Mass spectrometry of neutral, mono- and disialoglycosphingolipids

Wrapping axons in mammals and Drosophila: Different lipids, same principle

Plasma membranes of axon-wrapping glial cells develop specific cylindrical bilayer membranes that surround thin individual axons or axon bundles. Axons are wrapped with single layered glial cells in lower organisms whereas in the mammalian nervous system, axons are surrounded with a characteristic complex multilamellar myelin structure. The high content of lipids in myelin suggests that lipids play crucial roles in the structure and function of myelin. The most striking feature of myelin lipids is the high content of galactosylceramide (GalCer). Serological and genetic studies indicate that GalCer plays a key role in the formation and function of the myelin sheath in mammals. In contrast to mammals, Drosophila lacks GalCer. Instead of GalCer, ceramide phosphoethanolamine (CPE) has an important role to ensheath axons with glial cells in Drosophila. GalCer and CPE share similar physical properties: both lipids have a high phase transition temperature and high packing, are immiscible with cholesterol and form helical liposomes. These properties are caused by both the strong headgroup interactions and the tight packing resulting from the small size of the headgroup and the hydrogen bonds between lipid molecules. These results suggest that mammals and Drosophila wrap axons using different lipids but the same conserved principle.

MASS SPECTROMETRIC FRAGMENTATION OF TRIMETHYLSILYL AND RELATED ALKYLSILYL DERIVATIVES

This review describes the mass spectral fragmentation of trimethylsilyl (TMS) and related alkylsilyl derivatives used for preparing samples for analysis, mainly by combined gas chromatography and mass spectrometry (GC/MS). The review is divided into three sections. The first section is concerned with the TMS derivatives themselves and describes fragmentation of derivatized alcohols, thiols, amines, ketones, carboxylic acids and bifunctional compounds such as hydroxy- and amino-acids, halo acids and hydroxy ethers. More complex compounds such as glycerides, sphingolipids, carbohydrates, organic phosphates, phosphonates, steroids, vitamin D, cannabinoids, and prostaglandins are discussed next. The second section describes intermolecular reactions of siliconium ions such as the TMS cation and the third section discusses other alkylsilyl derivatives. Among these latter compounds are di- and trialkyl-silyl derivatives, various substituted-alkyldimethylsilyl derivatives such as the tert-butyldimethylsilyl ethers, cyclic silyl derivatives, alkoxysilyl derivatives, and 3-pyridylmethyldimethylsilyl esters used for double bond location in fatty acid spectra. © 2019 Wiley Periodicals, Inc. Mass Spec Rev 0000:1-107, 2019.

Measuring brain lipids

The rapid development of analytical technology has made lipidomics an exciting new area and this review will focus more on modern approaches to lipidomics than on earlier technology. Although not fully comprehensive for all possible brain lipids, the intent is to at least provide a reference for the analysis of classes of lipids found in brain and nervous tissue. We will discuss problems posed by the brain because of its structural and functional heterogeneity, the development changes it undergoes (myelination, aging, pathology etc.) and its cellular heterogeneity (neurons, glia etc.). Section 2 will discuss the various ways in which brain tissue can be extracted to yield lipids for analysis and section 3 will cover a wide range of techniques used to analyze brain lipids such as chromatography and mass-spectrometry. In Section 4 we will discuss ways of analyzing some of the specific biologically active brain lipids found in very small amounts except in pathological conditions and section 5 looks to the future of experimental lipidomic modification in the brain. This article is part of a Special Issue entitled Brain Lipids.

Sphingolipidomics: methods for the comprehensive analysis of sphingolipids

Sphingolipids comprise a highly diverse and complex class of molecules that serve as both structural components of cellular membranes and signaling molecules capable of eliciting apoptosis, differentiation, chemotaxis, and other responses in mammalian cells. Comprehensive or "sphingolipidomic" analyses (structure specific, quantitative analyses of all sphingolipids, or at least all members of a critical subset) are required in order to elucidate the role(s) of sphingolipids in a given biological context because so many of the sphingolipids in a biological system are inter-converted structurally and metabolically. Despite the experimental challenges posed by the diversity of sphingolipid-regulated cellular responses, the detection and quantitation of multiple sphingolipids in a single sample has been made possible by combining classical analytical separation techniques such as high-performance liquid chromatography (HPLC) with state-of-the-art tandem mass spectrometry (MS/MS) techniques. As part of the Lipid MAPS consortium an internal standard cocktail was developed that comprises the signaling metabolites (i.e. sphingoid bases, sphingoid base-1-phosphates, ceramides, and ceramide-1-phosphates) as well as more complex species such as mono- and di-hexosylceramides and sphingomyelin. Additionally, the number of species that can be analyzed is growing rapidly with the addition of fatty acyl Co-As, sulfatides, and other complex sphingolipids as more internal standards are becoming available. The resulting LC-MS/MS analyses are one of the most analytically rigorous technologies that can provide the necessary sensitivity, structural specificity, and quantitative precision with high-throughput for "sphingolipidomic" analyses in small sample quantities. This review summarizes historical and state-of-the-art analytical techniques used for the identification, structure determination, and quantitation of sphingolipids from free sphingoid bases through more complex sphingolipids such as sphingomyelins, lactosylceramides, and sulfatides including those intermediates currently considered sphingolipid "second messengers". Also discussed are some emerging techniques and other issues remaining to be resolved for the analysis of the full sphingolipidome.

