Phosphatidylcholines and -ethanolamines can be easily mistaken in phospholipid mixtures: a negative ion MALDI-TOF MS study with 9-aminoacridine as matrix and egg yolk as selected example

A Five-Year Update on Matrix Compounds for MALDI-MS Analysis of Lipids

Matrix-assisted laser desorption and ionization (MALDI) is a widely used soft-ionization technique of modern mass spectrometry (MS). MALDI enables the analysis of nearly all chemical compounds—including polar and apolar (phospho)lipids—with a minimum extent of fragmentation. MALDI has some particular advantages (such as the possibility to acquire spatially-resolved spectra) and is competitive with the simultaneously developed ESI (electrospray ionization) MS. Although there are still some methodological aspects that need to be elucidated in more detail, it is obvious that the careful selection of an appropriate matrix plays the most important role in (lipid) analysis. Some lipid classes can be detected exclusively if the optimum matrix is used, and the matrix determines the sensitivity by which a particular lipid is detected within a mixture. Since the matrix is, thus, crucial for optimum results, we provide here an update on the progress in the field since our original review in this journal in 2018. Thus, only the development during the last five years is considered, and lipids are sorted according to increasing complexity, starting with free fatty acids and ending with cardiolipins and phosphoinositides.

Nutritional lipidomics for the characterization of lipids in food

Lipids represent one out of three major macronutrient classes in the human diet. It is estimated to account for about 15-20% of the total dietary intake. Triacylglycerides comprise the majority of them, estimated 90-95%. Other lipid classes include free fatty acids, phospholipids, cholesterol, and plant sterols as minor components. Various methods are used for the characterization of nutritional lipids, however, lipidomics approaches become increasingly attractive for this purpose due to their wide coverage, comprehensiveness and holistic view on composition. In this chapter, analytical methodologies and workflows utilized for lipidomics profiling of food samples are outlined with focus on mass spectrometry-based assays. The chapter describes common lipid extraction protocols, the distinct instrumental mass-spectrometry based analytical platforms for data acquisition, chromatographic and ion-mobility spectrometry methods for lipid separation, briefly mentions alternative methods such as gas chromatography for fatty acid profiling and mass spectrometry imaging. Critical issues of important steps of lipidomics workflows such as structural annotation and identification, quantification and quality assurance are discussed as well. Applications reported over the period of the last 5years are summarized covering the discovery of new lipids in foodstuff, differential profiling approaches for comparing samples from different origin, species, varieties, cultivars and breeds, and for food processing quality control. Lipidomics as a powerful tool for personalized nutrition and nutritional intervention studies is briefly discussed as well. It is expected that this field is significantly growing in the near future and this chapter gives a short insight into the power of nutritional lipidomics approaches.

A new update of MALDI-TOF mass spectrometry in lipid research

Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS) is an indispensable tool in modern lipid research since it is fast, sensitive, tolerates sample impurities and provides spectra without major analyte fragmentation. We will discuss some methodological aspects, the related ion-forming processes and the MALDI MS characteristics of the different lipid classes (with the focus on glycerophospholipids) and the progress, which was achieved during the last ten years. Particular attention will be given to quantitative aspects of MALDI MS since this is widely considered as the most serious drawback of the method. Although the detailed role of the matrix is not yet completely understood, it will be explicitly shown that the careful choice of the matrix is crucial (besides the careful evaluation of the positive and negative ion mass spectra) in order to be able to detect all lipid classes of interest. Two developments will be highlighted: spatially resolved Imaging MS is nowadays well established and the distribution of lipids in tissues merits increasing interest because lipids are readily detectable and represent ubiquitous compounds. It will also be shown that a combination of MALDI MS with thin-layer chromatography (TLC) enables a fast spatially resolved screening of an entire TLC plate which makes the method competitive with LC/MS.

Phosphorus deprivation affects composition and spatial distribution of membrane lipids in legume nodules

In legumes, symbiotic nitrogen (N) fixation (SNF) occurs in specialized organs called nodules after successful interactions between legume hosts and rhizobia. In a nodule, N-fixing rhizobia are surrounded by symbiosome membranes, through which the exchange of nutrients and ammonium occurs between bacteria and the host legume. Phosphorus (P) is an essential macronutrient, and N2-fixing legumes have a higher requirement for P than legumes grown on mineral N. As in the previous studies, in P deficiency, barrel medic (Medicago truncatula) plants had impaired SNF activity, reduced growth, and accumulated less phosphate in leaves, roots, and nodules compared with the plants grown in P sufficient conditions. Membrane lipids in M. truncatula tissues were assessed using electrospray ionization-mass spectrometry. Galactolipids were found to increase in P deficiency, with declines in phospholipids (PL), especially in leaves. Lower PL losses were found in roots and nodules. Subsequently, matrix-assisted laser desorption/ionization-mass spectrometry imaging was used to spatially map the distribution of the positively charged phosphatidylcholine (PC) species in nodules in both P-replete and P-deficient conditions. Our results reveal heterogeneous distribution of several PC species in nodules, with homogeneous distribution of other PC classes. In P poor conditions, some PC species distributions were observed to change. The results suggest that specific PC species may be differentially important in diverse nodule zones and cell types, and that membrane lipid remodeling during P stress is not uniform across the nodule.

