Structural determination of picomole amounts of phospholipids via electrospray ionization tandem mass spectrometry

Fragmentation study of short-chain products derived from oxidation of diacylphosphatidylcholines by electrospray tandem mass spectrometry: identification of novel short-chain products

Lineloyl-palmitoyl (PLPC) and arachidonoyl-palmitoyl (PAPC) phosphatidylcholine were oxidized under Fenton reaction conditions (H2O2 and Fe2+), and the short-chain products formed were identified by electrospray ionization mass spectrometry (ESI-MS). The short-chain products resulted from beta-cleavage of oxygen-centered radicals and comprised aldehydes, hydroxyaldehydes and dicarboxylic acids that yielded both [MH]+ and [MNa]+ ions. The fragmentation of the [MH]+ and [MNa]+ ions of the peroxidation products was studied by tandem mass spectrometry (MS/MS). The MS/MS spectra of both ions showed ions resulting from characteristic losses of glycerophosphatidylcholine. Other product ions, resulting from C-C cleavages occurring in the vicinity of the functional group, and fragmentations involving the hydroxy groups, were the most informative since they allowed us to obtain structural information relating to the sn-2 acyl residue. Both fragmentation pathways are due to charge-remote fragmentation occurring by a 1,4-hydrogen elimination mechanism and/or by homolytic cleavage. Furthermore, the fragmentation pathway of some ions observed in the ESI-MS spectrum was not consistent with the fragmentation behavior expected for some of the short-chain species identified in the literature and allowed the reassignment of the ions as different structures. Isobaric ions were observed in the ESI-MS spectra of both oxidized phospholipids, and were differentiated based on distinct fragmentation. The detailed knowledge of lipid peroxidation degradation products is of major importance and should be very valuable in providing new markers for oxidative stress signaling and for disease states monitoring.

Changes of phospholipid composition within the dystrophic muscle by matrix-assisted laser desorption/ionization mass spectrometry and mass spectrometry imaging

Duchenne muscular dystrophy (DMD) is a neuromuscular disease linked to the lack of the dystrophin, a submembrane protein, leading to muscle weakness and associated with a defect of the lipid metabolism. A study of the fatty acid composition of glycerophosphatidylcholines by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS) and tandem mass spectrometry (MS/MS) enabled us to characterize a change of the lipid composition of dystrophic cells at the time of the differentiation. This modification has been used as a marker to identify with profiling and imaging MALDI-ToF MS regenerating areas in sections of an mdx mouse leg muscle. It is the first time that such a slight change in fatty acid composition has been observed directly on tissue slices using mass spectrometry. This approach will be useful in monitoring the treatment of muscular regeneration.

The Effect of Eicosapentaenoic Acid on Rat Lymphocyte Proliferation Depends Upon Its Position in Dietary Triacylglycerols

Animal and human studies have shown that greatly increasing the amount of fish oil [rich in long-chain (n-3) PUFA] in the diet can decrease lymphocyte functions. The effects of a more modest provision of long-chain (n-3) PUFA and whether eicosapentaenoic acid (20:5) and docosahexaenoic acid (22:6) have the same effects as one another are unclear. Whether the position of 20:5 or 22:6 in dietary triacylglycerols (TAG) influences their incorporation into immune cells and their subsequent functional effects is not known. In this study, male weanling rats were fed for 6 wk one of 9 diets that contained 178 g lipid/kg and that differed in the type of (n-3) PUFA and in the position of these in dietary TAG. The control diet contained 4.4 g alpha-linolenic acid (18:3)/100 g total fatty acids. In the other diets, 20:5 or 22:6 replaced a portion (50 or 100%) of 18:3, and were in the sn-2 or the sn-1(3) position of dietary TAG. There were significant dose-dependent increases in the proportion of 20:5 or 22:6 in spleen mononuclear cell phospholipids when 20:5 or 22:6 was fed. These increases were at the expense of arachidonic acid and were largely independent of the position of 20:5 or 22:6 in dietary TAG. Spleen lymphocyte proliferation increased dose dependently when 20:5 was fed in the sn-1(3) position of dietary TAG. There were no significant differences in interleukin-2, interferon-gamma or interleukin-10 production among spleen cells from rats fed the different diets. Prostaglandin E(2) production by spleen mononuclear cells was decreased by inclusion of either 20:5 or 22:6 in the diet in the sn-1(3) position. Thus, incorporation of 20:5 or 22:6 into spleen mononuclear cell phospholipids is not influenced by the position in dietary TAG. However, the pattern of incorporation may be influenced, and there are some differential functional effects of the position of long-chain (n-3) PUFA in dietary TAG. A moderate increase in the intake of 20:5 at the sn-1(3) position of dietary TAG increases lymphocyte proliferation.

Charting molecular composition of phosphatidylcholines by fatty acid scanning and ion trap MS3 fragmentation

The molecular composition of phosphatidylcholines (PCs) in total lipid extracts was characterized by a combination of multiple precursor ion scanning on a hybrid quadrupole time-of-flight mass spectrometer and MS3 fragmentation on an ion trap mass spectrometer. Precursor ion spectra for 50 acyl anion fragments of fatty acids (fatty acid scanning) acquired in parallel increased the specificity and the dynamic range of the detection of PCs and identified the fatty acid moieties in individual PC species. Subsequent analysis of detected PC peaks by MS3 fragmentation on an ion trap mass spectrometer quantified the relative amount of their positional isomers, thus providing the most detailed and comprehensive characterization of the molecular composition of the pool of PCs at the low-picomole level. The method is vastly simplified, compared with conventional approaches, and does not require preliminary separation of lipid classes or of individual molecular species, enzymatic digestion, or chemical derivatization. The approach was validated by the comparative analysis of the molecular composition of PCs from human red blood cells. In the total lipid extract of Madin-Darby canine kidney II cells, we detected 46 PC species with unique fatty acid composition and demonstrated that the presence of positional isomers almost doubled the total number of individual molecular species.

Characterization of alkylacyl, alk-1-enylacyl and lyso subclasses of glycerophosphocholine by tandem quadrupole mass spectrometry with electrospray ionization

Positive ion tandem quadrupole mass spectrometric methods for structural characterization of the subclasses of sn-glycero-3-phosphocholine (PC), including alkylacyl- and alk-1-enylacylphosphocholine and lysophosphatidylcholine (LPC), are described. Following collisionally activated dissociation, the [M + Li](+) ions generated by electrospray ionization yield abundant informative fragment ions that permit structural determination, and distinction of regioisomers among lysophosphatidylcholine can be easily achieved. In contrast, structurally informative ions arising from [M + H](+) or [M + Na](+) ions are less prominent. The most abundant ion observed in the product-ion spectra of the [M + Li](+) ions of plasmenyl- and plasmanyl-PC and of LPC arises from loss of N(CH(3))(3) ([M + Li - 59](+)). This feature permits their distinction from a product-ion spectrum arising from a diacylphosphatidylcholine, in which the [M + Li - 183](+) ion reflecting loss of phosphocholine is the most prominent. Examples for identification of various subclasses of PC in biological extracts by tandem mass spectrometry applying various constant neutral loss scannings are also shown.

