Analysis of cholesteryl esters and diacylglycerols using lithiated adducts and electrospray ionization-tandem mass spectrometry

Looking in Depth at Oxidized Cholesteryl Esters by LC-MS/MS: Reporting Specific Fragmentation Fingerprints and Isomer Discrimination

Cholesteryl esters (CE) are prone to oxidation under increased oxidative stress conditions, but little is known about oxidized CE species (oxCE). To date, only a few oxCE have been identified, however, mainly based on the detection of molecular ions by mass spectrometry (MS) or target approaches for specific oxCE. The study of oxCE occurring from radical oxidation is still scarcely addressed. In this work, we made a comprehensive assessment of oxCE derivatives and their specific fragmentation patterns to identify detailed structural features and isomer differentiation using high-resolution C18 HPLC-MS- and MS/MS-based lipidomic approaches. The LC-MS/MS analysis allowed us to pinpoint oxCE structural isomers of long-chain and short-chain species, eluting at different retention times (t_R). Data analysis revealed that oxCE can be modified either in the fatty acyl moiety or in the cholesterol ring. The location of the hydroxy/hydroperoxy group originates characteristic fragment ions, namely the unmodified cholestenyl cation (m/z 369) for the isomer with oxidation in the fatty acyl chain or ions at m/z 367 and m/z 385 (369 + 16) when oxygenation occurs in the cholesterol ring. Additionally, we identified CE 18:2 and 20:4 aldehydic and carboxylic short-chain products that showed a clear fragmentation pattern that confirmed the modification in the fatty acyl chain. Specific fragmentation fingerprinting allowed discrimination of the isobaric short-chain species, namely carboxylic short-chain products, from hydroxy aldehyde short-chain products, with a hydroxycholesterol moiety. This new information is important to identify different oxCE in biological samples and will contribute to unraveling their role in biological conditions and diseases such as cardiovascular disease.

Native mass spectrometry-based metabolomics identifies metal-binding compounds

Although metals are essential for the molecular machineries of life, systematic methods for discovering metal-small molecule complexes from biological samples are limited. Here, we describe a two-step native electrospray ionization-mass spectrometry method, in which post-column pH adjustment and metal infusion are combined with ion identity molecular networking, a rule-based data analysis workflow. This method enabled the identification of metal-binding compounds in complex samples based on defined mass (m/z) offsets of ion species with the same chromatographic profiles. As this native electrospray metabolomics approach is suited to the use of any liquid chromatography-mass spectrometry system to explore the binding of any metal, this method has the potential to become an essential strategy for elucidating metal-binding molecules in biology.

Hybrid 213 nm photodissociation of cationized Sterol lipid ions yield [M]+. Radical products for improved structural characterization using multistage tandem mass spectrometry

Sterols are a class of lipid molecules that include cholesterol, oxysterols, and sterol esters. Sterol lipids play critical functional roles in mammalian biology, including the dynamic regulation of cell membrane fluidity, as precursors for the synthesis of bile acids, steroid hormones and vitamin D, as regulators of gene expression in lipid metabolism, and for cholesterol transport and storage. The most common method employed for sterol analysis is high performance liquid chromatography coupled with tandem mass spectrometry (MS/MS). However, conventional collision induced dissociation (CID) methods used for ion activation during MS/MS typically fail to provide sufficient structural information for unambiguous assignment of sterol species based on their fragmentation behaviour alone. This places a significant burden on the efficiency of the chromatographic separation methods for the effective separation of isomeric sterols. Here, toward developing an improved analysis strategy for sterol lipids, we have explored the novel use of 213 nm photodissociation MS/MS and hybrid multistage-MS/MS (i.e., MSⁿ) data acquisition approaches for the improved structural characterization of cholesterol, representative isomeric oxysterols, and cholesteryl esters. Most notably, UVPD-MS/MS of ammoniated, lithiated and sodiated adducts of cholesterol, several representative oxysterol species, and an oxosterol lipid, are shown to give rise to abundant [M]^+. radical cation products, that subsequently fragment during collision induced MS³ to yield extensive structurally informative product ions, similar to those observed by Electron Ionization, and that enable their unambiguously assignment, including isomeric differentiation of oxysterols. For cholesterol esters, a reversed hybrid collision induced-MS/MS and UVPD-MS³ approach is shown to enable assignment of the sterol backbone, and localization of the site(s) of unsaturation within esterified fatty acyl chains.

