Determination of eicosanoids, phospholipids and related compounds by thermospray liquid chromatography-mass spectrometry

Atmospheric pressure photoionization as a powerful tool for large-scale lipidomic studies

Lipidomic studies often use liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) for separation, identification, and quantification. However, due to the wide structural diversity of lipids, the most apolar part of the lipidome is often detected with low sensitivity in ESI. Atmospheric pressure (APPI) can be an alternative ionization source since normal-phase solvents are known to enhance photoionization of these classes. In this paper, we intend to show the efficiency of APPI to identify different lipid classes, with a special interest on sphingolipids. In-source APPI fragmentation appears to be an added value for the structural analysis of lipids. It provides a detailed characterization of both the polar head and the non polar moiety of most lipid classes, and it makes possible the detection of all lipids in both polarities, which is not always possible with ESI.

Identification of biological tissues by rapid evaporative ionization mass spectrometry

The newly developed rapid evaporative ionization mass spectrometry (REIMS) provides the possibility of in vivo, in situ mass spectrometric tissue analysis. The experimental setup for REIMS is characterized in detail for the first time, and the description and testing of an equipment capable of in vivo analysis is presented. The spectra obtained by various standard surgical equipments were compared and found highly specific to the histological type of the tissues. The tissue analysis is based on their different phospholipid distribution; the identification algorithm uses a combination of principal component analysis (PCA) and linear discriminant analysis (LDA). The characterized method was proven to be sensitive for any perturbation such as age or diet in rats, but it was still perfectly suitable for tissue identification. Tissue identification accuracy higher than 97% was achieved with the PCA/LDA algorithm using a spectral database collected from various tissue species. In vivo, ex vivo, and post mortem REIMS studies were performed, and the method was found to be applicable for histological tissue analysis during surgical interventions, endoscopy, or after surgery in pathology.

Electrospray/tandem mass spectrometry for quantitative analysis of lipid remodeling in essential fatty acid deficient mice

A method utilizing electrospray ionization coupled with tandem mass spectrometry was developed as a facile and rapid method to identify and quantify lipid remodeling in vivo. Electrospray/tandem mass spectrometric analyses were performed on lipids isolated from liver tissue and resident peritoneal cells from essential fatty acid sufficient and deficient mice. Essential fatty acid deficiency was chosen as the paradigm to evaluate the methodology because it epitomizes the most extreme dietary means of altering fatty acid composition of virtually all cellular lipid species. Qualitative and quantitative changes were measured in the phospholipid and cholesterol ester species directly in the chloroform/methanol lipid extract without any prior chromatographic separation. Lipid remodeling in liver and peritoneal cells from essential fatty acid deficient mice was qualitatively similar in cholesterol ester, phosphatidylcholine, and phosphatidylethanolamine. The monoenoic fatty acids palmitoleic acid (16:1 n-7) and oleic acid (18:1 n-9) were increased markedly, whereas all n-6 and n-3 polyunsaturated fatty acids were nearly depleted in phospholipid and cholesterol ester species. The n-9 polyunsaturated fatty acid surrogate, Mead acid (20:3 n-9), substituted for arachidonic acid (20:4 n-6) and docosahexaenoic acid (22:6 n-3) in phospholipid, but not in cholesterol ester, species. Another notable difference was that adrenic acid (22:4 n-6) and docosapentaenoic acid (22:5 n-6), both metabolites of arachidonic acid, accumulated in phospholipid and cholesterol ester species of peritoneal cells, but not in liver cells, of essential fatty acid sufficient mice. The overall body of data presented illustrates the implementation of electrospray/tandem mass spectrometry as a method for facile and direct quantification of changes in lipid species during lipid metabolic studies.