Global analyses of cellular lipidomes directly from crude extracts of biological samples by ESI mass spectrometry: a bridge to lipidomics

Lipidomics is a rapidly expanding research field in which multiple techniques are utilized to quantitate the hundreds of chemically distinct lipids in cells and determine the molecular mechanisms through which they facilitate cellular function. Recent developments in electrospray ionization mass spectrometry (ESI/MS) have made possible, for the first time, the precise identification and quantification of alterations in a cell's lipidome after cellular perturbations. This review provides an overview of the essential role of ESI/MS in lipidomics, presents a broad strategy applicable for the generation of lipidomes directly from cellular extracts of biological samples by ESI/MS, and summarizes salient examples of strategies utilized to conquer the lipidome in physiologic signaling as well as pathophysiologically relevant disease states. Because of its unparalleled sensitivity, specificity, and efficiency, ESI/MS has provided a critical bridge to generate highly accurate data that fingerprint cellular lipidomes to facilitate insight into the functional role of subcellular membrane compartments and microdomains in mammalian cells. We believe that ESI/MS-facilitated lipidomics has now opened a critical door that will greatly increase our understanding of human disease.

Structural analysis of phosphatidylcholines by post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The utility of post-source decay (PSD) matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was investigated for the structural analysis of phosphatidylcholine (PC). PC did not produce detectable negative molecular ion from MALDI, but positive ions were observed as both [PC+H](+) and [PC+Na](+). The PSD spectra of the protonated PC species contained only one fragment corresponding to the head group (m/z 184), while the sodiated precursors produced many fragment ions, including those derived from the loss of fatty acids. The loss of fatty acid from the C-1 position (sn-1) of the glycerol backbone was favored over the loss of fatty acid from the C-2 position (sn-2). Ions emanating from the fragmentation of the head group (phosphocholine) included [PC+Na-59](+), [PC+Na-183](+) and [PC+Na-205](+), which corresponded to the loss of trimethylamine (TMA), non-sodiated choline phosphate and sodiated choline phosphate, respectively. Other fragments reflecting the structure of the head group were observed at m/z 183, 146 and 86. The difference in the fragmentation patterns for the PSD of [PC+Na](+) compared to [PC+H](+) is attributed to difference in the binding of Na(+) and H(+). While the proton binds to a negatively charged oxygen of the phosphate group, the sodium ion can be associated with several regions of the PC molecule. Hence, in the sodiated PC, intermolecular interaction of the negatively charged oxygen of the phosphate group, along with sodium association at multiple sites, can lead to a complex and characteristic ion fragmentation pattern. The preferential loss of sn-1 fatty acid group could be explained by the formation of an energetically favorable six-member ring intermediate, as apposed to the five-member ring intermediate formed prior to the loss of sn-2 fatty acid group.

Electrospray ionization/tandem quadrupole mass spectrometric studies on phosphatidylcholines: the fragmentation processes

We applied low-energy collisionally activated dissociation (CAD) tandem quadrupole mass spectrometry to study the fragmentation pathways of the [M + H](+) and [M + Li](+) ions of phosphatidylcholine (PC), generated by electrospray ionization (ESI). It is revealed that the fragmentation pathways leading to loss of the polar head group and of the fatty acid substituents do not involve the hydrogens attached to the glycerol backbone as previously reported. The pathway for formation of the major ion of m/z 184 by loss of the polar head group from the [M + H](+) precursor of a diacyl PC involves the participation of the alpha-hydrogen of the fatty acyl substituents, whereas the H(+) participates in the loss of fatty acid moieties. The alpha-hydrogens of the fatty acid substituents also participate in the major fragmentation processes, including formation of [M + Li-R(x)CO(2)H](+) and [M + Li-59-R(x)CO(2)H](+) ions for the [M + Li](+) ions of diacyl PCs, when subjected to low-energy CAD. These fragmentation processes are deterred by substitution of the fatty acyl moieties with alkyl, alkenyl, or hydroxyl groups and consequentially, result in a distinct product-ion spectrum for various PC, including diacyl-, plasmanyl- plasmenyl-, and lyso-PC isomers. The alpha-hydrogens of the fatty acyl substituents at sn-2 are more labile than those at sn-1. This is reflected by the preferential loss of the R(1)CO(2)H over the R(2)CO(2)H observed for the [M + Li](+) ions of diacyl PCs. The spectrum features resulting from the preferential losses permit identification and assignment of the fatty acid moieties in the glycerol backbone. The new fragmentation pathways established by tandem and source CAD tandem mass spectra of various PC molecules, including deuterium-labeling analogs, were proposed. These pathways would clarify the mechanisms underlying the ion formations that lead to the structural characterization of PC molecules.

Characterisation of a potential biomarker of phospholipidosis from amiodarone-treated rats

A novel and relatively simple analytical method for the separation, characterisation and semi-quantitation of phospholipids (PLs) from extracts of complex biological samples has been developed. This methodology allows PL extracts from cells and tissues to be analysed by liquid chromatography (LC) coupled to electrospray ionisation mass spectrometry (ESI-MS). Complex mixtures of PLs were separated on a high-performance liquid chromatography (HPLC) system using 0.5% ammonium hydroxide in methanol/water/hexane/formate mixture with UV detection at 205 nm. Identification and structural characterisation of molecular species were carried out utilising ESI-MS and MS/MS in the negative ion mode. The abnormal accumulation of PLs (phospholipidosis) was induced in male Sprague-Dawley rats by administration of the cationic amphiphilic drug (CAD), amiodarone. Analysis of the PL profile of liver and lung tissues, lymphocytes and serum from treated rats was carried out using this analytical procedure (LC-ESI/MS/MS). Differences in PL profiles between treated and untreated animals were highlighted by principal component analysis (PCA). This led to the selection of a potential metabolic marker of phospholipidosis (PLD) identified as a lyso-bis-phosphatidic acid (LBPA) derivative, also known as bis(monoglycero)phosphate (BMP). This PL was absent in control animals but was present in quantifiable amounts in all samples from amiodarone-treated rats.

Molecular species compositions of lung and pancreas phospholipids in the cftr(tm1HGU/tm1HGU) cystic fibrosis mouse

Fatty acid analysis of phospholipid compositions of lung and pancreas cells from a cystic fibrosis transmembrane regulator (CFTR) negative mouse (cftr(-/-))suggested that a decreased concentration of docosahexaenoate (22:6(n-3)) and increased arachidonate (20:4(n-6)) may be related to the disease process in cystic fibrosis (CF). Consequently, we have determined compositions of the major phospholipids of lung, pancreas, liver, and plasma from a different mouse model of CF, the cftr(tm1HGU/tm1HGU) mouse, compared with ZTM:MF-1 control mice. Electrospray ionization mass spectrometry permitted the quantification of all of the individual molecular species of phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylglycerol (PtdGly), phosphatidylserine (PtdSer), and phosphatidylinositol (PtdIns). There was no deficiency of 22:6(n-3) in any phospholipid class from lung, pancreas, or liver from mice with the cftr(tm1HGU/tm1HGU). Instead, the concentration of 20:4(n-6) was significantly decreased in plasma PtdCho species and in pancreas and lung species of PtdEtn, PtdSer, and PtdIns. These results demonstrate the variability of membrane phospholipid compositions in different mouse models of CF and suggest that in cftr(tm1HGU/tm1HGU) mice, the apparent deficiency was of 20:4n-6- rather than of 22:6n-3-containing phospholipid species. They highlight a need for detailed phospholipid molecular species analysis of cells expressing mutant CFTR from children with CF before the therapeutic effects of administering high doses of 22:6(n-3)-containing oils to children with CF can be fully evaluated.