Toward the Rational Design of Universal Dual Polarity Matrix for MALDI Mass Spectrometry

A series of novel anthranilic acid derivatives I-IV, of which COOH-NH₂ (I) and COOH-NHMe (IV) are endowed with acid and base bifunctionality, were designed and synthesized for matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry applications in dual polarity molecular imaging of biological samples, particularly for lipids. The heat of protonation, deprotonation, and proton transfer reaction as well as the capability of analyzing biomolecules in both positive and negative ion modes for I-IV were systematically investigated under standard 355 nm laser excitation. The results indicate correlation between dual polarity and acid-base property. Further, COOH-NHMe (IV) showed a unique performance and was successfully applied as the matrix for MALDI-TOF mass spectrometry imaging (MSI) for studying the mouse brain. Our results demonstrate the superiority of COOH-NHMe (IV) in detecting more lipid and protein species compared to commercially available matrices. Moreover, MALDI-TOF MSI results were obtained for lipid distributions, making COOH-NHMe (IV) a potential next generation universal matrix.

Uncovering matrix effects on lipid analyses in MALDI imaging mass spectrometry experiments

The specific matrix used in matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) can have an effect on the molecules ionized from a tissue sample. The sensitivity for distinct classes of biomolecules can vary when employing different MALDI matrices. Here, we compare the intensities of various lipid subclasses measured by Fourier transform ion cyclotron resonance (FT-ICR) IMS of murine liver tissue when using 9-aminoacridine (9AA), 5-chloro-2-mercaptobenzothiazole (CMBT), 1,5-diaminonaphthalene (DAN), 2,5-Dihydroxyacetophenone (DHA), and 2,5-dihydroxybenzoic acid (DHB). Principal component analysis and receiver operating characteristic curve analysis revealed significant matrix effects on the relative signal intensities observed for different lipid subclasses and adducts. Comparison of spectral profiles and quantitative assessment of the number and intensity of species from each lipid subclass showed that each matrix produces unique lipid signals. In positive ion mode, matrix application methods played a role in the MALDI analysis for different cationic species. Comparisons of different methods for the application of DHA showed a significant increase in the intensity of sodiated and potassiated analytes when using an aerosol sprayer. In negative ion mode, lipid profiles generated using DAN were significantly different than all other matrices tested. This difference was found to be driven by modification of phosphatidylcholines during ionization that enables them to be detected in negative ion mode. These modified phosphatidylcholines are isomeric with common phosphatidylethanolamines confounding MALDI IMS analysis when using DAN. These results show an experimental basis of MALDI analyses when analyzing lipids from tissue and allow for more informed selection of MALDI matrices when performing lipid IMS experiments.

Influence of Lipid Fragmentation in the Data Analysis of Imaging Mass Spectrometry Experiments

Imaging mass spectrometry (IMS) is becoming an essential technique in lipidomics. Still, many questions remain open, precluding it from achieving its full potential. Among them, identification of species directly from the tissue is of paramount importance. However, it is not an easy task, due to the abundance and variety of lipid species, their numerous fragmentation pathways, and the formation of a significant number of adducts, both with the matrix and with the cations present in the tissue. Here, we explore the fragmentation pathways of 17 lipid classes, demonstrating that in-source fragmentation hampers identification of some lipid species. Then, we analyze what type of adducts each class is more prone to form. Finally, we use that information together with data from on-tissue MS/MS and MS³ to refine the peak assignment in a real experiment over sections of human nevi, to demonstrate that statistical analysis of the data is significantly more robust if unwanted peaks due to fragmentation, matrix, and other species that only introduce noise in the analysis are excluded.

Application of Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging for Food Analysis

Food contains various compounds, and there are many methods available to analyze each of these components. However, the large amounts of low-molecular-weight metabolites in food, such as amino acids, organic acids, vitamins, lipids, and toxins, make it difficult to analyze the spatial distribution of these molecules. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging is a two-dimensional ionization technology that allows the detection of small metabolites in tissue sections without requiring purification, extraction, separation, or labeling. The application of MALDI-MS imaging in food analysis improves the visualization of these compounds to identify not only the nutritional content but also the geographical origin of the food. In this review, we provide an overview of some recent applications of MALDI-MS imaging, demonstrating the advantages and prospects of this technology compared to conventional approaches. Further development and enhancement of MALDI-MS imaging is expected to offer great benefits to consumers, researchers, and food producers with respect to breeding improvement, traceability, the development of value-added foods, and improved safety assessments.