Lipoxygenation of docosahexaenoic acid by the rat pineal body

Based on the inhibitor profile, production rate, and stereochemical purity of the hydroxylated products, it was demonstrated that lipoxygenation in rat brain occurs only in the pineal. Both positional and stereochemical specificities of the hydroxylation were observed only in pineal, clearly indicating that only the pineal is capable of lipoxygenating polyunsaturated fatty acids among the rat brain regions examined. Cerebral cortex also produced hydroxy products; however, they were racemic mixtures, indicating that peroxidation was responsible for their production. Rat pineal homogenate, obtained after the brain was perfused, metabolized [14C]docosahexaenoic acid ([1-14C]22:6n3) to monohydroxy derivatives, primarily by the 12- and, to a lesser extent, by the 15-lipoxygenase (LO) reaction. The resulting metabolites were 14(S)- and 17(S)-hydroxydocosahexaenoic acid (HDoHE), as determined by reversed-phase HPLC, chiral-phase HPLC, thermospray liquid chromatography-mass spectrometry, and gas chromatography-mass spectrometry. Because blood was removed by perfusion of the brain before incubation, it was clear that the observed LO activity was not due to contamination with blood cell components. The production rate of 17-HDoHE from 22:6n3 was higher than that of 15-hydroxyperoxy-5,8,11,13-eicosatetraenoic acid from 20:4n6, whereas 12-LO activity toward these two substrates was comparable. These monohydroxy metabolites were also detected in the pineal body lipid extract using negative ion chemical ionization mass spectrometry. This is the first observation of endogenous production of hydroxylated compounds in pineal. The ratio of endogenous 15-LO to 12-LO products was considerably higher than that of the in vitro production from exogenous substrate. In some cases, 15-LO products were the major LO metabolites present in the lipid extract of pineal body for both 20:4n6 and 22:6n3. Both 12- and 15-LO activities were recovered mainly in the microsomal plus cytosolic fraction. In addition to monohydroxy products, epoxy, hydroxy derivatives were formed from 22:6n3 by the pineal. The major isomer was identified as 12-hydroxy-13,14-epoxy-22:5n3.

Structural analysis of hydroxy fatty acids by thermospray liquid chromatography/tandem mass spectrometry

Thermospray liquid chromatography/tandem mass spectrometry provides a sensitive and convenient technique for the structural analysis of oxygenated polyunsaturates. Analysis of pentafluorobenzyl derivatives in the negative ion mode under the filament- or discharge-on condition generated abundant [M - PFB]- ions. These ions were further fragmented by collision with argon and detected in the negative ion mode. The negative ion fragmentation pattern was examined for various oxygenated polyunsaturated fatty acid standards as well as their deuterated and/or hydrogenated forms. Characteristic fragmentation occurs at the oxygenated C-C bonds, allowing unambiguous determination of the sites of oxygenation. The sample amount required is typically in the low tens of nanogram range. Using this method the structures of epoxy, hydroxy derivatives of 4,7,10,13,16,19-docosahexaenoic acid (22:6w3) formed by soybean lipoxygenase were determined. They were 13-hydroxy-16,17-epoxy-22:5w3 and 15-hydroxy-16,17-epoxy-22:5w3.

Analysis of vitamin D and its metabolites using thermospray liquid chromatography/mass spectrometry

A new method is described for the analysis of vitamin D and its metabolites utilizing thermospray (TSP) mass spectrometry as an on-line detector for high performance liquid chromatography. Ionization conditions were optimized for use with isocratic reversed phase chromatography. TSP mass spectrometry was employed in series with a UV absorbance detector to facilitate comparisons between the two methods of detection. Positive ion TSP mass spectra were recorded for vitamin D2, vitamin D3, 25-hydroxyvitamin D3 (25(OH)D3), 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 24,25-dihydroxyvitamin D3 (24,25(OH)2D3). The spectra contained protonated molecular ions, ammonium adduct ions and fragment ions due to the loss of one or more molecules of water. A comparison of quantitative precision was made by determining UV absorbance and TSP standard curves for vitamin D3 using two different methods: (1) External standard method with post-column (post UV detector) addition of ammonium acetate. (2) As (1) but using the method of internal standards with a closely eluting internal standard (vitamin D2). In each case the quantitative precision (correlation coefficient) for UV absorbance detection was superior owing to intrinsic instability of the TSP ion beam. A stable isotopically labelled internal standard was employed in the development of an assay for 1,25(OH)2D3. The assay was used to quantify in vitro enzymic conversion of 25(OH)D3 to 1,25(OH)2D3 in guinea pig and sheep renal mitochondrial incubations. TSP LC/MS was also applied to analysis of an extract of human blood plasma in which D3 and each of its principal metabolites were identified in a single analysis.

Plasma lipid profiling by liquid chromatography with chloride-attachment mass spectrometry

A sensitive high-performance liquid chromatographic assay was developed using chloride attachment negative chemical ionization mass spectrometry for detection of glyceryl esters and ceramides, and positive chemical ionization mass spectrometry for detection of free cholesterol and cholesteryl esters in minimal quantities of plasma. The novel technique was validated by high temperature gas-liquid chromatography with flame ionization detection. Sample preparation was achieved by phospholipase C digestion of whole plasma, total lipid extraction and derivatization of any free carboxyl and hydroxyl groups by trimethyl- or tert-butyldimethyl-chlorosilane. The lipids were separated by reverse phase HPLC with 20-90% propionitrile in acetonitrile containing 1% dichloromethane, which served as the reagent and the source of chloride. Negative chemical ionization with chloride attachment is estimated to provide about 100 times higher response for the triacylglycerols and the trimethylsilyl or tert-butyldimethylsilyl ethers of diacylglycerols, and about 500 times higher response for the trimethylsilyl or tert-butyldimethylsilyl ethers of ceramides than positive chemical ionization mass spectrometry. Determination of the full negative chemical ionization mass spectra showed that each glycerolipid and ceramide species yielded a single ionic species corresponding to the chloride-attachment product of the parent ion. The cholesteryl esters and ethers failed to attach chloride and remained undetected by negative chemical ionization. However, the cholesteryl esters and ethers gave a high response for the steroid nucleus in positive chemical ionization mass spectrometry. Chloride attachment negative chemical ionization mass spectrometry is suitable for the unequivocal identification of plasma glycerolipids and ceramides in high-performance liquid chromatography and for the quantitation of molecular species in any unresolved peaks following appropriate calibration of the instrument response.

