A comprehensive and standardized system to report lipid structures analyzed by MS is essential for the communication and storage of lipidomics data. Herein, an update on both the LIPID MAPS ...classification system and shorthand notation of lipid structures is presented for lipid categories Fatty Acyls (FA), Glycerolipids (GL), Glycerophospholipids (GP), Sphingolipids (SP), and Sterols (ST). With its major changes, i.e., annotation of ring double bond equivalents and number of oxygens, the updated shorthand notation facilitates reporting of newly delineated oxygenated lipid species as well. For standardized reporting in lipidomics, the hierarchical architecture of shorthand notation reflects the diverse structural resolution powers provided by mass spectrometric assays. Moreover, shorthand notation is expanded beyond mammalian phyla to lipids from plant and yeast phyla. Finally, annotation of atoms is included for the use of stable isotope-labeled compounds in metabolic labeling experiments or as internal standards. This update on lipid classification, nomenclature, and shorthand annotation for lipid mass spectra is considered a standard for lipid data presentation.
Mass spectrometry has played a critical role in the identification and quantitation of neutral lipids such as cholesteryl esters and triacylglycerols present in biological extracts. Various ...strategies have emerged in order to carry out such lipidomics studies since a large number of neutral lipid molecular species exist in tissues. These include both shotgun approaches as well as those engaging liquid chromatographic separation of species prior to mass spectrometric analysis. Nonetheless challenges remain at every level of the lipidomics experiment, including extraction of lipids, identification of specific species, and quantitation of the vast array of lipids present in the sample extract. Unambiguous identification of molecular species present (qualitative analysis) as well as precise quantitation remains as significant challenges. The relative quantitation enables quite accurate assessment of fold changes of complex lipid species without exact quantitation. The availability of reference standard material as well as relevant internal standards continue to be limited.
Leukotrienes are metabolites of arachidonic acid derived from the action of 5-LO (5-lipoxygenase). The immediate product of 5-LO is LTA4 (leukotriene A4), which is enzymatically converted into either ...LTB4 (leukotriene B4) by LTA4 hydrolase or LTC4 (leukotriene C4) by LTC4 synthase. The regulation of leukotriene production occurs at various levels, including expression of 5-LO, translocation of 5-LO to the perinuclear region and phosphorylation to either enhance or inhibit the activity of 5-LO. Several other proteins, including cPLA2a (cytosolic phospholipase A2a) and FLAP (5-LO-activating protein) also assemble at the perinuclear region before production of LTA4. LTC4 synthase is an integral membrane protein that is present at the nuclear envelope; however, LTA4 hydrolase remains cytosolic. Biologically active LTB4 is metabolized by w-oxidation carried out by specific cytochrome P450s (CYP4F) followed by b-oxidation from the w-carboxy position and after CoA ester formation. Other specific pathways of leukotriene metabolism include the 12-hydroxydehydrogenase/15-oxo-prostaglandin-13-reductase that forms a series of conjugated diene metabolites that have been observed to be excreted into human urine. Metabolism of LTC4 occurs by sequential peptide cleavage reactions involving a g-glutamyl transpeptidase that forms LTD4 (leukotriene D4) and a membrane-bound dipeptidase that converts LTD4 into LTE4 (leukotriene E4) before w-oxidation. These metabolic transformations of the primary leukotrienes are critical for termination of their biological activity, and defects in expression of participating enzymes may be involved in specific genetic disease.
Leukotrienes (LTs) are autacoids derived from the precursor arachidonic acid (AA) via the action of five-lipoxygenase (5-LO). When inflammatory cells are activated, 5-LO translocates to the nuclear ...membrane to initiate oxygenation of AA released by cytosolic phospholipase A2 (cPLA2) into leukotriene A4 (LTA4). LTA4 can also be exported from an activated donor cell into an acceptor cell by the process of transcellular biosynthesis. When thimerosal is added to cells, the level of free AA increases by inhibition of lysophospholipid acyltransferases of the Lands pathway of phospholipid remodeling. Another arachidonate phospholipid cycle involves phosphatidylinositol (PI) in the plasma membrane that undoubtedly intersects with the Lands pathway of phospholipid remodeling. The highest abundance of PI occurs between the ER and the plasma membrane and is probably a result of the importance of the PI signaling cascade in cellular biochemistry. Because transport proteins mediate the rapid intracellular movement of phospholipids, largely as result of physical membrane contact, 5-LO-dependent production of LTA4 could be mediated by the disappearance of free AA from the nuclear membrane, transfer to the ER for Lands cycle reesterification into PI, and population of PI(18:0/20:4) for cell membrane signaling. Display omitted
Arachidonic acid and esterified arachidonate are ubiquitous components of every mammalian cell. This polyunsaturated fatty acid serves very important biochemical roles, including being the direct ...precursor of bioactive lipid mediators such as prostaglandin and leukotrienes. This 20 carbon fatty acid with four double bonds was first isolated and identified from mammalian tissues in 1909 by Percival Hartley. This was accomplished prior to the advent of chromatography or any spectroscopic methodology (MS, infrared, UV, or NMR). The name, arachidonic, was suggested in 1913 based on its relationship to the well-known arachidic acid (C20:0). It took until 1940 before the positions of the four double bonds were defined at 5,8,11,14 of the 20-carbon chain. Total synthesis was reported in 1961 and, finally, the configuration of the double bonds was confirmed as all-cis-5,8,11,14. By the 1930s, the relationship of arachidonic acid within the family of essential fatty acids helped cue an understanding of its structure and the biosynthetic pathway. Herein, we review the findings leading up to the discovery of arachidonic acid and the progress toward its complete structural elucidation.