A new solution for an old problem: the regiochemical distribution of the acyl chains in galactolipids can be established by electrospray ionization tandem mass spectrometry

Electrospray ionization-quadrupole ion trap mass spectrometry (ESI-QITMS), either in positive- or in negative-ion mode, has been used to establish the chemical structures (chain length, degree of unsaturation, positional distribution) of the fatty acids attached to the primary (sn-1) and secondary (sn-2) hydroxyl groups of the glycerol moiety of natural monogalactosyl- (MGDG) and digalactosyldiacylglycerols (DGDG), isolated from the freshwater dinoflagellate Glenodinium sanguineum and from a marine diatom belonging to the genus Chaetoceros. Fragmentation by collision-induced dissociation of a single component in MGDG and DGDG mixtures, separated by high-performance liquid chromatography (HPLC) and detected on-line by tandem positive-ion ESI-MS, leads to a clear-cut determination of the positional distribution of the sn-glycerol-bound fatty acyl chains. Reversed-phase liquid chromatography allowed a partial resolution of the component mixture before ESI-MS/MS analysis. These results were validated by comparison with ESI-MS data obtained for the sn-2 lysoglyceroglycolipids synthesized via regiospecific enzymatic hydrolysis of the corresponding diacylglycerols by Rhizopus arrhizus lipase.

Metal ion complexes in the structural analysis of phospholipids by electrospray ionization tandem mass spectrometry

The effects of metal cationization on collisionally activated dissociation (CAD) of phospholipids were investigated by electrospray ionization with quadrupole ion trap tandem mass spectrometry. The metal ions include Li(+), Na(+), K(+), Sr(2+), Ba(2+), and the first transition series. CAD of the transition metal ion-bound lipid complexes gave significant yields of product ions that identify the positions of the two fatty acyl substituents on the glycerophospholipid backbone. The cobalt(II) ion, which has a single naturally occurring isotope, was expected to be a better cationization reagent as it produces simpler mass spectra than other transition metal ions. CAD of the cobalt(II) ion complexes of glycerophosphoethanolamines, glycerophosphoglycerols and glycerophosphoserines yielded product ions that revealed information regarding both the lipid classes and the regiospecific positions of the two fatty acyl substituents.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of lipids: ionization and prompt fragmentation patterns

Ionization and prompt fragmentation patterns of triacylglycerols, phospholipids (PLs) and galactolipids were investigated using matrix-assisted laser desorption/ionization (MALDI). Positive ions of non-nitrogen-containing lipids appeared only in the sodiated form, while nitrogen-containing lipids were detected as both sodiated and protonated adducts. Lipids containing acidic hydroxyls were detected as multiple sodium adducts or deprotonated ions in the positive and negative modes, respectively, with the exception of phosphatidylcholines. The positive MALDI spectra of triacylglycerols contained prompt fragments equivalent to the loss of RCOO(-) from the neutral molecules. Prompt fragment ions [PL-polar head](+) were observed in the positive MALDI spectra of all phospholipids except phosphatidylcholines. The phosphatidylcholines produced only a minor positive fragment corresponding to the head group itself (m/z 184). Galactolipids did not undergo prompt fragmentation. Post-source decay (PSD) was used to examine the source of prompt fragments. PSD fragment patterns indicated that the lipid prompt fragment ions did not originate from the observed molecular ions (sodiated or protonated), and suggested that the prompt fragmentation followed the formation of highly unstable, probably protonated, precursor ions. Pathways leading to the formation of prompt fragment ions are proposed.

Electrospray ionisation mass spectrometric analysis of lipid restructuring in the carp (Cyprinus carpio L.) during cold acclimation

Cold acclimation of carp from 30°C to 10°C causes a restructuring of liver microsomal phospholipids characterised by increased proportions of monounsaturated fatty acid in phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Here, we have used electrospray ionisation mass spectrometry (ESI-MS) to determine the patterns of alteration to individual molecular species compositions of PC, PE and phosphatidylinositol (PI) in response to gradually decreasing temperature. The results demonstrate that cold induces precise changes to a limited number of phospholipid species, and that these changes are distinct and different for each phospholipid class. The major change for PC was increased 16:1/22:6, but for PE the species that increased was 18:1/22:6. By contrast, the PI species that increased during cold acclimation were characterised by an sn-1 monounsaturated fatty acid in combination with arachidonoyl or eicosapentaenoyl fatty acid at the sn-2 position. Analysis of acyl distribution indicates that cold only caused the accumulation of monounsaturated fatty acids at the sn-1 and not at the sn-2 position of phospholipids. These results highlight the tight and restricted range of modifications that membranes make to their phospholipid composition in response to thermal stress.

Evidence that accumulation of ceramides and cholesterol esters mediates oxidative stress-induced death of motor neurons in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of motor neurons in the spinal cord resulting in progressive paralysis and death. The pathogenic mechanism of ALS is unknown but may involve increased oxidative stress, overactivation of glutamate receptors, and apoptosis. We report abnormalities in sphingolipid and cholesterol metabolism in the spinal cords of ALS patients and in a transgenic mouse model (Cu/ZnSOD mutant mice), which manifest increased levels of sphingomyelin, ceramides, and cholesterol esters; in the Cu/ZnSOD mutant mice, these abnormalities precede the clinical phenotype. In ALS patients and Cu/Zn-SOD mutant mice, increased oxidative stress occurs in association with the lipid alterations, and exposure of cultured motor neurons to oxidative stress increases the accumulation of sphingomyelin, ceramides, and cholesterol esters. Pharmacological inhibition of sphingolipid synthesis prevents accumulation of ceramides, sphingomyelin, and cholesterol esters and protects motor neurons against death induced by oxidative and excitotoxic insults. These findings suggest a pivotal role for altered sphingolipid metabolism in the pathogenesis of ALS.

Substantial sulfatide deficiency and ceramide elevation in very early Alzheimer's disease: potential role in disease pathogenesis

In addition to pathology in the gray matter, there are also abnormalities in the white matter in Alzheimer's disease (AD). Sulfatide species are a class of myelin-specific sphingolipids and are involved in certain diseases of the central nervous system. To assess whether sulfatide content in gray and white matter in human subjects is associated with both the presence of Alzheimer's disease (AD) pathology as well as the stage of dementia, we analyzed the sulfatide content of brain tissue lipid extracts by electrospray ionization mass spectrometry from 22 subjects whose cognitive status at time of death varied from no dementia to very severe dementia. All subjects with dementia had AD pathology. The results demonstrate that: (i) sulfatides were depleted up to 93% in gray matter and up to 58% in white matter from all examined brain regions from AD subjects with very mild dementia, whereas all other major classes of lipid (except plasmalogen) in these subjects were not altered in comparison to those from age-matched subjects with no dementia; (ii) there was no apparent deficiency in the biosynthesis of sulfatides in very mild AD subjects as characterized by the examination of galactocerebroside sulfotransferase activities in post-mortem brain tissues; (iii) the content of ceramides (a class of potential degradation products of sulfatides) was elevated more than three-fold in white matter and peaked at the stage of very mild dementia. The findings demonstrate that a marked decrease in sulfatides is associated with AD pathology even in subjects with very mild dementia and that these changes may be linked with early events in the pathological process of AD.

Evidence of a tetradocosahexaenoic cardiolipin in some marine bivalves

Separation of phospholipid classes in lipid extracts from the scallop Pecten maximus, the Pacific oyster Crassostrea gigas, and the blue mussel Mytilus edulis was conducted using HPLC. An isolated polar lipid fraction was found to contain a very high level of DHA, up to 80 mol% of the total FA. MS with electrospray ionization in the positive-ion mode, tandem MS (MS-MS) and multidimensional NMR spectroscopy were used to analyze the detailed chemical structure of this polar lipid fraction. The isolated fraction contained exclusively cardiolipin (CL) molecules, predominantly in a form with four docosahexaenoyl chains (Do4CL). To the best of our knowledge, this is the first time that such a CL form has been analytically characterized and described in these three bivalve species. This tetradocosahexaenoic CL is presumed to reflect a specific adaptation in bivalves that enhances the structural and functional mechanisms of biomembranes in response to variations in environmental conditions (temperature, salinity, emersion).