Stereochemical analysis of hydroxylated docosahexaenoates produced by human platelets and rat brain homogenate

The stereochemical configuration of hydroxylated products of docosahexaenoic acid (22:6w3) formed by human platelets and rat brain homogenate were characterized for the first time. Chiral phase HPLC was employed along with autooxidized 22:6w3 as reference material. The 14- and 11-hydroxy 22:6w3 (HDHE) products produced by human platelets were in the S configuration. Rat brain homogenate produced all of the ten possible positional isomers when incubated with 22:6w3. Their retention behavior on the reversed and chiral phase HPLC columns and GC/MS/EI analysis indicated that they were 20-, 17-, 16-, 14-, 13-, 11-, 10-, 8-, 7- and 4-HDHE. However, stereochemical analysis revealed that each positional isomer was a racemic mixture, suggesting that these were not formed by lipoxygenation but mainly by peroxidation process.

Formation of 15-lipoxygenase product from docosahexaenoic acid (22:6w3) by human platelets

The metabolism of docosahexaenoic acid (22:6w3) by 15-lipoxygenase activity of washed human platelets was investigated. Platelets produced 17-hydroxydocosahexaenoic acid (HDHE) when incubated with 22:6w3. Similarly, 15-hydroxyeicosatetraenoic acid (HETE) and 13- and 9-hydroxyoctadecadienoic acids (HODD) were produced when incubated with 20:4w6 and 18:2w6, respectively. However, these products were observed only as minor components in the platelet incubation mixture. Control studies with carefully purified platelets and mononuclear cells indicated that these products were formed by the platelets. Chiral phase HPLC analysis indicated that these compounds were mainly in the S configuration with the exception of the 9-HODD, thus, confirming that a lipoxygenase is responsible for their production. The 9-HODD produced by platelets was a racemic mixture.

Determination of prostaglandins in human seminal fluid by solid-phase extraction, pyridinium dichromate derivatization and high-performance liquid chromatography

Prostaglandins of the E type (PGEs) in human seminal fluid have been determined by reversed-phase high-performance chromatography on a C18 column and ultraviolet detection at 230 nm after solid-phase extraction (C18) and oxidation to the corresponding 15-oxoprostaglandin derivatives by pyridinium dichromate in acetonitrile. Under optimized conditions, PGEs from 10-ml seminal samples were extracted into 4 ml of methyl formate with high recoveries (estimated at greater than 95%) and subsequently separated under mild chromatographic conditions (0.5 mM formic acid-acetonitrile, apparent pH 3.8). Comparable analytical sensitivities were obtained with detection at 230 nm with a conventional deuterium lamp spectrophotometer and a photometer equipped with a cadmium emission source, while with a diode-array spectrophotometer, signal-to-noise ratios were reduced with factors between 4.4 and 3.1, depending on the spectral bandwidth of the instrument. Theoretical aspects of signal-to-noise optimization of ultraviolet detectors are discussed. The stability of dilute standard solutions of PGE2 and PGD2 was measured, showing solutions in dichloromethane at 20 degrees C to be as stable as acetonitrile solutions at 5 degrees C over a period of thirty days. Absolute ethanol and acetonitrile were equally suited as solvents.

Identification of reaction products from the pyridinium dichromate derivatization of prostaglandins by high-performance liquid chromatography and direct chemical ionization mass spectrometry

The reaction products from the oxidation of prostaglandins with pyridinium dichromate have been identified by direct chemical ionization mass spectrometry of the underivatized compounds after separation by reversed-phase high-performance liquid chromatography and ultraviolet diode-array detection. The thermal influence on the reproducibility of the dehydration patterns of the mass spectra was studied. The main products from the prostaglandins E1, E2, F1 alpha and F2 alpha were the corresponding 15-oxo derivatives. Minor amounts of the 9,11,15-trioxoprostaglandin were formed from PGE, while the oxidation of PGF was less selective, yielding additional dioxo derivatives. Addition of water to the reagent reduced the reactivity, but increased the selectivity in favour of the formation of 15-oxo-PGF during the oxidation of PGF.