Comparative evaluation of the extraction and analysis of urinary phospholipids and lysophospholipids using MALDI-TOF/MS

Lipids, particularly phospholipids (PLs) and lysophospholipids (LPLs), are attracting increasing scientific interest for their biological functions in cells and their potential as disease biomarkers for Alzheimer's disease and several types of cancer. Urinary PLs and LPLs could be ideal clinical biomarkers, because urine can be collected easily and noninvasively. However, due to their very low concentrations in urine compared with the relatively large quantity of contaminants in this matrix, efficient extraction and sensitive detection are required for analyzing urinary PLs and LPLs. In this study, various methods for analyzing PLs and LPLs in urine were compared and optimized from a clinical perspective. An optimized lipid extraction method and a matrix for matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS) were established using two external ionization standards and an internal standard mix containing 13 human urinary lipids. 9-Aminoacridine (9-AA) was a useful and effective matrix for the MALDI-TOF/MS analysis of all the internal standard lipids in both positive and negative ion modes. However, it was necessary to determine the proportional lipid concentrations from the balance between the extracted lipid and the matrix. The extraction efficiency and reproducibility of the acidified Bligh and Dyer method were excellent for both positively and negatively charged lipids. Analysis of small volumes of urine was the most efficient with the 9-AA MALDI matrix at concentrations of or below 5 mM. The combined analytical procedures allowed rapid and comprehensive screening of low concentrations of PLs and LPLs in clinical samples.

Polydopamine-capped AgNPs as a novel matrix overcoming the ion suppression of phosphatidylcholine for MALDI MS comprehensive imaging of glycerophospholipids and sphingolipids in impact-induced injured brain

In this study, AgNPs@PDA was synthesized as a matrix for the analysis of lipids in both positive and negative ion modes.

Recent Developments of Useful MALDI Matrices for the Mass Spectrometric Characterization of Lipids

Matrix-assisted laser desorption/ionization (MALDI) is one of the most successful “soft” ionization methods in the field of mass spectrometry and enables the analysis of a broad range of molecules, including lipids. Although the details of the ionization process are still unknown, the importance of the matrix is commonly accepted. Both, the development of and the search for useful matrices was, and still is, an empirical process, since properties like vacuum stability, high absorption at the laser wavelength, etc. have to be fulfilled by a compound to become a useful matrix. This review provides a survey of successfully used MALDI matrices for the lipid analyses of complex biological samples. The advantages and drawbacks of the established organic matrix molecules (cinnamic or benzoic acid derivatives), liquid crystalline matrices, and mixtures of common matrices will be discussed. Furthermore, we will deal with nanocrystalline matrices, which are most suitable to analyze small molecules, such as free fatty acids. It will be shown that the analysis of mixtures and the quantitative analysis of small molecules can be easily performed if the matrix is carefully selected. Finally, some basic principles of how useful matrix compounds can be “designed” de novo will be introduced.

Visualizing phosphatidylcholine via mass spectrometry imaging: relevance to human health

INTRODUCTION

Mass spectrometry imaging (MSI) techniques are nowadays widely used to obtain spatially resolved metabolite information from biological tissues. Since (phospho)lipids occur in all animal tissues and are very sensitively detectable, they are often in the focus of such studies. This particularly applies for phosphatidylcholines (PC) which are very sensitively detectable as positive ions due to the permanent positive charge of their choline headgroup. Areas covered: After a short introduction of lipid species occurring in biological systems and approaches normally used to obtain spatially resolved mass spectra (with the focus on matrix-assisted laser desorption/ionization coupled to time-of-flight (MALDI-TOF) MSI) a survey will be given which diseases have so far been characterized by changes of the PC composition. Expert commentary: Since PC species are very sensitively detectable by MS, sensitivity is not a major issue. However, spatial resolution is still limited and cellular dimensions can be hardly resolved by MALDI-TOF MSI, which is a critical point of the available approaches. Due to lacks of reproducibility and standardization further development is required.

Combined Use of MALDI-TOF Mass Spectrometry and 31P NMR Spectroscopy for Analysis of Phospholipids

Lipids are important and abundant constituents of all biological tissues and body fluids. In particular, phospholipids (PL) constitute a major part of the cellular membrane, play a role in signal transduction, and some selected PL are increasingly considered as potential disease markers. However, methods of lipid analysis are less established in comparison to techniques of protein analysis. Mass spectrometry (MS) is an increasingly used technique to analyze lipids, especially in combination with electrospray ionization (ESI) MS which is the so far best established ionization method. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS has itself proven to be also useful in the field of lipid analysis. ³¹P nuclear magnetic resonance (NMR) spectroscopy is another powerful method of PL analysis, represents a direct quantitative method, and does not suffer from suppression effects.This chapter gives an overview of methodological aspects of MALDI-TOF MS and ³¹P NMR in lipid research and summarizes the specific advantages and drawbacks of both methods. In particular, suppression effects in MS will be highlighted and possible ways to overcome this problem (use of different matrices, separation of the relevant lipid mixture prior to analysis) will be discussed.