Berry et al examine matrix-assisted laser desorption ionization (MALDI) imaging. They focus on MALDI with regard to of lipid biochemistry in tissues by mass spectrometry.
Common organic matrix-assisted laser desorption/ionization (MALDI) matrices, 2,5-dihydroxybenzoic acid, 3,5-dimethoxy-4-hydroxycinnamic acid, and α-cyano-4-hydroxycinnamic acid, were found to undergo ...sublimation without decomposition under conditions of reduced pressure and elevated temperature. This solid to vapor-phase transition was exploited to apply MALDI matrix onto tissue samples over a broad surface in a solvent-free application for mass spectrometric imaging. Sublimation of matrix produced an even layer of small crystals across the sample plate. The deposition was readily controlled with time, temperature, and pressure settings and was highly reproducible from one sample to the next. Mass spectrometric images acquired from phospholipid standards robotically spotted onto a MALDI plate yielded a more intense, even signal with fewer sodium adducts when matrix was applied by sublimation relative to samples where matrix was deposited by an electrospray technique. MALDI matrix could be readily applied to tissue sections on glass slides and stainless steel MALDI plate inserts as long as good thermal contact was made with the condenser of the sublimation device. Sections of mouse brain were coated with matrix applied by sublimation and were imaged using a Q-q-TOF mass spectrometer to yield mass spectral images of very high quality. Image quality is likely enhanced by several features of this technique including the microcrystalline morphology of the deposited matrix, increased purity of deposited matrix, and evenness of deposition. This inexpensive method was reproducible and eliminated the potential for spreading of analytes arising from solvent deposition during matrix application.
Recent developments in MALDI have enabled direct detection of lipids as intact molecular species present within cellular membranes. Abundant lipid-related ions are produced from the direct analysis ...of thin tissue slices when sequential spectra are acquired across a tissue surface that has been coated with a MALDI matrix. The lipid-derived ions can often be distinguished from other biomolecules because of the significant mass defect that these ions present due to the large number of covalently bound hydrogen atoms in hydrophobic molecules such as lipids. Collisional activation of the molecular ions can be used to determine the lipid family and often structurally define the molecular species. Specific examples in the detection of phospholipids, sphingolipids, and glycerolipids are presented with images of mouse brain and kidney tissue slices. Regional distribution of many different lipid molecular species and Na+ and K+ attachment ions often define anatomical regions within the tissues.
Summary Background Since late 2015, an epidemic of yellow fever has caused more than 7334 suspected cases in Angola and the Democratic Republic of the Congo, including 393 deaths. We sought to ...understand the spatial spread of this outbreak to optimise the use of the limited available vaccine stock. Methods We jointly analysed datasets describing the epidemic of yellow fever, vector suitability, human demography, and mobility in central Africa to understand and predict the spread of yellow fever virus. We used a standard logistic model to infer the district-specific yellow fever virus infection risk during the course of the epidemic in the region. Findings The early spread of yellow fever virus was characterised by fast exponential growth (doubling time of 5–7 days) and fast spatial expansion (49 districts reported cases after only 3 months) from Luanda, the capital of Angola. Early invasion was positively correlated with high population density (Pearson's r 0·52, 95% CI 0·34–0·66). The further away locations were from Luanda, the later the date of invasion (Pearson's r 0·60, 95% CI 0·52–0·66). In a Cox model, we noted that districts with higher population densities also had higher risks of sustained transmission (the hazard ratio for cases ceasing was 0·74, 95% CI 0·13–0·92 per log-unit increase in the population size of a district). A model that captured human mobility and vector suitability successfully discriminated districts with high risk of invasion from others with a lower risk (area under the curve 0·94, 95% CI 0·92–0·97). If at the start of the epidemic, sufficient vaccines had been available to target 50 out of 313 districts in the area, our model would have correctly identified 27 (84%) of the 32 districts that were eventually affected. Interpretation Our findings show the contributions of ecological and demographic factors to the ongoing spread of the yellow fever outbreak and provide estimates of the areas that could be prioritised for vaccination, although other constraints such as vaccine supply and delivery need to be accounted for before such insights can be translated into policy. Funding Wellcome Trust.
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