Separation and quantitation of phospholipids and lysophospholipids by high-performance liquid chromatography

Role of cardiolipin alterations in mitochondrial dysfunction and disease

Cardiolipin (CL) is a structurally unique dimeric phospholipid localized in the inner mitochondrial membrane where it is required for optimal mitochondrial function. In addition to its role in maintaining membrane potential and architecture, CL is known to provide essential structural and functional support to several proteins involved in mitochondrial bioenergetics. A loss of CL content, alterations in its acyl chain composition, and/or CL peroxidation have been associated with mitochondrial dysfunction in multiple tissues in a variety of pathological conditions, including ischemia, hypothyroidism, aging, and heart failure. Recently, aberrations in CL metabolism have been implicated as a primary causative factor in the cardioskeletal myopathy known as Barth syndrome, underscoring an important role of CL in human health and disease. The purpose of this review is to provide an overview of evidence that has linked changes in the CL profile to mitochondrial dysfunction in various pathological conditions. In addition, a brief overview of CL function and biosynthesis, and a discussion of methods used to examine CL in biological tissues are provided.

Characterization of cardiolipin as the sodiated ions by positive-ion electrospray ionization with multiple stage quadrupole ion-trap mass spectrometry

The application of multiple-stage ion-trap (IT) mass spectrometric methods for the structural characterization of cardiolipin (CL), a 1,3-bisphosphatidyl-sn-glycerol that consists of four fatty acyl chains and three glycerol backbones (designated as A, B, and central glycerol, respectively), as the sodiated adduct ions in the positive-ion mode was evaluated. Following collisionally activated dissociation (CAD), the [M - 2H + 3Na]+ ions of CL yield two prominent fragment ion pairs that consist of the phosphatidyl moieties attached to the 1'- and 3'-position of the central glycerol, respectively, resulting from the differential losses of the diacylglycerol moieties containing A and B glycerol, respectively. The results are consistent with those previously described for the [M - H]- and [M - 2H + Na]- ions in the negative-ion mode, thus permitting assignment of the two phosphatidyl moieties attached to the 1'- or 3'-position of the central glycerol. The identities of the fatty acyl substituents and their positions on the glycerol backbones (glycerol A and B) are deduced from further degradation of the above ion pairs that give the fragment ions reflecting the fatty acid substituents at the sn-1 (or sn-1') and sn-2 (or sn-2') positions. The ions that arise from losses of the fatty acid substituents at sn-1 and sn-1', respectively, are prominent, but the analogous ions from losses of the fatty acid substituents at sn-2 and sn-2', respectively, are of low abundance in the MS2 product-ion spectra. This feature further confirms the assignment of the positions of the fatty acid substituents. The similar IT multiple-stage mass spectrometric approaches including MS2 and MS3 for structural characterization of CL using its [M + Na]+ and the [M - H + 2Na]+ ions are also readily applicable. However, their uses for structural characterization are less desirable because formation of the [M + Na]+ and the [M - H + 2Na]+ ions for CL is not predictable.

A review of chromatographic methods for the assessment of phospholipids in biological samples

Phospholipids are important constituents of all living cell membranes. Lipidomics is a rapidly growing field that provides insight as to how specific phospholipids play roles in normal physiological and disease states. There are many analytical methods available for the qualitative and quantitative determination of phospholipids. This review provides a summary of the methods that were historically used such as thin layer chromatography, gas chromatography and high-performance liquid chromatography. In addition, an introduction to applications of interfacing these traditional chromatographic techniques with mass spectrometry is provided.

Analysis of phospholipid species in rat peritoneal surface layer by liquid chromatography/electrospray ionization ion-trap mass spectrometry

The main phospholipids in rat peritoneal surface layer were analyzed by normal-phase high-performance liquid chromatography (HPLC) coupled with electrospray ionization (ESI) ion-trap mass spectrometry (MS). By using a silica gel column and a gradient of hexane/isopropanol/water as mobile phase containing 5 mmol/L ammonium formate as modifiers, a baseline separation of glycerophosphoehtanolamine (PE), phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylcholine (PC), sphingomyelin (SM) and lyso-phosphatidylcholine (LPC) was obtained and more than 90 phospholipid constituents in rat peritoneal surface were identified and determined by on-line ion-trap MS detection. The major ethanolamine glycerophospholipids in rat peritoneal surfaces were plasmalogens that were highly enriched in polyunsaturated fatty acids at the sn-2 position. In addition, the fragmentation patterns for each phospholipid class by the ion-trap MS were discussed.