A novel structural analysis of glycerophosphocholines as TFA/K(+) adducts by electrospray ionization ion trap tandem mass spectrometry

When collisionally activated dissociation (CAD) of glycerophosphocholine (GPC) species is examined using quadrupole ion trap mass spectrometry (QITMS), the spectral patterns differ from those obtained using sector or quadrupole mass spectrometry. Methods employed in the structural analysis of GPCs using a sector or quadrupole mass spectrometers are not necessarily useful for an ion trap mass spectrometer. A novel method is presented for structurally analyzing GPCs that involves the CAD of trifluoroacetic acid (TFA) adducts of kaliated GPCs. Solutions of GPCs in 0.1% TFA/methanol were electrosprayed to produce precursor ions by attaching a trifluoroacetic acid (TFA) molecule to a kaliated GPC molecule. The CAD-MS/MS spectra obtained by QITMS revealed a dramatic increase in the abundance of fragment ions, corresponding to the losses of sn-1 and sn-2 fatty acyl substituents. A preferential loss of the sn-1 fatty acyl group over the loss of the sn-2 fatty acyl group was observed among the GPC standards examined. A GPC extract from egg yolk was directly analyzed by this method without prior separation. The identities and positions of fatty acyl substituents of over 20 GPC species were identified. Some isomers present in very low relative abundance, which could not be analyzed by QITMS/MS using other ions as precursors, were identified by the TFA attachment method.

Quantitative analysis and molecular species fingerprinting of triacylglyceride molecular species directly from lipid extracts of biological samples by electrospray ionization tandem mass spectrometry

Herein we describe a rapid, simple, and reliable method for the quantitative analysis and molecular species fingerprinting of triacylglycerides (TAG) directly from chloroform extracts of biological samples. Previous attempts at direct TAG quantitation by positive-ion electrospray ionization mass spectrometry (ESI/MS) were confounded by the presence of overlapping peaks from choline glycerophospholipids requiring chromatographic separation of lipid extracts prior to ESI/MS analyses. By exploiting the rapid loss of phosphocholine from choline glycerophospholipids, in conjunction with neutral-loss scanning for individual fatty acids, overlapping peaks in the ESI mass spectrum were deconvoluted generating a detailed molecular species fingerprint of individual TAG molecular species directly from chloroform extracts of biological samples. This method readily detects as little as 0.1 pmol of each TAG molecular species from chloroform extracts and is linear over a 1000-fold dynamic range. The sensitivity of individual TAG molecular species to ESI/MS/MS analyses correlated with the unsaturation index and inversely correlated with total aliphatic chain length of TAG. An algorithm was developed which identifies sensitivity factors, thereby allowing the rapid quantitation and molecular species fingerprinting of TAG molecular species directly from chloroform extracts of biological samples.

Electrospray ionization mass spectrometry analysis of changes in phospholipids in RBL-2H3 mastocytoma cells during degranulation

Biological membranes contain an extraordinary diversity of lipids. Phospholipids function as major structural elements of cellular membranes, and analysis of changes in the highly heterogeneous mixtures of lipids found in eukaryotic cells is central to understanding the complex functions in which lipids participate. Phospholipase-catalyzed hydrolysis of phospholipids often follows cell surface receptor activation. Recently, we demonstrated that granule fusion is initiated by addition of exogenous, nonmammalian phospholipases to permeabilized mast cells. To pursue this finding, we use positive and negative mode Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) to measure changes in the glycerophospholipid composition of total lipid extracts of intact and permeabilized RBL-2H3 (mucosal mast cell line) cells. The low energy of the electrospray ionization results in efficient production of molecular ions of phospholipids uncomplicated by further fragmentation, and changes were observed that eluded conventional detection methods. From these analyses we have spectrally resolved more than 130 glycerophospholipids and determined changes initiated by introduction of exogenous phospholipase C, phospholipase D, or phospholipase A2. These exogenous phospholipases have a preference for phosphatidylcholine with long polyunsaturated alkyl chains as substrates and, when added to permeabilized mast cells, produce multiple species of mono- and polyunsaturated diacylglycerols, phosphatidic acids, and lysophosphatidylcholines, respectively. The patterns of changes of these lipids provide an extraordinarily rich source of data for evaluating the effects of specific lipid species generated during cellular processes, such as exocytosis.

Analysis of phospholipid molecular species by liquid chromatography--atmospheric pressure chemical ionisation mass spectrometry of diacylglycerol nicotinates

A method using liquid chromatography - atmospheric pressure chemical ionisation mass spectrometry was evaluated for determining the molecular species composition of phospholipids (phosphatidylcholines from soybean, egg yolk and bovine liver) after conversion to diacylglycerol nicotinate derivatives. The structures could be deduced from pseudo-molecular ions ([MH-123](+)) and three pairs of monoacyl containing fragment ions. All molecular species in mixed peaks were readily identified and many minor components, earlier not encountered in the samples under investigation, were identified. Acyl chain regioisomers were readily distinguished by the ratio of the [MH-RCHCO](+) ions. Molecular species differing only in the position of the double bonds in one polyunsaturated acyl chain were separated on the basis of retention times. A half quantitative estimation of the molecular species composition of complex samples was achieved by a combination of UV detection and, for mixed peaks, the areas of [MH-123](+) ions.

Highly saturated endonuclear phosphatidylcholine is synthesized in situ and colocated with CDP-choline pathway enzymes

Chromatin-associated phospholipids are well recognized. A report that catalytically active endonuclear CTP:choline-phosphate cytidylyltransferase alpha is necessary for cell survival questions whether endonuclear, CDP-choline pathway phosphatidylcholine synthesis may occur in situ. We report that chromatin from human IMR-32 neuroblastoma cells possesses such a biosynthetic pathway. First, membrane-free nuclei retain all three CDP-choline pathway enzymes in proportions comparable with the content of chromatin-associated phosphatidylcholine. Second, following supplementation of cells with deuterated choline and using electrospray ionization mass spectrometry, both the time course and molecular species labeling pattern of newly synthesized endonuclear and whole cell phosphatidylcholine revealed the operation of spatially separate, compositionally distinct biosynthetic routes. Specifically, endogenous and newly synthesized endonuclear phosphatidylcholine species are both characterized by a high degree of diacyl/alkylacyl chain saturation. This unusual species content and synthetic pattern (evident within 10 min of supplementation) are maintained through cell growth arrest by serum depletion and when proliferation is restored, suggesting that endonuclear disaturated phosphatidylcholine enrichment is essential and closely regulated. We propose that endonuclear phosphatidylcholine synthesis may regulate periodic nuclear accumulations of phosphatidylcholine-derived lipid second messengers. Furthermore, our estimates of saturated phosphatidylcholine nuclear volume occupancy of around 10% may imply a significant additional role in regulating chromatin structure.