Alterations in the Human Plasma Lipidome in Response to Tularemia Vaccination

Tularemia is a highly infectious and contagious disease caused by the bacterium Francisella tularensis. To better understand human response to a live-attenuated tularemia vaccine and the biological pathways altered post-vaccination, healthy adults were vaccinated, and plasma was collected pre- and post-vaccination for longitudinal lipidomics studies. Using tandem mass spectrometry, we fully characterized individual lipid species within predominant lipid classes to identify changes in the plasma lipidome during the vaccine response. Separately, we targeted oxylipins, a subset of lipid mediators involved in inflammatory pathways. We identified 14 differentially abundant lipid species from eight lipid classes. These included 5-hydroxyeicosatetraenoic acid (5-HETE) which is indicative of lipoxygenase activity and, subsequently, inflammation. Results suggest that 5-HETE was metabolized to a dihydroxyeicosatrienoic acid (DHET) by day 7 post-vaccination, shedding light on the kinetics of the 5-HETE-mediated inflammatory response. In addition to 5-HETE and DHET, we observed pronounced changes in 34:1 phosphatidylinositol, anandamide, oleamide, ceramides, 16:1 cholesteryl ester, and other glycerophospholipids; several of these changes in abundance were correlated with serum cytokines and T cell activation. These data provide new insights into alterations in plasma lipidome post-tularemia vaccination, potentially identifying key mediators and pathways involved in vaccine response and efficacy.

A Review of Efforts to Improve Lipid Stability during Sample Preparation and Standardization Efforts to Ensure Accuracy in the Reporting of Lipid Measurements

Lipidomics is a rapidly growing field, fueled by developments in analytical instrumentation and bioinformatics. To date, most researchers and industries have employed their own lipidomics workflows without a consensus on best practices. Without a community-wide consensus on best practices for the prevention of lipid degradation and transformations through sample collection and analysis, it is difficult to assess the quality of lipidomics data and hence trust results. Clinical studies often rely on samples being stored for weeks or months until they are analyzed, but inappropriate sampling techniques, storage temperatures, and analytical protocols can result in the degradation of complex lipids and the generation of oxidized or hydrolyzed metabolite artifacts. While best practices for lipid stability are sample dependent, it is generally recommended that strategies during sample preparation capable of quenching enzymatic activity and preventing oxidation should be considered. In addition, after sample preparation, lipid extracts should be stored in organic solvents with antioxidants at -20 °C or lower in an airtight container without exposure to light or oxygen. This will reduce or eliminate sublimation, and chemically and physically induced molecular transformations such as oxidation, enzymatic transformation, and photon/heat-induced degradation. This review explores the available literature on lipid stability, with a particular focus on human health and/or clinical lipidomic applications. Specifically, this includes a description of known mechanisms of lipid degradation, strategies, and considerations for lipid storage, as well as current efforts for standardization and quality insurance of protocols.

Profiling of Cholesteryl Esters by Coupling Charge-Tagging Paternò-Büchi Reaction and Liquid Chromatography-Mass Spectrometry

The profile of cholesteryl esters (CEs) is increasingly used in metabolic disease monitoring due to the roles of CE in regulating the cholesterol level. While electrospray ionization-tandem mass spectrometry is routinely applied for the identification and quantitation of CE, it has a limitation of not being able to provide the location of carbon-carbon double bond (C═C) within unsaturated fatty acyls. In this study, we paired offline 2-acetylpyridine (2-AP) Paternò-Büchi (PB) reaction and reversed-phase liquid chromatography-tandem mass spectrometry to achieve highly sensitive and structural informative CE analysis from complex mixtures. The 2-AP PB reactions of CE standards provided 20-30% conversion but resulted in enhanced ion signal relative to that of intact CE detected as ammonium adduct ions. MS/MS of 2-AP derivatized CE via collision-induced dissociation produced two abundant diagnostic ions for each C═C in a fatty acyl, leading to both sensitive identification and quantitation of C═C location isomers. Twelve saturated and twenty-seven unsaturated CEs were profiled in pooled human plasma; of the latter group, relative quantitation of 6 groups of C═C location isomers was achieved. A dehydrocholesteryl ester, DHE 18:2 (Δ9,12), was confidently differentiated from coexisting compositional isomers: CE 18:3 (Δ9,12,15) and CE 18:3 (Δ6,9,12). The above results represented improved CE coverage at the C═C location level over those reported by gas chromatography MS or acetone PB-MS/MS methods.