Quantification of lysophosphatidylcholines and phosphatidylcholines using liquid chromatography-tandem mass spectrometry in neonatal serum

We established an improved method for quantification of phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) molecular species in neonatal serum using high-performance liquid chromatography coupled tandem mass spectrometry (LC-MS/MS). A multiple reaction monitoring (MRM) mode of positive ionization for MS/MS was used. The method involved purification of phospholipids by solid phase extraction (SPE) from a 20-microl minimum specimen of serum. The assayed values of authentic 16:0-LPC and 18:0-LPC showed a linear response, and our quantitative results showed high precision for the all species of PC and LPC. Then, we quantified PC and LPC in adult and neonatal serum and compared them. Day 0-1 neonatal serum 16:0-, 18:0-, 18:1-, 18:2-LPC levels were significantly lower than adult ones. All species LPC levels in the day 0-1 neonates were significantly lower than day 4-8 neonates. Day 0-1 neonatal serum 16:0/18:2-, 18:0/18:2-PC levels were significantly lower than adult ones. Our method is advantageous for precise assessments of the relationships between PCs/LPCs levels and neonatal infectious diseases.

Characterization of cardiolipin from Escherichia coli by electrospray ionization with multiple stage quadrupole ion-trap mass spectrometric analysis of [M - 2H + Na]- ions

We report a multiple-stage ion-trap (IT) mass spectrometric approach with electrospray ionization (ESI) for structural characterization of the [M - 2H + Na]- ion of cardiolipin (CL), a 1,3-bisphosphatidyl-sn-glycerol that consists of four fatty acyl chains and three glycerol backbones designated as A, B, and central glycerol, respectively (see Scheme 1). Following collisionally activated dissociation (CAD), the [M - 2H + Na]- ions of CL yield two prominent fragment ions that arise from the differential losses of the diacylglycerol moieties containing A or B glycerol, respectively. The tentative assignment of the two phosphatidyl moieties attached to the 1'- or 3'-position of the central glycerol is based on the observation that the ions arising from loss of the diacylglycerol moiety containing glycerol B is more abundant than that containing glycerol A. The structures of the above two ions, including the identities of the fatty acyl substituents and the position of fatty acyl substituents on the glycerol backbones (glycerol A and B) are determined by MS3 experiments that give spectra comprising several sets of prominent ions informative for the structural assignment of the fatty acyl substituents on the glycerol A and glycerol B. This method permits the structures of CL in a mixture isolated from Escherichia coli, including species that consist of various isomers, to be unveiled in detail.

LC-MS-based method for the qualitative and quantitative analysis of complex lipid mixtures

A simple and robust LC-MS-based methodology for the investigation of lipid mixtures is described, and its application to the analysis of human lipoprotein-associated lipids is demonstrated. After an optional initial fractionation on Silica 60, normal-phase HPLC-MS on a YMC PVA-Sil column is used first for class separation, followed by reversed-phase LC-MS or LC-tandem mass spectrometry using an Atlantis dC18 capillary column, and/or nanospray MS, to fully characterize the individual lipids. The methodology is applied here for the analysis of human apolipoprotein B-associated lipids. This approach allows for the determination of even low percentages of lipids of each molecular species and showed clear differences between lipids associated with apolipoprotein B-100-LDL isolated from a normal individual and those associated with a truncated version, apolipoprotein B-67-containing lipoproteins, isolated from a homozygote patient with familial hypobetalipoproteinemia. The methods described should be easily adaptable to most modern MS instrumentation.