Characterization of N-acylphosphatidylethanolamine and acylphosphatidylglycerol in oats

Two polar lipid classes, both with three acyl groups, were isolated from an extract of oats and characterized by nuclear magnetic resonance spectroscopy, electrospray mass spectrometry (MS), and electron ionization MS (EIMS). Distortionless enhancement by polarization transfer (DEPT) and the two-dimensional correlation experiments 1H-detected heteronuclear multiple quantum coherence spectroscopy, heteronuclear multiple bond correlation spectroscopy, double quantum filtered correlation spectroscopy, and total correlation spectroscopy provided sufficient information for determination of the structure of the two lipid classes. The polar lipid classes were found to be N-acylphosphatidylethanolamine [1,2-diacyl-sn-glycero-3-phospho-(N-acyl)-1'-ethanolamine; N-acyl-PE] and acylphosphatidylglycerol [1,2-diacyl-sn-glycero-3-phospho-(3'-acyl)-1'-sn-glycerol]. High-performance liquid chromatography with electrospray ionization MS (HPLC-ESMS) and with electrospray ionization tandem MS (HPLC-MS/MS) were utilized for the separation and subsequent determination of molecular species. With HPLC-ESMS, ions of deprotonated molecules were obtained and with HPLC-MS/MS carboxylate ions (representing acyl groups) were obtained as well as other structurally significant ions. Fifty molecular species of N-acylphosphatidylethanolamine and 24 molecular species of acylphosphatidylglycerol were found, with a molecular mass range of 924-1032 Da and 959-1035 Da, respectively. Identification of the fatty acid isomers, as picolinyl ester derivatives, was done with gas chromatography with EIMS. Three isomers of 16:1 fatty acids were found in N-acyl-PE, and their double bond positions were determined to 6, 9, and 11 with a relative abundance of 4:10:1.

Characterization of sodiated glycerol phosphatidylcholine phospholipids by mass spectrometry

Fast atom bombardment mass spectrometry in the positive mode was used for the characterization of sodiated glycerol phosphatidylcholines. The relative abundance (RA) of the protonated species is similar to the RA of the sodiated molecular species. The sodiated fragment ion, [M + Na - 59](+), corresponding to the loss of trimethylamine, and other sodiated fragment ions, were also observed. The decomposition of the sodiated molecule is very similar for all the studied glycerol phosphatidylcholines, in which the most abundant ion corresponds to a neutral loss of 59 Da. Upon collision-induced dissociation (CID) of the [M + Na](+) ion informative ions are formed by the losses of the fatty acids in the sn-1 and sn-2 positions. Other major fragment ions of the sodiated molecule result from loss of non-sodiated and sodiated choline phosphate, [M + Na - 183](+), [M + Na - 184](+.) and [M + Na - 205](+), respectively. The main CID fragmentation pathway of the [M + Na - 59](+) ion yields the [M + Na - 183](+) ion, also observed in the CID spectra of the [M + Na](+) molecular ion. Other major fragment ions are [M + Na - 205](+) and the fragment ion at m/z 147. Collisional activation of [M + Na - 205](+) results in charge site remote fragmentation of both fatty acid alkyl chains. The terminal ions of these series of charge remote fragmentations result from loss of part of the R(1) or R(2) alkyl chain. Other major informative ions correspond to acylium ions.

Charge-remote and charge-driven fragmentation processes in diacyl glycerophosphoethanolamine upon low-energy collisional activation: a mechanistic proposal

A mechanistic study of diacyl glycerophosphoethanolamine fragmentation under low energy collision-activated dissociation with electrospray ionization tandem mass spectrometry is reported. The fragmentation pathways leading to the formation of carboxylate anions (RxCO2-) (x = 1, 2) and the formation of the ions representing neutral loss of ketene ([M - H - Rx'CH=C=O]-) are charge-driven processes, which are governed by the gas-phase basicity and the steric configuration of the molecules. The fragmentation pathway for the formation of the [M - H - RxCO2H]- ions, reflecting neutral loss of fatty acid, is a charge-remote process, which involves the participation of the hydrogens at C-1 and C-2 of the glycerol, resulting in [M - H - R2CO2H]- > [M - H - R1CO2H]-. The preferential formations of R2CO2- > R1CO2-, and of [M - H - R2'CH=C=O]- > [M - H - R1'CH=C=O]- are attributed to the findings that charge-driven processes are sterically more favorable at sn-2. The observation of the abundance of [M - H - Rx'CH=C=O]- > [M - H - RxCO2H]- is attributed to the fact that the [M - H]- ions of GPE are basic precursor ions, which undergo preferential loss of ketene than loss of acid. The major pathway for the formation of RxCO2- ions arises from the nucleophilic attack of the anionic charge site of the phosphate on the C-1 or C-2 of the glycerol to render a charge transfer. The sterically more favorable attack on the C-2 than C-2 of the glycerol results in the abundance of R2CO2- > R1CO2-. These features of tandem spectra readily identify and locate the fatty acid substituents of GPE in the glycerol backbone.

Effect of soybean oil and fish oil on individual molecular species of Atlantic salmon head kidney phospholipids determined by normal-phase liquid chromatography coupled to negative ion electrospray tandem mass spectrometry

The effect of soybean oil (SO) and fish oil (FO) on the relative molecular species distribution of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS) in Atlantic salmon head kidney was studied using normal-phase liquid chromatography coupled with negative ion electrospray tandem mass spectrometry. The conformation of identity of the phospholipid species was based on retention time, the mass of the [M-H]- ([M-15]- for PC) molecular ions and the carboxylate anion fragments in the product ion spectrum. The intensity ratio of sn-1/sn-2 fragment ions increased with increasing number of double bonds in the sn-2 acyl chain but was not affected by increasing number of double bonds in the sn-1 acyl chain of the species examined. The relative distribution of the molecular species was determined by multiple reaction monitoring of the carboxylate anion fragment from the sn-1 position. A total of 68 different phospholipid species were determined in the head kidney and the largest amount was found in PE (22 species). Depending on the diet, the main species identified in the different phospholipid classes were; PC 16:0/18:1, PE 16:0/22:6, PI 18:0/20:4 and PS 16:0/22:6. The SO diet significantly increased the 18:2, 20:3 and most 20:4 containing species and significantly reduced the 14:0 and most 20:5 and 22:6 fatty acid containing species. The increase of the 20:4 and the decrease of the 20:5 and 22:6 containing species were dependent on the fatty acid combination of the species.

Determinants of bioactivity of oxidized phospholipids. Specific oxidized fatty acyl groups at the sn-2 position

We previously described 3 bioactive oxidation products of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (PAPC) containing oxovaleroyl (POVPC), glutaroyl (PGPC), and epoxyisoprostane (PEIPC) groups at the sn-2 position that were increased in minimally modified/oxidized low density lipoprotein (MM-LDL) and rabbit atherosclerotic lesions. We demonstrated specific and contrasting effects of POVPC and PGPC on leukocyte-endothelial interactions and described an effect of PEIPC on monocyte binding. The major purpose of the present study was to determine the effects of structural changes on the bioactivities of these 3 lipids. We demonstrate herein that the group at the sn-2 position determines the specific bioactivity and that the substitution of stearoyl for palmitoyl at the sn-1 position or ethanolamine for choline at the sn-3 position of the phospholipid did not alter bioactivity. Oxidized PAPC, oxidized 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine, and oxidized 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphorylethanolamine stimulated monocyte binding and inhibited lipopolysaccharide-induced expression of the neutrophil-binding molecule E-selectin. Furthermore, all oxovaleroyl phospholipids but not the glutaroyl phospholipids induced monocyte binding without an increase in vascular cell adhesion molecule-1 (VCAM-1) expression and inhibited lipopolysaccharide-induced E-selectin expression. In contrast, glutaroyl phospholipids but not oxovaleroyl phospholipids stimulated E-selectin and VCAM-1 expression. We further demonstrate that all parts of the phospholipid molecules are required for these bioactivities. Hydrolysis with phospholipase (PL) A(1), PLA(2), and PLC strongly reduced the bioactivities of POVPC, PGPC, and mixed isomers of PEIPC. PLD had a smaller but still significant effect. The effects of POVPC and PEIPC could be abolished by sodium borohydride treatment, indicating the importance of the reducible groups (carbonyl and epoxide) in these molecules. In summary, these studies identify 6 new bioactive, oxidized phospholipids that are increased in MM-LDL and, where measured, in atherosclerotic lesions. They thus suggest that a family of phospholipid oxidation products containing oxovaleroyl, glutaroyl, and epoxyisoprostane at the sn-2 position play an important role in the regulation of leukocyte-endothelial interactions, bioactivity being in part controlled by several types of phospholipid hydrolases.