Data Processing Pipeline for Lipid Profiling of Carotid Atherosclerotic Plaque with Mass Spectrometry Imaging

Atherosclerosis is a lipid and inflammation-driven disease of the arteries that is characterized by gradual buildup of plaques in the vascular wall. A so-called vulnerable plaque, consisting of a lipid-rich necrotic core contained by a thin fibrous cap, may rupture and trigger thrombus formation, which can lead to ischemia in the heart (heart attack) or in the brain (stroke). In this study, we present a protocol to investigate the lipid composition of advanced human carotid plaques using matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI), providing a framework that should enable the discrimination of vulnerable from stable plaques based on lipid composition. We optimized the tissue preparation and imaging methods by systematically analyzing data from three specimens: two human carotid endarterectomy samples (advanced plaque) and one autopsy sample (early stage plaque). We show a robust data reduction method and evaluate the variability of the endarterectomy samples. We found diacylglycerols to be more abundant in a thrombotic area compared to other plaque areas and could distinguish advanced plaque from early stage plaque based on cholesteryl ester composition. We plan to use this systematic approach to analyze a larger dataset of carotid atherosclerotic plaques.

Comparison of liquid chromatography-mass spectrometry and direct infusion microchip electrospray ionization mass spectrometry in global metabolomics of cell samples

In this study, the feasibility of direct infusion electrospray ionization microchip mass spectrometry (chip-MS) was compared to the commonly used liquid chromatography-mass spectrometry (LC-MS) in non-targeted metabolomics analysis of human foreskin fibroblasts (HFF) and human induced pluripotent stem cells (hiPSC) reprogrammed from HFF. The total number of the detected features with chip-MS and LC-MS were 619 and 1959, respectively. Approximately 25% of detected features showed statistically significant changes between the cell lines with both analytical methods. The results show that chip-MS is a rapid and simple method that allows high sample throughput from small sample volumes and can detect the main metabolites and classify cells based on their metabolic profiles. However, the selectivity of chip-MS is limited compared to LC-MS and chip-MS may suffer from ion suppression.

Shotgun Analysis of Diacylglycerols Enabled by Thiol-ene Click Chemistry

Diacylglycerols (DAGs) are a subclass of neutral lipids actively involved in cell signaling and metabolism. Alteration in DAG metabolism has been associated with onset and progression of several human-related diseases. The structural diversity of DAGs and their low concentrations in biological samples call for the development of methods that are capable of sensitive identification and quantitation of each DAG species as well as rapid profiling when a biochemical pathway is perturbed. In this work, the thiol-ene click chemistry has been employed to introduce a charge-tag, namely, cysteamine (CA), at a carbon-carbon double bond (C═C) of unsaturated DAGs. This one-pot photochemical derivatization is fast (within 1 min), universal (monotagging) for DAGs varying in fatty acyl chain lengths and the number of C═Cs, and suitable for small sample volume (e.g., 1-50 μL plasma). Because of the presence of the amine group in CA, tagged DAGs showed at least 10 times increase in response to electrospray ionization as compared to conventional ammonium adduct formation. Low-energy collision-induced dissociation of CA tagged DAGs allowed confident assignment of fatty acyl composition. A neutral loss scan based on characteristic 95 Da loss (a combined loss of CA and H₂O) of tagged DAGs has been established as a sensitive means for unsaturated DAG detection (limit of detection = 100 pM) and quantitation from mixtures. The analytical utility of CA tagging was demonstrated by shotgun analysis of unsaturated DAGs in human plasma, including samples from type 2 diabetes mellitus patients.