Phospholipids as cancer biomarkers: Mass spectrometry-based analysis

Lipids, particularly phospholipids (PLs), are key components of cellular membrane. PLs play important and diverse roles in cells such as chemical-energy storage, cellular signaling, cell membranes, and cell-cell interactions in tissues. All these cellular processes are pertinent to cells that undergo transformation, cancer progression, and metastasis. Thus, there is a strong possibility that some classes of PLs are expected to present in cancer cells and tissues in cellular physiology. The mass spectrometric soft-ionization techniques, electrospray ionization (ESI), and matrix-assisted laser desorption/ionization (MALDI) are well-established in the proteomics field, have been used for lipidomic analysis in cancer research. This review focused on the applications of mass spectrometry (MS) mainly on ESI-MS and MALDI-MS in the structural characterization, molecular composition and key roles of various PLs present in cancer cells, tissues, blood, and urine, and on their importance for cancer-related problems as well as challenges for development of novel PL-based biomarkers. The profiling of PLs helps to rationalize their functions in biological systems, and will also provide diagnostic information to elucidate mechanisms behind the control of cancer, diabetes, and neurodegenerative diseases. The investigation of cellular PLs with MS methods suggests new insights on various cancer diseases and clinical applications in the drug discovery and development of biomarkers for various PL-related different cancer diseases. PL profiling in tissues, cells and body fluids also reflect the general condition of the whole organism and can indicate the existence of cancer and other diseases. PL profiling with MS opens new prospects to assess alterations of PLs in cancer, screening specific biomarkers and provide a basis for the development of novel therapeutic strategies. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 37:107-138, 2018.

The combination of 2,5-dihydroxybenzoic acid and 2,5-dihydroxyacetophenone matrices for unequivocal assignment of phosphatidylethanolamine species in complex mixtures

Unequivocal assignment of phospholipid peaks in complex mixtures is difficult if only the m/z values but no tandem mass spectrometry (MS/MS) data are available. This is usually the case for matrix-assisted laser/desorption ionization time-of-flight (MALDI-TOF) MS imaging experiments and the analysis has normally to be performed without prior separation. Another problem might be the often matrix-induced loss of one methyl group in phosphatidylcholine (PC) species, which makes them detectable as negative ions becoming isomers of some phosphatidylethanolamines (PEs). Selected lipid mixtures of known compositions were investigated by negative ion MALDI-TOF MS and various imaging experiments. In addition to common matrices such as 2,5-dihydroxybenzoic acid (DHB) and 9-aminoacridine (9-AA), different binary matrices, including 2,5-dihydroxyacetophenone (2,5-DHAP) as matrix additive to DHB, were tested to probe their performance in both ionization modes. Beside artificial PC and PE mixtures of known compositions, egg yolk and liver extracts as well as cryosections from liver and pancreas tissue were selected as biologically relevant systems. The majority of the binary MALDI matrices used here leads to the loss of a methyl group from PC in the negative ion mode, which makes the clear identification of PE species ambiguous. However, this problem does not apply if a mixture of DHB and 2,5-DHAP is used. Therefore, the application of DHB/2,5-DHAP as matrix is a simple method to unequivocally identify PEs even in complex mixtures and tissue sections as negative ions and without the necessity to separate the individual lipid classes prior to MS detection. Graphical abstract Many common MALDI matrices (such as 9-AA) induce the loss of a methyl group from PC rendering the PC detectable as negative ion. These ions (m/z 744.6 in the upper trace) represent isomers of typical PE species. It will be shown that this problem can be avoided if mixtures between DHB and 2,5-DHAP are applied. At these conditions, POPC is exclusively detectable as a matrix adduct with DHB (at m/z 912.6, lower trace) and does not interfere with PE. This approach can also be used in MALDI MS imaging.

1,6-Diphenyl-1,3,5-hexatriene (DPH) as a Novel Matrix for MALDI MS Imaging of Fatty Acids, Phospholipids, and Sulfatides in Brain Tissues

1,6-Diphenyl-1,3,5-hexatriene (DPH) is a commonly used fluorescence probe for studying cell membrane-lipids due to its affinity toward the acyl chains in the phospholipid bilayers. In this work, we investigated its use in matrix-assisted laser desorption/ionization (MALDI) as a new matrix for mass spectrometry imaging (MSI) of mouse and rat brain tissue. DPH exhibits very minimal matrix-induced background signals for the analysis of small molecules (below m/z of 1000). In the negative ion mode, DPH permits the highly sensitive detection of small fatty acids (m/z 200-350) as well as a variety of large lipids up to m/z of 1000, including lyso-phospholipid, phosphatidic acid (PA), phosphoethanolamine (PE), phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidylinositol (PI), and sulfatides (ST). The analytes were mostly detected as the deprotonated ion [M - H]^-. Our results also demonstrate that sublimated DPH is stable for at least 24 h under the vacuum of our MALDI mass spectrometer. The ability to apply DPH via sublimation coupled with its low volatility allows us to perform tissue imaging of the above analytes at high spatial resolution. The degree of lipid fragmentation was determined experimentally at varying laser intensities. The results illustrated that the use of relatively low laser energy is important to minimize the artificially generated fatty acid signals. On the other hand, the lipid fragmentation obtained at higher laser energies provided tandem MS information useful for lipid structure elucidation.