Phospholipid hydrolysis in a pharmaceutical emulsion assessed by physicochemical parameters and a new analytical method

The aim of this work was to develop a simple high-performance liquid chromatography (HPLC) technique with evaporative light scattering detection (ELSD) for the separation and quantification of the major phospholipid (PL) and lysophospholipid (LPL) classes contained in a pharmaceutical phospholipid-based emulsion. In the established method, phosphatidylcholine (PC), phosphatidylethanolamine (PE), sphingomyeline (SM), lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) were separated with a PVA-Sil stationary phase and a binary gradient from pure chloroform to methanol:water (94:6 v/v) at 3.4%/min. The ELSD detection was enhanced using 0.1% triethylamine and formic acid in each gradient mobile phases. Factors such as stationary phase and ELSD drift tube temperature were optimized, concluding in optimal temperatures of 25 degrees C for separation and 50 degrees C for evaporation. This HPLC-ELSD method was then applied to a PL-emulsion exposed to autoclaving and accelerated thermal conditions at 50 degrees C. Hydrolysis of PC and PE followed first-order kinetics, representing only 45% of the total lipid mass after 3 months. The chemical stability was correlated to commonly measured formulation physical and physico-chemical parameters such as droplet size, emulsion pH and zeta-potential.

Structural characterization of cardiolipin by tandem quadrupole and multiple-stage quadrupole ion-trap mass spectrometry with electrospray ionization

We report negative-ion electrospray tandem mass spectrometric methods for structural characterization of cardiolipin (CL), a four-acyl-chain phospholipid containing two distinct phosphatidyl moieties, of which structural assignment of the fatty acid residues attached to the glycerol backbones performed by low-energy CAD tandem mass spectrometry has not been previously described. The low-energy MS2-spectra of the [M - H]- and [M - 2H]2- ions obtained with ion-trap or with tandem quadrupole instrument combined with ion-trap MS3-spectra or with source CAD product-ion spectra provide complete structural information for CL characterization. The MS2-spectra of the [M - H]- ions contain two sets of prominent fragment ions that comprise a phosphatidic acid, a dehydrated phosphatidylglycerol, and a (phosphatidic acid + 136) anion. The substantial differences in the abundances of the two distinct phosphatidic anions observed in the MS2-spectra of the [M -H]- ions lead to the assignment of the phosphatidyl moieties attached to the 1' or 3' position of central glycerol. Upon further collisional dissociation, the MS3-spectra of the phosphatidic anions provide information to identify the fatty acyl substituents and their position in the glycerol backbone. The MS2-spectra of the [M - 2H]2- ions obtained with TSQ or ITMS contain complementary information to confirm structural assignment. The applications of the above methods in the differentiation of cardiolipin isomers and in the identification of complex cardiolipin species consisting of multiple molecular structures are also demonstrated.

High-throughput quantification of phosphatidylcholine and sphingomyelin by electrospray ionization tandem mass spectrometry coupled with isotope correction algorithm

The choline head group containing phosphatidylcholine (PC) and sphingomyelin (SPM) are major eukaryotic lipid components playing an important role in forming membrane microdomains and serve as precursor of signaling molecules. Both lipids can be monitored by positive ion mode electrospray tandem mass spectrometry using a parent ion scan of m/z 184. Although PC species appear at even m/z and SPM species at odd m/z, there may be a significant overlap of their isotopes. In order to separate PC and SPM species, an isotope correction algorithm was established, which utilizes calculated isotope percentages to correct the measured peak intensities for their isotopic overlap. We could demonstrate that this approach was applicable to correct the isotope overlap resulting from spiked PC and SPM species. Quantification was achieved by addition of different PC and SPM species prior to lipid extraction. The developed assay showed a precision, detection limit and robustness sufficient for routine analysis. Furthermore, an analysis time of only 1.3 min combined with automated data analysis using self-programmed Excel Macros allows high-throughput analysis. In summary, this assay may be a valuable tool for detailed lipid analysis of PC and SPM species in a variety of sample materials.

Blockade of Electron Transport during Ischemia Protects Cardiac Mitochondria

Subsarcolemmal mitochondria sustain progressive damage during myocardial ischemia. Ischemia decreases the content of the mitochondrial phospholipid cardiolipin accompanied by a decrease in cytochrome c content and a diminished rate of oxidation through cytochrome oxidase. We propose that during ischemia mitochondria produce reactive oxygen species at sites in the electron transport chain proximal to cytochrome oxidase that contribute to the ischemic damage. Isolated, perfused rabbit hearts were treated with rotenone, an irreversible inhibitor of complex I in the proximal electron transport chain, immediately before ischemia. Rotenone pretreatment preserved the contents of cardiolipin and cytochrome c measured after 45 min of ischemia. The rate of oxidation through cytochrome oxidase also was improved in rotenone-treated hearts. Inhibition of the electron transport chain during ischemia lessens damage to mitochondria. Rotenone treatment of isolated subsarcolemmal mitochondria decreased the production of reactive oxygen species during the oxidation of complex I substrates. Thus, the limitation of electron flow during ischemia preserves cardiolipin content, cytochrome c content, and the rate of oxidation through cytochrome oxidase. The mitochondrial electron transport chain contributes to ischemic mitochondrial damage that in turn augments myocyte injury during subsequent reperfusion.

Matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry in lipid and phospholipid research

The interest in the analysis of lipids and phospholipids is continuously increasing due to the importance of these molecules in biochemistry (e.g. in the context of biomembranes and lipid second messengers) as well as in industry. Unfortunately, commonly used methods of lipid analysis are often time-consuming and tedious because they include previous separation and/or derivatization steps. With the development of "soft-ionization techniques" like electrospray ionization (ESI) or matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF), mass spectrometry became also applicable to lipid analysis. The aim of this review is to summarize so far available experiences in MALDI-TOF mass spectrometric analysis of lipids. It will be shown that MALDI-TOF MS can be applied to all known lipid classes and the characteristics of individual lipids will be discussed. Additionally, some selected applications in medicine and biology, e.g. mixture analysis, cell and tissue analysis and the determination of enzyme activities will be described. Advantages and disadvantages of MALDI-TOF MS in comparison to other established lipid analysis methods will be also discussed.

Determination of lipid profile in meningococcal polysaccharide using reversed-phase liquid chromatography

A fast and sensitive HPLC method using fluorescence detection is developed to quantitate 1-pyrenyldiazomethane (PDAM) derivatized fatty acids derived from the lipid components of both the capsular meningococcal polysaccharide and other impurities such as endotoxin in various meningococcal vaccine samples. The HPLC method is capable of well resolving 13 relevant fatty acids within 40min by using a multi-stage acetonitrile/water gradient. Endotoxin values measured by HPLC well correlated with results from the standard Limulus amebocyte lysate (LAL) assay. Furthermore, the fatty acid profiles of various process intermediate samples as well as final purified polysaccharide products were determined to better understand and characterize the purification process.

Ischemia, rather than reperfusion, inhibits respiration through cytochrome oxidase in the isolated, perfused rabbit heart: role of cardiolipin

Ischemia and reperfusion result in mitochondrial dysfunction, with decreases in oxidative capacity, loss of cytochrome c, and generation of reactive oxygen species. During ischemia of the isolated perfused rabbit heart, subsarcolemmal mitochondria, located beneath the plasma membrane, sustain a loss of the phospholipid cardiolipin, with decreases in oxidative metabolism through cytochrome oxidase and the loss of cytochrome c. We asked whether additional injury to the distal electron chain involving cardiolipin with loss of cytochrome c and cytochrome oxidase occurs during reperfusion. Reperfusion did not lead to additional damage in the distal electron transport chain. Oxidation through cytochrome oxidase and the content of cytochrome c did not further decrease during reperfusion. Thus injury to cardiolipin, cytochrome c, and cytochrome oxidase occurs during ischemia rather than during reperfusion. The ischemic injury leads to persistent defects in oxidative function during the early reperfusion period. The decrease in cardiolipin content accompanied by persistent decrements in the content of cytochrome c and oxidation through cytochrome oxidase is a potential mechanism of additional myocyte injury during reperfusion.

Ischemia–reperfusion injury in the aged heart: role of mitochondria

The aged heart sustains greater injury during ischemia and reperfusion compared to the adult heart. Aging decreases oxidative phosphorylation and the activity of complexes III and IV only in interfibrillar mitochondria (IFM) that reside among the myofibrils, whereas subsarcolemmal mitochondria (SSM), located beneath the plasma membrane, remain unaltered. The peptide subunit composition of complexes III and IV is intact in aging. The aging defect in complex IV is in the inner membrane lipid environment. The defect in complex III is within the ubiquinol binding site of the cytochrome b subunit. Following ischemia, in the aged heart both SSM and IFM sustain additional decreases in complex III and complex IV activity. In contrast to the aging defect, with ischemia the subunits of complex IV appear to be damaged. Ischemia inactivates the iron-sulfur peptide subunit in complex III. Mitochondria are the major source of the reactive oxygen species that are generated during myocardial ischemia. Complex III is the major site of mitochondrial oxyradical production during ischemia in the adult heart. The role of complex III in the oxidative damage sustained by the aged heart during ischemia, as well as the potential contribution of aging defects in electron transport to ischemic damage in the aged heart, deserves further study. We propose that following ischemic damage to the electron transport chain, the production and release of reactive oxygen species increases from mitochondria in the aged heart, leading to additional damage during reperfusion.