Uncovering Structural Diversity of Unsaturated Fatty Acyls in Cholesteryl Esters via Photochemical Reaction and Tandem Mass Spectrometry

Mass spectrometry analysis of cholesteryl esters (CEs) faces several challenges, with one of them being the determination of the carbon-carbon double bond (C=C) locations within unsaturated fatty acyl chains. Paternὸ-Büchi (PB) reaction, a photochemical reaction based on the addition of acetone to C=C, is capable of C=C location determination when coupled with tandem mass spectrometry (MS/MS). In this study, the PB reaction conditions were tailored for CEs and subsequent nanoelectrospray ionization (nanoESI). A solvent system containing acetone/methanol/dichloromethane/water (40/30/20/10, volume ratios) and 100 μM LiOH was determined to be optimal, resulting in reasonable PB reaction yield (~30%) and good ionization efficiency (forming lithium adduct of CEs). Collision-induced dissociation (CID) of the PB reaction products produced characteristic fragment ions of CE together with those modified by the PB reactions, such as lithiated fatty acyl ([FA + Li]+) and its PB product ([FA - PB + Li]+). MS3 CID of [FA - PB + Li]+ led to abundant C=C diagnostic ion formation, which was used for C=C location determination and isomer quantitation. A PB-MS3 CID approach was developed and applied for CE analysis from human plasma. A series of unsaturated CEs was identified with specific C=C locations within fatty acyl chains. Absolute quantitation for each CE species was achieved including coexisting C=C location isomers, such as Δ9 and Δ11 isomers of CE 18:1 and ω-6 and ω-3 isomers of CE 18:3. These results show that PB-MS/MS is useful in uncovering structural diversity of CEs due to unsaturation in fatty acyls, which is often undetected from current lipid analysis approach. Graphical Abstract ᅟ.

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

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

PON3 knockout mice are susceptible to obesity, gallstone formation, and atherosclerosis

We report the engineering and characterization of paraoxonase-3 knockout mice (Pon3KO). The mice were generally healthy but exhibited quantitative alterations in bile acid metabolism and a 37% increased body weight compared to the wild-type mice on a high fat diet. PON3 was enriched in the mitochondria-associated membrane fraction of hepatocytes. PON3 deficiency resulted in impaired mitochondrial respiration, increased mitochondrial superoxide levels, and increased hepatic expression of inflammatory genes. PON3 deficiency did not influence atherosclerosis development on an apolipoprotein E null hyperlipidemic background, but it did lead to a significant 60% increase in atherosclerotic lesion size in Pon3KO mice on the C57BL/6J background when fed a cholate-cholesterol diet. On the diet, the Pon3KO had significantly increased plasma intermediate-density lipoprotein/LDL cholesterol and bile acid levels. They also exhibited significantly elevated levels of hepatotoxicity markers in circulation, a 58% increase in gallstone weight, a 40% increase in hepatic cholesterol level, and increased mortality. Furthermore, Pon3KO mice exhibited decreased hepatic bile acid synthesis and decreased bile acid levels in the small intestine compared with wild-type mice. Our study suggests a role for PON3 in the metabolism of lipid and bile acid as well as protection against atherosclerosis, gallstone disease, and obesity.

Inhibiting monoacylglycerol acyltransferase 1 ameliorates hepatic metabolic abnormalities but not inflammation and injury in mice

Abnormalities in hepatic lipid metabolism and insulin action are believed to play a critical role in the etiology of nonalcoholic steatohepatitis. Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol, which is the penultimate step in one pathway for triacylglycerol synthesis. Hepatic expression of Mogat1, which encodes an MGAT enzyme, is increased in the livers of mice with hepatic steatosis, and knocking down Mogat1 improves glucose metabolism and hepatic insulin signaling, but whether increased MGAT activity plays a role in the etiology of nonalcoholic steatohepatitis is unclear. To examine this issue, mice were placed on a diet containing high levels of trans fatty acids, fructose, and cholesterol (HTF-C diet) or a low fat control diet for 4 weeks. Mice were injected with antisense oligonucleotides (ASOs) to knockdown Mogat1 or a scrambled ASO control for 12 weeks while remaining on diet. The HTF-C diet caused glucose intolerance, hepatic steatosis, and induced hepatic gene expression markers of inflammation, macrophage infiltration, and stellate cell activation. Mogat1 ASO treatment, which suppressed Mogat1 expression in liver and adipose tissue, attenuated weight gain, improved glucose tolerance, improved hepatic insulin signaling, and decreased hepatic triacylglycerol content compared with control ASO-treated mice on HTF-C chow. However, Mogat1 ASO treatment did not reduce hepatic diacylglycerol, cholesterol, or free fatty acid content; improve histologic measures of liver injury; or reduce expression of markers of stellate cell activation, liver inflammation, and injury. In conclusion, inhibition of hepatic Mogat1 in HTF-C diet-fed mice improves hepatic metabolic abnormalities without attenuating liver inflammation and injury.