MALDI Orbitrap Mass Spectrometry Profiling of Dysregulated Sulfoglycosphingolipids in Renal Cell Carcinoma Tissues

Matrix-assisted laser desorption/ionization coupled with Orbitrap mass spectrometry (MALDI-Orbitrap-MS) is used for the clinical study of patients with renal cell carcinoma (RCC), as the most common type of kidney cancer. Significant changes in sulfoglycosphingolipid abundances between tumor and autologous normal kidney tissues are observed. First, sulfoglycosphingolipid species in studied RCC samples are identified using high mass accuracy full scan and tandem mass spectra. Subsequently, optimization, method validation, and statistical evaluation of MALDI-MS data for 158 tissues of 80 patients are discussed. More than 120 sulfoglycosphingolipids containing one to five hexosyl units are identified in human RCC samples based on the systematic study of their fragmentation behavior. Many of them are recorded here for the first time. Multivariate data analysis (MDA) methods, i.e., unsupervised principal component analysis (PCA) and supervised orthogonal partial least square discriminant analysis (OPLS-DA), are used for the visualization of differences between normal and tumor samples to reveal the most up- and downregulated lipids in tumor tissues. Obtained results are closely correlated with MALDI mass spectrometry imaging (MSI) and histologic staining. Important steps of the present MALDI-Orbitrap-MS approach are also discussed, such as the selection of best matrix, correct normalization, validation for semiquantitative study, and problems with possible isobaric interferences on closed masses in full scan mass spectra. Graphical Abstract ᅟ.

Addition of CsCl reduces ion suppression effects in the matrix-assisted laser desorption/ionization mass spectra of triacylglycerol/phosphatidylcholine mixtures and adipose tissue extracts

RATIONALE

Ion suppression is a known disadvantage in mixture analysis. Matrix-assisted laser desorption/ionization (MALDI) mass spectra of crude adipose tissue extracts are dominated by triacylglycerol (TAG) signals while less abundant phospholipids such as phosphatidylcholines (PC) and particularly phosphatidylethanolamines (PE) are suppressed. It is suggested that addition of an excess of cesium (Cs) ions helps to overcome this problem.

METHODS

Selected lipid mixtures of known compositions and organic adipose tissue extracts were investigated by positive ion MALDI-time-of-flight mass spectrometry (TOF MS). 2,5-Dihydroxybenzoic acid (DHB) in methanol was used as the matrix. In selected cases the methanolic DHB solution was saturated by the addition of different solid alkali chlorides (such as NaCl, KCl, RbCl and CsCl). Studies on the solubilities of these salts in methanol and the interaction with DHB (by ¹³ C NMR) were also performed.

RESULTS

Saturation of the DHB matrix with solid CsCl leads to tremendous intensity differences, i.e. the intensities of the TAG signals (which otherwise dominate the mass spectra) are significantly reduced. In contrast, the intensity of small signals of phospholipids increases considerably. Decrease in the TAG signal intensity is particularly caused by the considerable size of the Cs⁺ ion which prevents successful analyte ionization.

CONCLUSIONS

The addition of CsCl improves the detectability of otherwise invisible or weak phospholipid ions. This is a simple approach to detect small amounts of phospholipids in the presence of an excess of TAG. No laborious and time-consuming separation of the total lipid extract into the individual lipid classes is required. Copyright © 2016 John Wiley & Sons, Ltd.

A comparison of PC oxidation products as detected by MALDI-TOF and ESI-IT mass spectrometry

Oxidized (phospho)lipids are of paramount interest for different reasons: besides their in vivo relevance as markers of inflammatory diseases, they are often needed in the laboratory to study the response of selected cells to oxidized lipids. Mass spectrometry (MS) is nowadays one of the most powerful methods to identify lipid oxidation products. Although MALDI and ESI MS are both widely used, it is so far not clear whether all potential phospholipid oxidation products can be detected by both methods This aspect will be studied here using NaMnO₄-oxidized phosphatidylcholine 16:0/18:1 and 16:0/18:2 as simple, but reliable model systems. We will show that chain-shortened products such as aldehydes and carboxylic acids (generated by cleavage at the double bond position) can be easily detected by both ionization methods: without the need of any derivatization. However, primary oxidation products such as hydroperoxides can be predominantly detected by ESI MS while MALDI-TOF MS detects secondary oxidation products derived thereof more sensitively. Potential reasons for these differences will be discussed and put in the context of biological mixture analysis.

Norharmane Matrix Enhances Detection of Endotoxin by MALDI-MS for Simultaneous Profiling of Pathogen, Host, and Vector Systems

The discovery of novel pathogenic mechanisms engaged during bacterial infections requires the evolution of advanced techniques. Here, we evaluate the dual polarity matrix norharmane (NRM) to improve detection of bacterial lipid A (endotoxin), from host and vector tissues infected withFrancisella novicida (Fn). We evaluated NRM for improved detection and characterization of a wide range of lipids in both positive and negative polarities, including lipid A and phospholipids across a range of matrix-assisted laser desorption-ionization-coupled applications. NRM matrix improved the limit of detection (LOD) for monophosphoryl lipid A (MPLA) down to picogram level representing a 10-fold improvement of LOD versus 2,5-dihydroxybenzoic acid and 100-fold improvement of LOD versus 9-aminoacridine (9-AA). Improved LOD for lipid A subsequently facilitated detection of theFn lipid A major ion (m/z 1665) from extracts of infected mouse spleen and the temperature-modifiedFn lipid A atm/z 1637 from infectedDermacentor variabilis ticks. Finally, we simultaneously mapped bacterial phospholipid signatures within anFn-infected spleen along with an exclusively host-derived inositol-based phospholipid (m/z 933) demonstrating coprofiling of the host-pathogen interaction. Expanded use of NRM matrix in other infection models and endotoxin-targeting imaging experiments will improve our understanding of the lipid interactions at the host-pathogen interface.