Chemical diagnosis of Fabry's disease by fluorometric assay and fast atom bombardment/mass spectrometry

We report the results of a fluorometric assay for alpha-galactosidase A (EC.3.2.1.22) in plasma and leukocytes, and fast atom bombardment/mass spectrometry (FAB/MS) analysis of glycosphingolipids in urine sediments from a patient with Fabry's disease. In plasma, this patient had only 5.0% of the normal amount of alpha-galactosidase A, and his brother and mother had 11.0% and 25.0%, respectively. In leukocytes, the activities were below 8.0%. Glycosphingolipids from urine sediments were partially purified using a Sep-Pack C18 cartridge. The chemical diagnosis of Fabry's disease can be made more rapidly and accurately using fluorometric and FAB/MS analyses.

Analysis of native neutral glycosphingolipids by combined high performance liquid chromatography/mass spectrometry

Conditions for normal phase high performance liquid chromatography-chemical ionization mass spectrometry of underivatized glycosphingolipids with the use of a moving belt transport interface were investigated. Chromatography was performed on a 2 mm ID X 3 cm column packed with 5 micron spherical silica. A gradient of increasing methanol and water in methylene chloride was used to resolve mono-, di-, tri-, and tetraglycosylceramides and sulfatides in less than 15 min. A polyimide belt was used to transport the sample residues to the mass spectrometer where ammonia chemical ionization mass spectrometry was used to obtain spectra. One to 5 micrograms of each component was sufficient to obtain full spectral scans. Mono- and dihexosylceramide spectra showed [M + H]+ ions of good abundance, while the higher glycosphingolipids have molecular weights that exceed the range of the mass spectrometer utilized. All glycosphingolipids gave ions characteristic of their ceramide, fatty acid, long chain base and carbohydrate components. Sequence information which reflected the relative position of hexose and N-acetylhexosamine residues was also obtained.

Structure of a glycolipid reacting with monoclonal IgM in neuropathy and with HNK-1

An acidic glycolipid antigen that reacts with monoclonal IgM in patients with demyelinating neuropathy and with the mouse monoclonal antibody, HNK-1, was purified from human peripheral nerves. This lipid sharing antigenic determinants with the myelin-associated glycoprotein was shown to be an unusual glucuronic acid-containing sulfated glycosphingolipid with five sugars, but without sialic acid. Mild acid methanolysis converted the GlcUA to its methyl ester, removed the acidic sulfate group and abolished the antigenicity. Results from chemical, enzymatic, infrared, and mass spectral analysis suggested the following structure with a sulfate in a position that remains to be determined: GlcUA beta 1----3Gal beta 1----4GlcNAc beta 1----3Gal beta 1----4Glc beta 1----1 ceramide.

Structural characterization of glycosphingolipids by direct chemical ionization mass spectrometry

This report describes the use of direct chemical ionization mass spectrometry with ammonia as the reagent gas (NH3-DCI) for structure analysis of underivatized, permethylated and permethylated and reduced glycosphingolipids. In contrast to ionization by electron impact, the NH3-DCI mass spectra exhibit intense molecular and carbohydrate sequence-related ions using microgram amounts of sample. Underivatized glycosphingolipids with up to two sugar residues yield abundant protonated and ammonia-cationized molecular ions and structurally significant fragments. Permethylation in conjunction with NH3-DCI can be used to obtain molecular weight as well as oligosaccharide sequence and branching information on neutral, acidic and complex-type glycosphingolipids with up to five sugar residues. Reduction of the permethylated derivatives gives rise to several new, structurally significant fragments in the corresponding NH3-DCI mass spectra which enable fatty acid and base compositions to be determined. Isotopically labeled reagent gases have been used to confirm the assignment of fragment structures and to demonstrate that the ions observed are unique to the NH3-DCI mass spectra.

Chemical-ionization mass spectra of the permethylated sialo-oligosaccharides liberated from gangliosides

Permethylated mono- and di-sialo-oligosaccharides liberated from several parent gangliosides have been examined by chemical-ionization mass spectrometry with ammonia as the reagent gas in order to elucidate their structures. Several major fragment-ions, in addition to both the protonated and ammonium adduct molecular-ions, may be readily assigned without interference from the ceramide moiety. Sialic acid-containing di-, tri-, and tetra-saccharide ions can be clearly observed and used to determine the sugar residue to which the sialic acid residue is attached. The neutral-sugar skeletons produced by the loss of sialic acid give rise to both the protonated and the ammonium adduct ions; in the case of tetrasaccharides, these are further degraded to produce di- and tri-saccharide ions. These characteristic ions are useful for the determination of the number of sugar residues and their sequence in an oligosaccharide structure. The chemical-ionization mass spectra of GM3- and GM1-oligosaccharides with isobutane show the ions corresponding to each monosaccharide residue. These results indicate that chemical-ionization mass spectrometry is highly useful in determining the complete sugar-sequence of gangliosides.