Analysis of sitosteryl oleate esters in phytosterols esters enriched foods by HPLC-ESI-MS(2.)

Phytosteryl esters (PE)-enriched spreads are marketed for eating and cooking purposes. Temperature and also light exposure are the major factors leading to the formation of PE oxides in food matrix. In this study a high-speed HPLC-MS(2) method was developed to analyze the major PE present in PE-enriched spreads: sitosteryl oleate (SO) and its oxidation products, by using synthesized compounds as standards. This analytical method was used to quantify seven SO oxides formed in PE-enriched spreads after heating at different temperatures for varying time periods and after prolonged exposure to sunlight. Quantification of remaining native SO was also performed after these different treatments. It was found that under specific heating conditions the decrease of the SO amount was much more important compared to the formation of SO oxides showing that many other products are formed. In contrast to heating, sunlight radiation did not result in the degradation of SO and very few oxides were formed.

A rapid and precise method for quantification of fatty acids in human serum cholesteryl esters by liquid chromatography and tandem mass spectrometry

We described a rapid and precise method for simultaneous quantification of eleven fatty acids in human serum cholesteryl esters (CEFAs) by liquid chromatography and tandem mass spectrometry (LC-MS/MS). After extraction of serum lipids with isopropanol, CEFAs were separated on reversed phase liquid chromatography and detected by mass spectrometry in positive ion mode with multiple reaction monitor. Individual CEFA was quantified by peak area normalization method and expressed as molar percent of total CEFAs. The run time was less than 5 min and detection limits were from 0.31 to 14.50 × 10(-5)mmol/L. Recoveries of the CEFAs ranged from 91.85% to 104.83% with a mean of 99.12%. The intra and total CVs for the measurement of CEFAs were 0.87-7.70% and 1.02-7.65%, respectively. This LC-MS/MS method required no internal standards, eliminated natural isotope interferences, and provided reproducible and reliable results for 11 major CEFAs in human serum. This method can be used in monitoring and evaluating dietary fatty acid intake. Additional studies are needed to evaluate the associations between serum CEFAs and cardiovascular disease risk factors.

Development of an automated multi-injection shotgun lipidomics approach using a triple quadrupole mass spectrometer

Shotgun lipidomics is a well-suited approach to monitor lipid alterations due to its ability to scan for varying lipid types on a global, class and individual species level. However, the ability to perform high-throughput shotgun lipidomics has remained challenging due to time-consuming data processing and hardware limitations. To increase the throughput nature of shotgun lipidomics, an automated shotgun lipidomics approach is described utilizing conventional low flow gradient liquid chromatography (LC) analysis (post-injection) coupled with multiple sample injections per sample (on a lipid scan per injection basis). The proposed automated multi-injection approach resulted in a reproducible lipid scanning period of 2.5 min (in a 4.5 min total data acquisition period), thereby providing a sufficient scanning period for performing either mass spectrometric or tandem mass spectrometric analyses. In addition to being simple, robust and reproducible, this approach was also constructed to be cost-effective by using common LC instrumentation and customizable as the data acquisition period can be tailored to perform different scan types, period lengths and scan numbers. Combined with a strategy to create multiple lipid-specific aliquots per sample, the overall approach provides a simple and efficient platform to perform high-throughput lipid profiling.

Lipidomics: analysis of the lipid composition of cells and subcellular organelles by electrospray ionization mass spectrometry

Lipidomics aims to quantitatively define lipid classes, including their molecular species, in biological systems. Lipidomics has experienced rapid progress, mainly because of continuous technical advances in instrumentation that are now enabling quantitative lipid analyses with an unprecedented level of sensitivity and precision. The still-growing category of lipids includes a broad diversity of chemical structures with a wide range of physicochemical properties. Reflecting this diversity, different methods and strategies are being applied to the quantification of lipids. Here, I review state-of-the-art electrospray ionization tandem mass spectrometric approaches and direct infusion to quantitatively assess lipid compositions of cells and subcellular fractions. Finally, I discuss a few examples of the power of mass spectrometry-based lipidomics in addressing cell biological questions.