Charge-driven fragmentation processes in diacyl glycerophosphatidic acids upon low-energy collisional activation. A mechanistic proposal

A mechanistic study of diacyl glycerophosphatidic acid (GPA) under low energy collisionally activated decomposition (CAD) with electrospray ionization tandem mass spectrometry is reported. The fragmentation pathways leading to the formation of carboxylate anions [RxCO2-], (x = 1, 2) and the formation of the ions representing neutral loss of fatty acid ([M-H-RxCO2H] ) and neutral loss of ketene ([M-H-R'xCH-C=O] ) (Rx=R'xCH2) are charge-driven processes that are governed by the gas-phase basicity and the steric configuration of the molecules. The preferential formation of the ions of [M-H-R2CO2H]- > [M-H-R1CO2H]- and [M-H-R'2CH=C=O]- > [M-H-R'1CH=C=O]- are attributed to the fact that loss of fatty acid and loss of ketene are sterically more favorable at sn-2. While the observation of the abundance of [M-H-RxCO2H]- > [M-H-R'xCH=C=O]- is attributed to the acidity of the gas phase ion of GPA, which undergoes a more facile neutral loss of acid than loss of ketene. The major pathway leading to the formation of RxCO2- ion under low energy CAD arises from further fragmentation of the [M-H-RxCO2H]- ions by neutral loss of 136, resulting in an abundance of R1CO2- > R2CO2-. The differential formation of the carboxylate anions permits accurate assignment of the regiospecificity of the fatty acid substituents of GPA molecules by tandem mass spectrometry.

Two-dimensional analysis of phospholipids by capillary liquid chromatography/electrospray ionization mass spectrometry

A phospholipid mixture extracted from cultured cells was directly analyzed by capillary (Cap) liquid chromatography (LC)/electrospray ionization (ESI) mass spectrometry (MS). Using a quadrupole mass spectrometer, we analyzed positive molecular ions, negative molecular ions, positive fragment ions and negative fragment ions under four different functions. In the analysis of the elution patterns of the phospholipids, a two-dimensional map, in which the first dimension is elution time and the second dimension is mass, proved useful. Consequently, four different maps can be obtained by each of four different functions. Among them, from negative fragment ions at high cone voltage in the negative ion mode, ions that originated from acyl fatty acid and phosphorylcholine, phosphorylethanolamine and cyclic inositol phosphate can be detected at specific elution times. The map from positive fragment ions at high cone voltage in the positive ion mode indicated ions such as diradylglycerol and derivatives of 1-alkyl or 1-alkenyl cyclic phosphatidic acid from phosphatidylethanolamine (PE), and phosphorylcholine from choline-containing phospholipids. The map produced from positive molecular ions indicated choline-containing phospholipids such as phosphatidylcholine, sphingomyelin, lysophosphatidylcholine and PE. The map of negative molecular ions effectively indicated acidic phospholipids such as phosphatidylinositol. We were able to obtain more than 500 molecular species of phospholipids by this method within a few hours immediately after extraction from culture cells using a mixture of chloroform and methanol (2:1). In this context, we concluded that the combination of Cap-LC and ESIMS seems to be very effective in the analysis of phospholipid classes and their molecular species.

Electrospray ionization mass spectrometric analyses of changes in tissue phospholipid molecular species during the evolution of hyperlipidemia and hyperglycemia in Zucker diabetic fatty rats

The Zucker diabetic fatty (ZDF) rat is a genetic model of type II diabetes mellitus in which males homozygous for nonfunctional leptin receptors (fa/fa) develop obesity, hyperlipidemia, and hyperglycemia, but rats homozygous for normal receptors (+/+) remain lean and normoglycemic. Insulin resistance develops in young fa/fa rats and is followed by evolution of an insulin secretory defect that triggers hyperglycemia. Because insulin secretion and insulin sensitivity are affected by membrane phospholipid fatty acid composition, we have determined whether metabolic abnormalities in fa/fa rats are associated with changes in tissue phospholipids. Electrospray ionization mass spectrometric analyses of glycerophosphocholine (GPC) and glycerophosphoethanolamine (GPE) molecular species from tissues of prediabetic (6 wk of age) and overtly diabetic (12 wk) fa/fa rats and from +/+ rats of the same ages indicate that arachidonate-containing species from heart, aorta, and liver of prediabetic fa/fa rats made a smaller contribution to GPC total ion current than was the case for +/+ rats. There was a correspondingly larger contribution from species with sn-2 oleate or linoleate substituents in fa/fa heart and aorta. The relative contributions of arachidonate-containing GPC species increased in these tissues as fa/fa rats aged and were equal to or greater than those for +/+ rats by 12 wk. For heart and aorta, relative contributions from GPE species with sn-2 arachidonate or docosahexaenoate substituents to the total ion current increased and those from species with sn-2 oleate or linoleate substituents fell as fa/fa rats aged, but these tissue lipid profiles changed little with age in +/+ rats. GPC and GPE profiles for brain, kidney, sciatic nerve, and red blood cells were similar among fa/fa and +/+ rats at 6 and 12 wk of age, and pancreatic islets from fa/fa and +/+ rats exhibited similar GPC and GPE profiles at 12 wk of age. Under-representation of arachidonate-containing GPC and GPE species in some fa/fa rat tissues at 6 wk could contribute to insulin resistance, but depletion of islet arachidonate-containing GPC and GPE species is unlikely to explain the evolution of the insulin secretory defect that is well-developed by 12 wk of age.

Characterization of phosphatidylethanolamine as a lithiated adduct by triple quadrupole tandem mass spectrometry with electrospray ionization

Structural characterization of the glycerophosphoethanolamine (GPE) molecule as a lithiated adduct ion by collisionally activated dissociation (CAD) tandem mass spectrometry with electrospray ionization is described. Abundant fragment ions reflecting polar head group and fatty acid constituents were observed in the product ion spectrum of GPE, which permits an unambiguous structural determination, including the regiospecificity of fatty acyl substituents. The pathways leading to the formation of fragment ions are proposed. The suggested mechanisms are supported by the tandem mass spectra of various deuterated analogs and source CAD of GPE followed by CAD tandem mass spectrometry. Identification of GPE molecular species and specific GPE subclasses in a biological mixture by tandem mass spectrometry with various constant neutral loss scannings is also described.

Electrospray ionization mass spectrometric analyses of phospholipids from INS-1 insulinoma cells: comparison to pancreatic islets and effects of fatty acid supplementation on phospholipid composition and insulin secretion

Insulin secretion by pancreatic islet beta-cells is impaired in diabetes mellitus, and normal beta-cells are enriched in phospholipids with arachidonate as sn-2 substituent. Such molecules may play structural roles in exocytotic membrane fusion or serve as substrates for phospholipases activated by insulin secretagogues. INS-1 insulinoma cells respond to secretagogues and permit the study of effects of culture with free fatty acids on phospholipid composition and secretion. INS-1 cell glycerophosphocholine (GPC) and glycerophosphoethanolamine (GPE) lipids are demonstrated here by electrospray ionization mass spectrometry to contain a lower fraction of molecules with arachidonate and a higher fraction with oleate as sn-2 substituent than native islets. Palmitic acid supplementation induces little change in these INS-1 cell lipids, but supplementation with linoleate or arachidonate induces a large rise in the fraction of INS-1 cell GPC species with polyunsaturated sn-2 substituents and a fall in oleate-containing species to yield a GPC profile similar to native islets. The fraction of GPE lipids comprised of plasmenylethanolamine species with polyunsaturated sn-2 substituents in early-passage INS-1 cells is similar to that of islets, but declines on serial passage. Such molecules might participate in exocytotic membrane fusion, and late-passage INS-1 cells have reduced insulin secretory responses. Arachidonate supplementation induces a rise in the fraction of INS-1 cell GPE lipids with polyunsaturated sn-2 substituents and partially restores responses to insulin secretagogues by late-passage INS-1 cells, but does not further amplify secretion by early-passage cells. Effects of extracellular free fatty acids on beta-cell phospholipid composition and secretory responses could be involved in changes in beta-cell function during the period of hyper-free fatty acidemia that precedes diabetes mellitus.

