxicity and the development of diabetes mellitus. Group VIA phospholipase A2 (iPLA2β) affects β-cell sensitivity to apoptosis, and here we examined iPLA2β effects on events that occur in β-cells incubated with C16:0. Such events in INS-1 insulinoma cells were found to include activation of caspase-3, expression of stress response genes (C/EBP homologous protein and activating transcription factor 4), accumulation of ceramide, loss of mitochondrial membrane potential, and apoptosis. All of these responses were blunted in INS-1 cells that overexpress iPLA2β, which has been proposed to facilitate repair of oxidized mitochondrial phospholipids, e.g. cardiolipin (CL), by excising oxidized polyunsaturated fatty acid residues, e.g. linoleate (C18:2), to yield lysophospholipids, e.g. monolysocardiolipin (MLCL), that can be reacylated to regenerate the native phospholipid structures. Here the MLCL content of mouse pancreatic islets was found to rise with increasing iPLA2β expression, and recombinant iPLA2β hydrolyzed CL to MLCL and released oxygenated C18:2 residues from oxidized CL in preference to native C18:2. C16:0 induced accumulation of oxidized CL species and of the oxidized phospholipid (C18:0/hydroxyeicosatetraenoic acid)-glycerophosphoethanolamine, and these effects were blunted in INS-1 cells that overexpress iPLA2β, consistent with iPLA2β-mediated removal of oxidized phospholipids. C16:0 also induced iPLA2β association with INS-1 cell mitochondria, consistent with a role in mitochondrial repair. These findings indicate that iPLA2β confers significant protection of β-cells against C16:0-induced injury.

Matrix-assisted laser desorption/ionization mass spectrometry imaging of cardiolipins in rat organ sections

Cardiolipin (CL) is a class of phospholipid tightly associated with the mitochondria functions and a prime target of oxidative stress. Peroxidation of CL dissociates its bound cytochrome C, a phenomenon that reflects oxidative stress sustained by the organ and a trigger for the intrinsic apoptotic pathway. However, CL distribution in normal organ tissues has yet to be documented. Fresh rat organs were snap-frozen, cut into cryosections that were subsequently desalted with ammonium acetate solution, and vacuum-dried. CL distribution in situ was determined using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) technique on sections sublimed with 2,5-dihydroxybenzoic acid. CL images in rat cardiac ventricular section showed a homogeneous distribution of a single m/z 1447.9 ion species that was confirmed as the (18:2)4 CL by tandem mass spectrometry. The presence of low abundant (18:2)3(18:1) CL with the bulk (18:2)4 CL in quadriceps femoris rendered the muscle CL exhibiting a slightly deviated isotopic pattern from that of cardiac muscle. In rat liver, MALDI-MSI unveiled three CL-containing mass ranges, each with a unique in situ distribution pattern. Co-registration of the CL ion images with its stained liver section image further revealed the association of CLs in each mass range with the functional zones in the liver parenchyma and suggests the participation of in situ CLs with localized hepatic functions such as oxidation, conjugation, and detoxification. The advances in CL imaging offer an approach with molecular accuracy to reveal potentially dysregulated metabolic machineries in acute and chronic diseased states.

Structural studies on archaeal phytanyl-ether lipids isolated from membranes of extreme halophiles by linear ion-trap multiple-stage tandem mass spectrometry with electrospray ionization

The structures of archaeal glycerophospholipids and glycolipids are unique in that they consist of phytanyl substituents ether linked to the glycerol backbone, imparting stability to the molecules. In this contribution, we described multiple-stage linear ion-trap combined with high resolution mass spectrometry toward structural characterization of this lipid family desorbed as lithiated adduct ions or as the [M-H](-) and [M-2H](2-) ions by ESI. MS(n) on various forms of the lithiated adduct ions yielded rich structurally informative ions leading to complete structure identification of this lipid family, including the location of the methyl branches of the phytanyl chain. By contrast, structural information deriving from MS(n) on the [M-H](-) and [M-2H](2-) ions is not complete. The fragmentation pathways in an ion-trap, including unusual internal loss of glycerol moiety and internal loss of hexose found for this lipid family were proposed. This mass spectrometric approach provides a simple tool to facilitate confident characterization of this unique lipid family.

Quantification of cardiolipin molecular species in Escherichia coli lipid extracts using liquid chromatography/electrospray ionization mass spectrometry

RATIONALE

Cardiolipin (CL), a glycerophospholipid containing four acyl chains, is found in most organisms including Gram-negative bacteria such as Escherichia coli. While CL composes only a fraction of the total glycerophospholipids, the four acyl chains lead to a large number of possible molecular species as defined by the total number of carbons and unsaturations in the acyl chains. Understanding the molecular composition of CL, and how it changes under different growth conditions, will aid in understanding the complex role of CL in E. coli.

METHODS

Normal-phase liquid chromatography/electrospray ionization mass spectrometry was used to quantify the CL molecular species (as defined by the total number of carbons:unsaturations in the acyl chains) in lipid extracts prepared from E. coli grown at 15 °C, 30 °C, 37 °C and 42 °C.

RESULTS

Fifty-six different CL species were identified as [M-2H](2-) ions in E. coli lipid extracts ranging from 60:0 to 72:4. CL species with an increased total number of unsaturations were more abundant in lipid extracts prepared from cells grown at 15 °C as compared to higher temperatures.

CONCLUSIONS

This work characterizes the CL composition of E. coli cells grown at various temperatures. By quantifying CL species at a molecular level we have illuminated the molecular complexity of the CL in this relatively simple model organism. This data will be useful for understanding CL function in E. coli and other organisms.

Bioinformatics tools and challenges in structural analysis of lipidomics MS/MS data

Lipidomics, the systematic study of the lipid composition of a cell or tissue, is an invaluable complement to knowledge gained by genomics and proteomics research. Mass spectrometry provides a means to detect hundreds of lipids in parallel, and this includes low abundance species of lipids. Nevertheless, frequently occurring isobaric and isomeric lipid species complicate lipidomics analyses from an analytical and bioinformatics perspective. Various MS/MS strategies have evolved to resolve ambiguous identifications of lipid species, and these strategies have been supported by corresponding bioinformatics analysis tools. This review intends to familiarize readers with available bioinformatics MS/MS analysis tools and databases, the structural information obtainable from these, and their applicability to different MS/MS strategies. Finally, future challenges in detecting double bond positions are investigated from a bioinformatics perspective.