Long-term Expression of Apolipoprotein B mRNA-specific Hammerhead Ribozyme via scAAV8.2 Vector Inhibits Atherosclerosis in Mice

Target substrate-specific hammerhead ribozyme cleaves the specific mRNA efficiently and results in the inhibition of gene expression. In humans, overproduction of apolipoprotein B (apoB) is positively associated with premature coronary artery diseases. The goal of this study is to demonstrate that long-term reduction of apoB gene expression using hammerhead ribozyme would result in inhibition of atherosclerosis development. We designed two hammerhead ribozymes targeted at the nucleotides of apoB mRNA GUC²³²⁶ (designated RB1) and GUA⁶⁶⁷⁹ (designated RB15), and we used self-complementary adeno-associated virus 8.2 (scAAV8.2) vector to deliver these active ribozymes of RB1, RB15, combination of RB1/RB15, and an inactive hammerhead ribozyme RB15 mutant to atherosclerosis-prone LDb mice (Ldlr^−/−Apobec1^−/−). LDb mice lack both low density lipoproteins (LDL) receptor (Ldlr^−/−) and apoB mRNA editing enzyme (Apobec1^−/−) genes and develop atherosclerosis spontaneously. After the RB1, RB15, or combination of RB1/RB15 ribozymes treatment, the LDb mice had significantly decreased plasma triglyceride and apoB levels, resulting in markedly decreased of atherosclerotic lesions, Furthermore, the active ribozymes treatment decreased the levels of diacylglycerol acyltransferase 1 (Dgat1) mRNA and the levels of multiple diacylglycerol (DAG) molecular species. These results provide the first evidence that decreased apoB levels results to reduction of Dgat1 expression and triglyceride levels (TAG), which had a significant impact on the development of atherosclerosis.

Quantitative profiling of major neutral lipid classes in human meibum by direct infusion electrospray ionization mass spectrometry

PURPOSE

The purpose of this investigation was to better understand lipid composition in human meibum.

METHODS

Intact lipids in meibum samples were detected by direct infusion electrospray ionization mass spectrometry (ESI-MS) analysis in positive detection mode using sodium iodide (NaI) as an additive. The peak intensities of all major types of lipid species, that is, wax esters (WEs), cholesteryl esters (CEs), and diesters (DEs) were corrected for peak overlapping and isotopic distribution; an additional ionization efficiency correction was performed for WEs and CEs, which was simplified by the observation that the corresponding ionization efficiency was primarily dependent on the specific lipid class and saturation degree of the lipids while independent of the carbon chain length. A set of WE and CE standards was spiked in meibum samples for ionization efficiency determination and absolute quantitation.

RESULTS

The absolute amount (μmol/mg) for each of 51 WEs and 31 CEs in meibum samples was determined. The summed masses for 51 WEs and 31 CEs accounted for 48 ± 4% and 40 ± 2%, respectively, of the total meibum lipids. The mass percentages of saturated and unsaturated species were determined to be 75 ± 2% and 25 ± 1% for CEs and 14 ± 1% and 86 ± 1% for WEs. The profiles for two types of DEs were also obtained, which include 42 α,ω Type II DEs, and 21 ω Type I-St DEs.

CONCLUSIONS

Major neutral lipid classes in meibum samples were quantitatively profiled by ESI-MS analysis with NaI additive.

Obesity-related alterations in cardiac lipid profile and nondipping blood pressure pattern during transition to diastolic dysfunction in male db/db mice

Obesity and a nondipping circadian blood pressure (BP) pattern are associated with diastolic dysfunction. Ectopic lipid accumulation is increasingly recognized as an important metabolic abnormality contributing to diastolic dysfunction. However, little is known about the contribution of different lipids and the composition of lipid analytes to diastolic dysfunction. We have performed functional and structural studies and analyzed cardiac lipid profile at two time points during progression to diastolic dysfunction in a genetic model of obesity. Serial cardiac magnetic resonance imaging and telemetric measures of BP between 12 and 15 wk of age in obese male db/db mice indicated a nondipping circadian BP pattern and normal diastolic function at 12 wk that progressed to a deteriorating nondipping pattern and onset of diastolic dysfunction at 15 wk of age. Lipidomic analysis demonstrated elevated fatty acids and ceramides in db/db at 12 wk, but their levels were decreased at 15 wk, and this was accompanied by persistent mitochondrial ultrastructural abnormalities in concert with evidence of increased fatty acid oxidation and enhanced production of reactive oxygen species. Triacylglyceride and diacylglyceride levels were elevated at both 12 and 15 wk, but their composition changed to consist of more saturated and less unsaturated fatty acyl at 15 wk. An increase in the lipid droplets was apparent at both time points, and this was associated with increases in phosphatidycholine. In conclusion, a distinct pattern of myocardial lipid remodeling, accompanied by oxidative stress, is associated with the onset of diastolic dysfunction in obese, insulin-resistant db/db mice.