Phospholipid molecular species of bronchoalveolar lavage fluid after local allergen challenge in asthma

Electrospray ionization mass spectrometry was used to quantify phosphatidylcholine (PC) and phosphatidylglycerol (PG) molecular species in bronchoalveolar lavage fluid (BALF) from control and mild asthmatic subjects after local allergen challenge. BALF was obtained from 5 control and 13 asthmatic subjects before and 24 h after segmental allergen and saline challenge. There were no differences in the ratio of total PC to total PG or in the molecular species composition of PC or PG between the asthmatic and control groups under basal conditions. Allergen challenge in asthmatic but not in control volunteers caused a significant increase in the PC-to-PG ratio because of increased concentrations of PC species containing linoleic acid (16:0/18:2 PC, 18:0/18:2 PC, and 18:1/18:2 PC). These molecular species were characteristic of plasma PC analyzed from the same subjects, strongly suggesting that the altered PC composition in BALF in asthmatic subjects after allergen challenge was due to infiltration of plasma lipoprotein, not to catabolism of surfactant phospholipid. Interactions between surfactant and lipoprotein infiltrate may contribute to surfactant dysfunction and potentiate disease severity in asthma.

Characterisation of membrane phospholipids and glycolipids from a halophilic archaebacterium by high-performance liquid chromatography/electrospray mass spectrometry

Combined high-performance liquid chromatography and electrospray mass spectrometry (LC/ES-MS) has been used for direct characterisation of the polar membrane lipids in total lipid extracts from Halobacterium salinarium, a species of halophilic archaebacterium. The principle phospholipids found were the diphytanyl archaeol phosphatidylglycerol and diphytanyl archaeol phosphatidylglycerolphosphate methyl ester. The application of LC/ES-MS revealed the additional presence of diphytanyl archaeol phosphatidylglycerol sulphate The extracts also contained an archaeol glycolipid, initially detected in preliminary offline ES-MS studies, which was further characterised by LC/ES-MS and by product ion tandem mass spectrometry (MS/MS) as a sulphate ester of diglycosyl-2,3-di-O-phytanyl-sn-glycerol. Whilst archaeol phospho- and glycolipids containing a (C(20)-C(20))-isopranyl glycerol ether core predominated, LC/ES-MS of the extracts from Halobacterium salinarium indicated the presence of an analogue containing one double bond in its isoprenyl ether core as a minor component of the phosphatidylglycerolphosphate methyl ester fraction, providing a further example of the previously recognised existence of isoprenologues of diphytanyl archaeols which occur as minor components of archaebacterial membrane lipids. The value of these techniques in compositional analysis of archaebacterial lipid extracts is discussed.

Characterization of the molecular species of phosphatidylethanolamine from kidney of the fresh water snail Lymneae stagnalis by mass spectrometry

The structural analysis of sixteen molecular species of diacyl glycerophosphoethanolamine from fresh water snail Lymneae stagnalis kidney using chromatography and mass spectrometry is described in this paper. 1-eicosadienoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 20:2-20:4), 1-eicosapentaenoyl-2-eicosadienoyl-sn-glycero-3-phosphoethanolamine (PE 20:5-20:2) and 1-eicosatrienoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 20:3-20:4) as well as 1-octadecanoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 18:0-20:4), 1-ocetadecenoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 18:1-20:4) and 1-octadecanoyl-2-eicosapentaneoyl-sn-glycero-3-phosphoethanolamine (PE 18:0-20:5) were found as the major molecular species, and the first three were tentatively identified as the novel species present in this biological material. The presence of a relatively high content of 1,2-dieicosenoyl-sn-glycero-3-phosphoethanolamine species (approximately 27% of total species) as well as the absence of 22-carbon fatty acid containing and plasmalogen PE molecular species are remarkable in healthy Lymneae stagnalis kidney. Copyright 1999 John Wiley & Sons, Ltd.

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

We describe features of tandem mass spectra of lithiated adducts of triacylglycerol (TAG) species obtained by electrospray ionization mass spectrometry (ms) with low-energy collisionally activated dissociation (CAD) on a triple stage quadrupole instrument. The spectra distinguish isomeric triacylglycerol species and permit assignment of the mass of each fatty acid substituent and positions on the glycerol backbone to which substituents are esterified. Source CAD-MS2 experiments permit assignment of double bond locations in polyunsaturated fatty acid substituents. The ESI/MS/MS spectra contain [M + Li - (RnCO2H)]+, [M + Li - (RnCO2Li)]+, and RnCO+ ions, among others, that permit assignment of the masses of fatty acid substituents. Relative abundances of these ions reflect positions on the glycerol backbone to which substituents are esterified. The tandem spectra also contain ions reflecting combined elimination of two adjacent fatty acid residues, one of which is eliminated as a free fatty acid and the other as an alpha, beta-unsaturated fatty acid. Such combined losses always involve the sn-2 substituent, and this feature provides a robust means to identify that substituent. Fragment ions reflecting combined losses of both sn-1 and sn-3 substituents without loss of the sn-2 substituent are not observed. Schemes are proposed to rationalize formation of major fragment ions in tandem mass spectra of lithiated TAG that are supported by studies with deuterium-labeled TAG and by source CAD-MS2 experiments. These schemes involve initial elimination of a free fatty acid in concert with a hydrogen atom abstracted from the alpha-methylene group of an adjacent fatty acid, followed by formation of a cyclic intermediate that decomposes to yield other characteristic fragment ions. Determination of double bond location in polyunsaturated fatty acid substituents of TAG is achieved by source CAD experiments in which dilithiated adducts of fatty acid substituents are produced in the ion source and subjected to CAD in the collision cell. Product ions are analyzed in the final quadrupole to yield information on double bond location.

Mass spectrometric analysis of arachidonyl-containing phospholipids in human U937 cells

The human histiocytic lymphoma U937 cell line contains a rich source of the 85 kDa cytosolic phospholipase A2 (cPLA2). DMSO-differentiated U937 cells were used as a model to investigate the free arachidonic acid release, the arachidonate distribution and the phospholipid source of arachidonate upon Ca2+ ionophore stimulation. A combination of several chromatographic and mass spectrometric techniques was employed in this study. The amount of free arachidonic acid (AA) released upon stimulation, the arachidonate content in total lipids and in each of the phospholipid classes were determined by gas chromatography/mass spectrometry (GC/MS). Glycerophosphoethanolamine (GPE) was found to be the major pool of arachidonate in differentiated human U937 cells (55%) and glycerophosphocholine (GPC) and glycerophosphoinositol (GPI) contributed 22 and 8%, respectively. Upon Ca2+ ionophore stimulation, GPE class lost the largest amount of arachidonate, followed by GPC class. GPI class, however, gained a substantial amount of arachidonate. Most of the arachidonate depleted from GPE and GPC was recovered as free AA, some of which was rapidly esterified into GPI species. GC/MS with electron capture negative chemical ionization provided excellent sensitivity for the measurement of arachidonic acid which was derivatized to its pentafluorobenzyl ester. Intact phospholipid molecular species including the arachidonyl-containing phospholipid species were identified using capillary high-performance liquid chromatography/continuous-flow liquid secondary ion mass spectrometry (CF-LSIMS). No specificity was found for releasing free AA among the arachidonyl-containing GPE and GPC species upon Ca2+ ionophore stimulation. CF-LSIMS provided a sensitive and effective means of detecting intact phospholipid species.