Biochemical applications of liquid chromatography-mass spectrometry

The current state-of-the-art liquid chromatography-mass spectrometry (LC-MS) is reviewed with particular attention to biomedical applications. The most common LC-MS interface designs are described and compared. These interfaces include transport, direct liquid introduction, thermospray, atmospheric pressure ionization, monodisperse aerosol generation, open-tubular LC and continuous-flow fast atom bombardment. The relative sensitivities of the techniques are compared as much as possible, as well as their tendencies to induce thermal decomposition of the sample. Applications of these various interface types to a variety of biomedically important compound classes, including peptides, nucleotides, steroids, lipids, carbohydrates, xenobiotic metabolites and drugs, are also reviewed.

Biosynthesis, catabolism, and biological properties of HPETEs, hydroperoxide derivatives of arachidonic acid

The oxygenation of arachidonic acid by lipoxygenases results in the formation of HPETEs (hydroperoxyeicosatetraenoic acids), the first products of the LOX pathway. These compounds are short lived and are catabolised into various families of more stable compounds of which the HETEs, hepoxilins, lipoxins and leukotrienes have been identified so far. The development of new techniques have helped to identify and understand the structures of various HPETEs and only recently the biological effects of HPETEs and their various catabolites are being unraveled. Although lipoxygenases are ubiquitous, not all tissues possess the same spectrum of lipoxygenase enzymes. Hence different HPETEs can be formed in different tissues. Recent studies have revealed that HPETEs or products derived from them possess a diversity of important biological properties including the regulation of electrolyte flux and eicosanoid and corticosterone syntheses, release of histamine, regulation of oocyte maturation and release of various reproductive hormones. HPETEs appear to be involved in some pathological conditions viz, skin psoriasis, Clarkson's disease, nerve injury and spinal cord ischemia. These novel eicosanoids are associated with the release of insulin as well as renin. Recently HPETEs have been suggested to act as second messengers in the Aplysia sensory neurons and its catabolite, hepoxilin, has been demonstrated to have effects on mammalian hippocampal neurons. The purpose of this review is to provide a brief summary of the formation of the HPETEs and the various families of compounds derived from them as well as the various types of biological activities for these products described so far.

Preparation and the structural determination of hydroperoxy derivatives of docosahexaenoic acid and other polyunsaturates by thermospray LC/MS

A new method to determine the structure of lipoxygenase reaction products is presented. Thermospray mass spectra of hydroperoxy derivatives of polyunsaturates contain both molecular ion species and fragments reflecting the position of oxygenation. Data are presented for hydroperoxy-docosahexaenoic, eicosapentaenoic, arachidonic and linoleic acids in this regard. Ten positional isomers of hydroperoxy docosahexaenoic acid were prepared by autooxidation and their structures were determined by thermospray LC/MS and confirmed by electron impact GC/MS after suitable derivatives were made. This technique was particularly useful in determining the structure of unknown metabolites by direct monitoring of the reaction mixture without derivatization. In this paper, the value of this approach is demonstrated using a soybean lipoxygenase reaction mixture as a simple example.

Highly sensitive determination and characterization of intact cellular ester-linked phospholipids using liquid chromatography-plasma spray mass spectrometry

Liquid chromatographic class separations of common cellular phospholipids combined with plasma spray ionization of the effluents were investigated. Comparison with true thermospray ionization involving ammonium acetate buffering revealed a gain in total ionization in the plasma spray of a factor of approximately 10 using a cation-exchange column and a solvent mixture consisting of acetonitrile-methanol-water (400:100:15, v/v). Plasma spray ionization studies of bovine brain polyphosphoinositides interrelated by the phosphate content in the inositol moiety showed almost identical monoglyceride and diglyceride ion clusters, indicating possibilities of studying the biochemical turnover of such phospholipids. Plasma spray ionization liquid chromatography-mass spectrometry of bacterial membrane phospholipids (Pseudomonas fluorescens) revealed possibilities of obtaining indications of individual fatty acid compositions from the spectra of the phosphatidylinositol and phosphatidylethanolamine fractions present. Conventional gas chromatographic fatty acid analysis agreed with the direct mass spectrometric structure elucidations. Interestingly, the two phospholipid classes had different relative fatty acid compositions with a significantly higher degree of cyclic fatty acids in the phosphatidyl ethanolamines. Plasma spray ionization yielded linear dose-response curves for both the monoglyceride and diglyceride fragment signals in the selected-ion monitoring mode. The detection limit for the monoglyceride and diglyceride species of phosphatidylcholine under the chromatographic and mass spectrometric conditions used was found to be in the picogram range.