Identification of amino sugars from bacterial lipopolysaccharides by gas chromatography electron impact and chemical ionization mass spectrometry

Amino sugars isolated from lipopolysaccharides of Brucella suis, Brucella abortus and Neisseria gonorrhoeae colony types 1 and 4 were identified using gas chromatography electron impact and chemical ionization mass spectrometry. Lipopolysaccharides were obtained by aqueous ether or aqueous phenol extraction. Isolated lipopolysaccharides were hydrolyzed in 1% acetic acid followed by hydrolysis of the polysaccharide moiety in 2 NHCl for 6 h at 100 degrees C. Amino sugars were first isolated by elution from Dowex 50 H+ and then N-acetylated, followed by trimethylsilylation. Trimethylsilyl ethers of 2-acetamido-2-deoxysugars; N-acetylglucosamine, N-acetylmannosamine, N-acetylgalactosamine, and a 2-acetamido-2.6-dideoxysugar, N-acetylquinovosamine, were identified by their fragmentation patterns. In the electron impact mode, N-acetylglucosamine and N-acetyl-galactosamine were distinguished from one another by comparing peak intensities at m/e 233 and 305. However, N-acetylglucosamine and N-acetylmannosamine could not be differentiated by electron impact mass spectrometry. In the chemical ionization mode, N-acetylglucosamine and N-acetylmannosamine both with base peaks at m/e 494, could be distinguished from N-acetylgalactosamine and N-acetylquinovosamine by their base peaks at m/e 420 and 332, respectively. N-Acetylglucosamine and N-acetylmannosamine were differentiated from one another by comparing peak intensities at m/e 330, 404, 420, and 510 [MH]+. This is the first report of chemical ionization mass spectrometry applied to the identification of amino sugars in bacterial lipopolysaccharides and shows that some 2-amino-2-deoxysugars can be differentiated by both electron impact and chemical ionization mass spectrometry.

The interpretation of the mass spectrum of an ornithine-containing lipid from Thiobacillus thiooxidans

The electron impact mass spectrum of a previously identified ornithine-containing lipid from Thiobacillus thiooxidans has been interpreted using exact mass measurements, low and high energy ionization, and defocused metastable studies. The spectrum, which did not contain a molecular ion for the intact lipid, was consistent with cyclization of the ornithine zwitterionic moiety with elimination of water to give 3[3'-(11,12-methylene-2-hydroxyoctadecanoxy)hexadecanylamine]-2-piperidone. Production of this sufficiently volatile species for mass spectral analysis was accomplished by gentle pyrolysis in the mass spectrometer source. The spectrum can be understood to arise by three primary decompositions which serve to separate the two fatty acid constituents. The remainder of the spectrum is consistent with the expected fragmentations of these constituents.

Mass spectrometric analysis of permethylated glycosphingolipids II. Comparative studies on different blood-group active and related erythrocyte membrane glycosphingolipids

Three isomeric ceramide tetrasaccharides--P blood-group active globoside, lacto-N-neotetraosyl ceramide as ABH blood-group precursor, both isolated from human erythrocytes and "asialo ganglioside" from human brain as reference standard--and two ceramide pentasaccharides--H blood-group active glycosphingolipid, obtained from blood-group B active ceramide hexasaccharide of human B erythrocytes after alpha-galactosidase treatment and ceramide pentasaccharide from rabbit erythrocytes with B-like blood-group activity--were investigated by mass spectrometry after permethylation. The carbohydrate moiety exhibits differences not only concerning the sugar sequence but also with regard to the position of some glycosidic linkages: Oligosaccharides containing N-acetylhexosamine substituted at position 4 produce spectra that are distinctly different from those containing C-3 substituted N-acetylhexosamines, thus allowing the differentation between type 1 and type 2 carbohydrate chains. Moreover, oligosaccharide ions with a hexose at the cleavage site exhibit a fragmentation pattern different from those with a N-acetylhexosamine at the "reducing terminal". The intensity ratio between parent ion and parent ion -32 mass units is Q greater than or equal to 3 in the first case, whereas in the latter case Q is less than 1. The Q-values are given for 14 oligosaccharide ions. Differences in the composition of the ceramide residues can also be deduced from the mass spectra.

Mass spectrometric analysis of permethylated glycosphingolipids I. Sequence analysis of two blood-group B active glycosphingolipids from human B erythrocyte membranes

Two blood group B active glycosphingolipids (B-I and B-II) formerly isolated and purified from human B erythrocytes (16) were investigated by mass spectrometry after permethylation. B-I yielded fragments up to m/e 1266 and B-II up to m/e 1495, showing the sequence of six and seven carbohydrate residues respectively. In combination with additional experimental evidence (18) the glycosphingolipids are demonstrated to be a gal-[ fuc ]-gal-glcNAc-gal-glc-ceramide (B-I) and a gal-[ fuc ]-gal-glcNAc-gal-glcNAc-gal-glc-ceramide (B-II). Mass spectrometric evidence for the ceramide residues are also obtained indicating besides spingosine C24-,C24:1-, and C22-fatty acids as main constituents.