Cholesterol efflux analyses using stable isotopes and mass spectrometry

Cholesterol efflux from macrophages and the vascular wall is the initial step of the cardiovascular protective reverse cholesterol transport process. This study demonstrates a mass spectrometry based assay to measure the cellular and medium content of [d(7)]cholesterol and unlabeled cholesterol that can be used to measure cholesterol efflux from cell lines. Using a triple-quadrupole electrospray ionization-MS instrument in direct infusion mode, product ion scanning for m/z 83, neutral loss (NL) 375.5 scanning, and NL 368.5 scanning were used to detect cholesterol (as an acetylated derivative), [d(7)]cholesteryl ester (CE), and unlabeled CE, respectively. The same mass of [d(7)]cholesterol was substituted for [(3)H]cholesterol under standard efflux assay conditions. At the end of [d(7)]cholesterol loading, the intracellular mass of [d(7)]cholesterol was twofold greater than that of unlabeled cholesterol, and the intracellular [d(7)]CE profile was similar to that of unlabeled CE. Efflux of cholesterol to apolipoprotein A-I and high-density lipoproteins was similar comparing efflux of either [d(7)]cholesterol or [(3)H]cholesterol as measured by following efflux of the tracers only. This technique also can be used to assess the efflux of unlabeled cholesterol to acceptors in medium that are initially cholesterol-free (e.g., apolipoprotein A-I). Taken together, this mass spectrometry-based assay provides new molecular detail to assess cholesterol efflux.

Gas phase chemistry of Li+ with amides: the observation of LiOH loss in mass spectrometry

Collision-induced dissociation (CID) of Li(+) adducts of three sets of compounds that contains an amide bond, including 2-(4, 6-dimethoxypyrimidin-2-ylsulfanyl)-N-phenylbenzamide, its derivatives and simpler structures was investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Observed fragment ions include those that reflect loss of LiOH. Other product ions result from the Smiles rearrangement and direct C-S bond cleavage. MS/MS of H/D exchange products demonstrated occurrence of a 1,3-H shift from the amide nitrogen atom to the phenyl ring of these compounds. The LiOH loss from Li(+) adducts of amides was further examined by CID of [M + Li](+) ions of N-phenylbenzamide and N-phenylcinnamide. Loss of LiOH was essentially the sole fragmentation reaction observed for the former. For the latter, both losses of LiOH and H(2)O were discovered. The presence of electron-donating substituents of the phenyl ring of these compounds was found to facilitate elimination of LiOH, while that loss was retarded by electron-withdrawing substituents. Proposed fragment ion structures were supported by elemental compositions deduced from ultrahigh resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR-MS/MS) m/z value determinations. Density functional theory-based (DFT) calculations were performed to evaluate potential mechanisms for these reactions.

Electrospray ionization tandem mass spectrometry of sodiated adducts of cholesteryl esters

Cholesteryl esters (CE) are important lipid storage molecules. The present study demonstrates that sodiated adducts of CE molecular species form positive ions that can be detected in both survey scan mode as well as by exploiting class-specific fragmentation in MS/MS scan modes. A common neutral loss for CE is the loss of cholestane (NL 368.5), which can be used to specifically quantify tissue CE molecular species. Using this MS/MS technique, CE molecular species were quantified in mouse monocyte-derived macrophages (J774 cells) incubated with either linoleic (18:2) or arachidonic acid (20:4). These studies revealed that arachidonic acid was not only incorporated into the CE pool, but also was elongated resulting in the accumulation of 22:4 and 24:4 CE molecular species in macrophages. Additionally, this technique was used to quantify CE molecular species present in crude lipid extracts from plasma of female mice fed a Western diet, which led to an enrichment in CE molecular species containing monounsaturated fatty acids compared to female mice fed a normal chow diet. Last, NL 368.5 spectra revealed the oxidation of the aliphatic fatty acid residues of CE molecular species containing polyunsaturated fatty acids. Taken together, these studies demonstrate the utility of using sodiated adducts of CE in conjunction with direct infusion electrospray ionization tandem mass spectrometry to rapidly quantify CE molecular species in biological samples.