Liquid chromatography with particle-beam electron-impact mass spectrometry of diacylglycerol nicotinates

Several synthetic diacylglycerols and natural mixtures derived from soybean, egg yolk, and bovine liver phosphatidylcholines were examined in the form of nicotinate derivatives by liquid chromatography with particle-beam electron-impact mass spectrometry. High-performance liquid chromatography was carried out in the reversed-phase mode with a base-deactivated octyl-/octadecylsilyl stationary phase. The nature (size and degree of unsaturation) of the acyl residues was readily determined from the mass spectra. Uniquely, the positions of the double bonds could be deduced, although this became increasingly difficult as the degree of unsaturation of diacylglycerols containing mixed acyl residues increased. Reverse 1,2-diacylglycerol isomers could be distinguished by their mass spectra.

Electrospray ionization tandem mass spectrometric analysis of sulfatide. Determination of fragmentation patterns and characterization of molecular species expressed in brain and in pancreatic islets

The sphingolipid sulfatide is a component of myelin and some non-neuronal cells. Antibodies to sulfatide occur in some patients with autoimmune neuropathies and in patients with insulin-dependent diabetes mellitus (IDDM) caused by immunologic destruction of insulin-secreting pancreatic islet beta-cells. Distinct sulfatide molecular species may differ in immunogenicity, and facile means to identify sulfatide species in islets and other tissues obtainable in only small amounts could be useful. Electrospray ionization mass spectrometry (ESI/MS) permits structural determination of small quantities of phospholipids and is applied here to sulfatide analysis. We find that sulfatide standards are readily analyzed by negative ion ESI/MS, and tandem mass spectra of individual species exhibit some ions common to all species and other ions that reflect distinct fatty acid substituents in different sulfatide molecules. A signature ion cluster resulting from cleavage directed by the alpha-hydroxy group of sulfatide species with a hydroxylated fatty acid substituent identifies such species. Sulfatide profiles in tissue lipid extracts can be obtained by ESI/MS/MS scanning for common sulfatide ions and for ions reflecting fatty acid substituents. Islets are demonstrated to contain sulfatide and to exhibit a profile of species different from that of brain.

Formation of lithiated adducts of glycerophosphocholine lipids facilitates their identification by electrospray ionization tandem mass spectrometry

Electrospray ionization (ESI) tandem mass spectrometry (MS) has simplified analysis of phospholipid mixtures, and, in negative ion mode, permits structural identification of picomole amounts of phospholipid species. Collisionally activated dissociation (CAD) of phospholipid anions yields negative ion tandem mass spectra that contain fragment ions representing the fatty acid substituents as carboxylate anions. Glycerophosphocholine (GPC) lipids contain a quaternary nitrogen moiety and more readily form cationic adducts than anionic species, and positive ion tandem mass spectra of protonated GPC species contain no abundant ions that identify fatty acid substituents. We report here that lithiated adducts of GPC species are readily formed by adding lithium hydroxide to the solution in which phospholipid mixtures are infused into the ESI source. CAD of [MLi+] ions of GPC species yields tandem mass spectra that contain prominent ions representing losses of the fatty acid substituents. These ions and their relative abundances can be used to assign the identities and positions of the fatty acid substituents of GPC species. Tandem mass spectrometric scans monitoring neutral losses of the head-group or of fatty acid substituents from lithiated adducts can be used to identify GPC species in tissue phospholipid mixtures. Similar scans monitoring parents of specific product ions can also be used to identify the fatty acid substituents of GPC species, and this facilitates identification of distinct isobaric contributors to ions observed in the ESI/MS total ion current.

Preferential decarboxylation of hydrophilic phosphatidylserine species in cultured cells. Implications on the mechanism of transport to mitochondria and cellular aminophospholipid species compositions

In baby hamster kidney and other cultured cells the majority of phosphatidylethanolamine (PE) is synthesized from phosphatidylserine (PS) in a process which involves transport of PS from the endoplasmic reticulum to mitochondria and decarboxylation therein by PS decarboxylase. To study the mechanism of this transport process, we first determined the molecular species composition of PE and PS from baby hamster kidney and Chinese hamster ovary cells. Interestingly, the hydrophilic diacyl molecular species were found to be much more abundant in PE than in PS, suggesting that hydrophilic PS species may be more readily transported to mitochondria than the hydrophobic ones. To study this, we compared the rates of decarboxylation of different PS molecular species in these cells. The cells were pulse labeled with [3H]serine whereafter the distribution of the labels among PS and PE molecular species was determined by reverse phase high performance liquid chromatography and liquid scintillation counting. The hydrophilic PE species contained relatively much more 3H label than those of PS, which indicates that they are more readily decarboxylated than the hydrophobic ones. Control experiments showed that differences in [3H]PS and -PE molecular species profiles are not due to (i) incorporation of 3H label to some PE species via alternative pathways, (ii) differences in degradation or remodeling among species, or (iii) selective decarboxylation of PS molecular species by the enzyme. Therefore, hydrophilic PS species are indeed decarboxylated faster than the hydrophobic ones. The rate of decarboxylation decreased systematically with hydrophobicity, strongly suggesting that formation of so called activated monomers, i.e. lipid molecules perpendicularly displaced from the membrane (Jones, J. D., and Thompson, T. E. (1990) Biochemistry 29, 1593-1600), is the rate-limiting step in the transport of PS from the endoplasmic reticulum to mitochondria. The formation of activated monomers and thus the rate of transfer is probably greatly enhanced by frequent collisions between the two membranes which tend to be closely associated. The present data also provides a feasible explanation why hydrophilic molecular species in these cells are much more abundant in PE as compared with PS, its immediate precursor.

An electrospray-ionization tandem mass spectrometry method for determination of the anomeric configuration of glycosyl 1-phosphate derivatives

A rapid, simple, and sensitive method is described for the determination of the anomeric configuration of sugar 1-phosphates, sugar nucleotides, and polyisoprenyl-phospho-sugars. Negative-ion electrospray ionization of picomole amounts of glycosyl 1-phosphate derivatives produces an intense signal of the [M-H]-deprotonated molecule which, by collision-induced dissociation, decomposes in a characteristic manner depending on cis/trans configuration of the 2-hydroxyl and phosphate groups of the glycosyl residue. A distinct feature of the product ion spectra of glycosyl 1-P and polyisoprenyl-P-sugars with cis configuration is the presence of abundant ions that correspond to the [M-H2O-H]- dehydration product and the [R-PO4-(C2H3O]- fragment arising from a cleavage across the sugar ring, where R is -H or -polyprenyl/dolichyl for glycosyl 1-P and polyisoprenyl-P-sugar, respectively. These two fragments, [M-H2O-H]- and [R-PO4-(C2H3O)]- are absent from the product ion spectra of sugar 1-P and polyisoprenyl-P-sugars with trans configuration. For sugar nucleotides, compounds with cis configuration produce, in tandem mass spectrometry, only one abundant fragment of nucleoside monophosphate, whereas those with trans configuration give nucleoside diphosphate as a major fragment ion. Accordingly, the anomeric configuration of a glycosyl 1-phosphate derivative can be easily determined by using electrospray-ionization tandem mass spectrometry provided that the glycosyl residue of known absolute configuration has a free 2-hydroxyl group and no other charge location.