Combination of ESI and MALDI mass spectrometry for qualitative, semi-quantitative and in situ analysis of gangliosides in brain

Gangliosides are a family of complex lipids that are abundant in the brain. There is no doubt the investigations about the distribution of gangliosides in brian and the relationship between gangliosides and Alzheimer’s disease is profound. However, these investigations are full of challenges due to the structural complexity of gangliosides. In this work, the method for efficient extraction and enrichment of gangliosides from brain was established. Moreover, the distribution of gangliosides in brain was obtained by matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI). It was found that 3-aminoquinoline (3-AQ) as matrix was well-suited for MALDI MS analysis of gangliosides in negative ion mode. In addition, the pretreatment by ethanol (EtOH) cleaning brain section and the addition of ammonium formate greatly improved the MS signal of gangliosides in the brain section when MALDI MSI analysis was employed. The distribution of ganliosides in cerebral cortex, hippocampus and cerebellum was respectively acquired by electrospray ionization (ESI) MS and MALDI MSI, and the data were compared for reliability evaluation of MALDI MSI. Further, applying MALDI MSI technology, the distribution of gangliosides in amyloid precursor protein transgenic mouse brain was obtained, which may provide a new insight for bioresearch of Alzheimer’s disease (AD).

Lipid-conjugated fluorescent pH sensors for monitoring pH changes in reconstituted membrane systems

Accurate real-time measurements of the dynamics of proton concentration gradients are crucial for detailed molecular studies of proton translocation by membrane-bound enzymes. To reduce complexity, these measurements are often carried out with purified, reconstituted enzyme systems. Yet the most paramount problem to detect pH changes in reconstituted systems is that soluble pH reporters leak out of the vesicle system during the reconstitution procedure. This requires loading of substantial amounts of pH-sensors into the lumen of unilamellar liposomes during reconstitution. Here, we report the synthesis and detailed characterisation of two lipid-linked pH sensors employing amine-reactive forms of seminaphthorhodafluors (SNARF®-1 dye) and rhodamine probes (pHrodo™ Red dye). Lipid-conjugation of both dyes allowed for efficient detergent-based reconstitution of these pH indicators into liposomes. Vesicle-embedded pHrodo™ displayed excellent photostability and an optimal pH-response between 4 and 7. The suitability of the lipid-linked pHrodo™ probe as a pH reporter was demonstrated by assaying the activity of a plant plasma membrane H(+)-ATPase (proton pump) reconstituted in proteoliposomes.

1,8-Di(piperidinyl)-naphthalene – rationally designed MAILD/MALDI matrix for metabolomics and imaging mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of small molecules requires special matrices, which do not generate interfering signals below m/z 500.

MALDI-TOF MS to monitor the kinetics of phospholipase A2-digestion of oxidized phospholipids

Free fatty acids (FFA) are released through phospholipase A2 (PLA2), which cleaves the fatty acyl residue at the sn-2 position of phospholipids (PL). During inflammatory diseases, reactive oxygen species (such as HOCl) lead to the formation of oxidatively modified PL (e.g., chlorohydrin generation). It is still widely unknown to which extent the oxidation of PL influences their digestibility by PLA2. Additionally, investigations on the impact of the position of the unsaturated fatty acyl residue (sn-1 versus sn-2 position) and modifications of the headgroup (for instance phosphatidylcholine (PC) versus phosphatidylethanolamine (PE)) are also lacking. Therefore, the aim of this study is the investigation of these aspects using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry to elucidate the PL/lysophospholipid (LPL) ratios as measures of the PLA2 digestibility. We will show that oxidative modifications of PL by HOCl have a considerable impact on the PLA2 digestibility, i.e., oxidation of the unsaturated fatty acyl residues leads to a reduced digestibility of both PC and PE. Besides, it will be shown that MALDI MS is a convenient and reliable tool to investigate the related changes.

Formation and Characterization of Supported Lipid Bilayers Composed of Hydrogenated and Deuterated Escherichia coli Lipids

Supported lipid bilayers are widely used for sensing and deciphering biomolecular interactions with model cell membranes. In this paper, we present a method to form supported lipid bilayers from total lipid extracts of Escherichia coli by vesicle fusion. We show the validity of this method for different types of extracts including those from deuterated biomass using a combination of complementary surface sensitive techniques; quartz crystal microbalance, neutron reflection and atomic force microscopy. We find that the head group composition of the deuterated and the hydrogenated lipid extracts is similar (approximately 75% phosphatidylethanolamine, 13% phosphatidylglycerol and 12% cardiolipin) and that both samples can be used to reconstitute high-coverage supported lipid bilayers with a total thickness of 41 ± 3 Å, common for fluid membranes. The formation of supported lipid bilayers composed of natural extracts of Escherichia coli allow for following biomolecular interactions, thus advancing the field towards bacterial-specific membrane biomimics.

Quantitative lipidomic analysis of plasma and plasma lipoproteins using MALDI-TOF mass spectrometry

Knowledge of the plasma lipid composition is essential to clarify the specific roles of different lipid species in various pathophysiological processes. In this study, we developed an analytical strategy combining high-performance liquid chromatography with evaporative light scattering detection (HPLC-ELSD) and off-line coupling with matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry (MALDI-TOF/MS) to determine the composition of plasma and major lipoproteins at two levels, lipid classes and lipid species. We confirmed the suitability of MALDI-TOF/MS as a quantitative measurement tool studying the linearity and repeatability for triglycerides (TG), phosphatidylethanolamine (PE) and phosphatidylcholine (PC). Moreover, data obtained with this method were correlated with other lipid classes and species measurements using currently available technologies. To establish the potential utility of our approach, human plasma very low density- (VLDL), low density- (LDL) and high density- (HDL) lipoproteins from 10 healthy donors were separated using ultracentrifugation, and compositions of nine lipid classes, cholesteryl esters (CE), TG, free cholesterol (FC), PE, phosphatidylinositol (PI), sulfatides (S), PC, lysophosphatidylcholine (LPC) and sphingomyelin (SM), analyzed. In total, 157 lipid species in plasma, 182 in LDL, 171 in HDL, and 148 in VLDL were quantified. The lipidomic profile was consistent with known differences in lipid classes, but also revealed unexpected differences in lipid species distribution of lipoproteins, particularly for LPC and SM. In summary, the methodology developed in this study constitutes a valid approach to determine the lipidomic composition of plasma and lipoproteins.

A simple method to generate oxidized phosphatidylcholines in amounts close to one milligram

Oxidized (phospho)lipids are of paramount interest from different reasons: beside their significant in vivo relevance, these products are often needed in the laboratory to study the response of selected cells to oxidized lipids. Unfortunately, the commercial availability of oxidized lipids is limited and scientists interested in studying the physiological impact of oxidized lipids are normally forced to prepare the required compounds by themselves. We will show here that chain-shortened products of oxidized phosphatidylcholines (PCs) such as aldehydes and carboxylic acids can be easily (and in nearly quantitative yields) generated by the Fenton reaction (H2O2+Fe(2+)) or the KMnO4-induced oxidation of the PC. Using the Fenton reaction and physiological saline, chlorinated oxidation products such as chlorohydrins are also readily available. Additionally, it will be shown that preparative thin-layer chromatography (TLC) is a convenient but simple method to isolate the individual oxidation products in reasonable yields and high purities: all relevant products could be successfully identified by matrix-assisted laser desorption and ionization (MALDI) mass spectrometry and the amounts of the oxidized products determined by a simple colorimetric assay.

Mass spectrometry methodology in lipid analysis

Lipidomics is an emerging field, where the structures, functions and dynamic changes of lipids in cells, tissues or body fluids are investigated. Due to the vital roles of lipids in human physiological and pathological processes, lipidomics is attracting more and more attentions. However, because of the diversity and complexity of lipids, lipid analysis is still full of challenges. The recent development of methods for lipid extraction and analysis and the combination with bioinformatics technology greatly push forward the study of lipidomics. Among them, mass spectrometry (MS) is the most important technology for lipid analysis. In this review, the methodology based on MS for lipid analysis was introduced. It is believed that along with the rapid development of MS and its further applications to lipid analysis, more functional lipids will be identified as biomarkers and therapeutic targets and for the study of the mechanisms of disease.

Detection of choline and phosphatidic acid (PA) catalyzed by phospholipase D (PLD) using MALDI-QIT-TOF/MS with 9-aminoacridine matrix

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine (PC), the most abundant phospholipids of plasma membrane, resulting in the production of choline and phosphatidic acid (PA). Choline is a precursor of the neurotransmitter acetylcholine, whereas PA functions as an intracellular lipid mediator of diverse biological functions. For assessing PLD activity in vitro, PLD-derived choline has been often analyzed with radioactive or non-radioactive methods. In this study, we have developed a new method for detecting choline and PA with MALDI-QIT-TOF/MS by using 9-aminoacridine as a matrix. The standard calibration curves showed that choline and PA could be detected with linearity over the range from 0.05 and 1 pmol, respectively. Importantly, this method enables the concomitant detection of choline and PA as a reaction product of PC hydrolysis by PLD2 proteins. Thus, our simple and direct method would be useful to characterize the enzymatic properties of PLD, thereby providing insight into mechanisms of PLD activation.

1,8-bis(dimethylamino)naphthalene/9-aminoacridine: a new binary matrix for lipid fingerprinting of intact bacteria by matrix assisted laser desorption ionization mass spectrometry

The effectiveness of a novel binary matrix composed of 1,8-bis(dimethylamino)naphthalene (DMAN; proton sponge) and 9-aminoacridine (9AA) for the direct lipid analysis of whole bacterial cells by matrix assisted laser desorption ionization mass spectrometry (MALDI MS) is demonstrated. Deprotonated analyte signals nearly free of matrix-related ions were observed in negative ion mode. The effect of the most important factors (laser energy, pulse voltage, DMAN/9AA ratio, analyte/matrix ratio) was investigated using a Box-Behnken response surface design followed by multi-response optimization in order to simultaneously maximize signal-to-noise (S/N) ratio and resolution. The chemical surface composition of single or mixed matrices was explored by X-ray photoelectron spectroscopy (XPS). Moreover, XPS imaging was used to map the spatial distribution of a model phospholipid in single or binary matrices. The DMAN/9AA binary matrix was then successfully applied to the analysis of intact Gram positive (Lactobacillus sanfranciscensis) or Gram negative (Escherichia coli) microorganisms. About fifty major membrane components (free fatty acids, mono-, di- and tri-glycerides, phospholipids, glycolipids and cardiolipins) were quickly and easily detected over a mass range spanning from ca. 200 to ca. 1600 m/z. Moreover, mass spectra with improved S/N ratio (compared to single matrices), reduced chemical noise and no formation of matrix-clusters were invariably obtained demonstrating the potential of this binary matrix to improve sensitivity.

Conservation of honey bee (Apis mellifera) sperm phospholipids during storage in the bee queen--a TLC/MALDI-TOF MS study

The honey bee (Apis mellifera) is characterized by a high degree of phenotypic plasticity of senescence-related processes, and has therefore become a model organism of gerontological research. Sperm of honey bee drones can remain fertile for several years within the storage organ of queens. The reason for this longevity is unknown, but the suppression of lipid peroxidation seems to play a decisive role. Here, we examined the questions of whether spermatheca- and in vitro-stored honey bee sperm are indeed resistant to lipid peroxidation, and whether the nature of sperm lipids could explain this resistance. The lipid composition of bee sperm was determined by matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) combined with thin-layer chromatography (TLC). The positive ion mass spectra of drone sperm lipids are dominated by two glycerophosphocholine (GPC) species, although small amounts of sphingomyelins (SM) and glycerophosphoethanolamines (GPE) are also detectable after TLC. Alkyl/acyl and alkenyl/acyl compounds of GPC, and alkyl/acyl as well as diacyl compounds of GPE were detected containing oleyl, oleoyl, palmityl and palmitoyl as the most abundant residues. Assignments of all compounds have been additionally verified by enzymatic digestion and exposition to HCl. During incubation of sperm in the presence of air, characteristic lipid oxidation products such as lysophosphatidylcholine (LPC) appear. Inside the spermatheca, however, sperm lipids are obviously protected from oxidation and their composition does not change, even if they are stored over years. Our data support the view that the membrane composition of honey bee sperm could help to explain the extraordinary longevity of these cells.

Analysis of phospolipids and glycolipids by thin-layer chromatography-matrix-assisted laser desorption and ionization mass spectrometry

Thin-layer chromatography-matrix-assisted laser desorption and ionization mass spectrometry (TLC-MALDI-MS) of organic extracts from biological samples allows untargeted analysis and structural characterization of phospholipids and glycolipids ionized from the near-surface region of a sample separated on a TLC-plate. In particular, MALDI-MS enables the sensitive detection of many analytes directly from the solid surface of an ordinary TLC-plate even without previous staining. It will be shown that the detailed fatty acyl composition of phospholipids can be determined solely by TLC-MALDI-MS without previous derivatization, enzymatic cleavage and/or reversed phase separation. MALDI-time-of-flight (TOF) MS is thus a powerful method in this field due to its high sensitivity, low extent of induced fragmentation and simple, user-friendly performance. This review summarizes the so far available knowledge about combined TLC-MALDI-MS for phospholipid and glycolipid characterization together with the technical workflow and a survey of applications. Finally a perspective on the future of TLC-MALDI-MS is given.

Enhanced sensitivity for high spatial resolution lipid analysis by negative ion mode matrix assisted laser desorption ionization imaging mass spectrometry

We have achieved enhanced lipid imaging to a ~10 μm spatial resolution using negative ion mode matrix assisted laser desorption ionization (MALDI) imaging mass spectrometry, sublimation of 2,5-dihydroxybenzoic acid as the MALDI matrix, and a sample preparation protocol that uses aqueous washes. We report on the effect of treating tissue sections by washing with volatile buffers at different pHs prior to negative ion mode lipid imaging. The results show that washing with ammonium formate, pH 6.4, or ammonium acetate, pH 6.7, significantly increases signal intensity and number of analytes recorded from adult mouse brain tissue sections. Major lipid species measured were glycerophosphoinositols, glycerophosphates, glycerolphosphoglycerols, glycerophosphoethanolamines, glycerophospho-serines, sulfatides, and gangliosides. Ion images from adult mouse brain sections that compare washed and unwashed sections are presented and show up to 5-fold increases in ion intensity for washed tissue. The sample preparation protocol has been found to be applicable across numerous organ types and significantly expands the number of lipid species detectable by imaging mass spectrometry at high spatial resolution.