Electrospray mass spectrometry of phospholipids

Phospholipids play a central role in the biochemistry of all living cells. These molecules constitute the lipid bilayer defining the outer confines of a cell, but also serve as the structural entities which confine subcellular components. Mass spectrometry has emerged as a powerful tool useful for the qualitative and quantitative analysis of complex phospholipids, including glycerophospholipids and the sphingolipid, sphingomyelin. Collision induced decomposition of both positive and negative molecular ion species yield rich information as to the polar head group of the phospholipid and the fatty-acyl substituents esterified to the glycerophospholipid backbone. This review presents the current level of understanding of the mechanisms involved in the formation of various product ions following collisional activation of molecular ion species generated by electrospray ionization of the common glycerophospholipids, including phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol, phosphatidylserine, cardiolipin, and sphingomyelin. Recent advances in the application of matrix assisted laser desorption ionization is also considered. Several applications of mass spectrometry applied to phospholipid analysis are presented as they apply to physiology as well as pathophysiology.

High-Throughput Quantification of Lysophosphatidylcholine by Electrospray Ionization Tandem Mass Spectrometry

Background: Lysophosphatidylcholine (LPC) has been suggested to play a functional role in various diseases, including atherosclerosis, diabetes, and cancer mediated by LPC-specific G-protein-coupled receptors. Initial studies provided evidence for a potential use of LPC as diagnostic maker. However, existing methodologies are of limited value for a systematic evaluation of LPC species concentrations because of complicated, time-consuming procedures. We describe a methodology based on electrospray ionization tandem mass spectrometry (ESI-MS/MS) applicable for high-throughput LPC quantification.

Methods: Crude lipid extracts of EDTA-plasma samples were used for direct flow injection analysis. LPC 13:0 and LPC 19:0 were added as internal standards, and the ESI-MS/MS was operated in the parent-scan mode for m/z 184. Quantification was achieved by standard addition. Data processing was highly automated by use of the mass spectrometer software and self-programmed Excel macros.

Results: The calibrators LPC 16:0, LPC 18:0, and LPC 22:0 showed a linear response independent of sample dilution and plasma cholesterol concentration for both internal standards. The within-run imprecision (CV) was 3% for the major and 12% for the minor species, whereas the total imprecision was ∼12% for the major and 25% for the minor species. The detection limit was <1 μmol/L.

Conclusion: The developed ESI-MS/MS methodology with an analysis time of 2 min/sample, simple sample preparation, and automated data analysis allows high-throughput quantification of distinct LPC species from plasma samples, which could be a valuable tool for the evaluation of LPC as diagnostic marker.

Comparative measurements of multicomponent phospholipid mixtures by electrospray mass spectroscopy: relating ion intensity to concentration

Electrospray mass spectrometry allows direct identification and sensitive detection of multiple phospholipids in non-derivatized cell extracts. However, quantitative analyses are not straightforward, and are confounded by analyte and mass discrimination effects, and non-linear dependence of the ion intensity on concentration. This non-linearity is particularly severe in the negative mode and precludes even comparative measurements of anion concentrations. Herein, we report a general method for relating negative electrospray ion intensity to concentration when analyzing multicomponent phospholipid samples. In this method, the intensity of individual ions is measured at several different concentrations of the total mixture and the slope (n(E)) of the double log plot of sample concentration vs. intensity for each analyte is determined. The n(E) is then used to map intensity data to a quantity proportional to concentration for each analyte. The method allows facile and accurate comparison of negative spectra of complex mixtures containing structurally different anions.

Phospholipid composition of cell-derived microparticles determined by one-dimensional high-performance thin-layer chromatography

Microparticles in the circulation activate the coagulation system and may activate the complement system via C-reactive protein upon conversion of membrane phospholipids by phospholipases. We developed a sensitive and reproducible method to determine the phospholipid composition of microparticles. Samples were applied to horizontal, one-dimensional high-performance thin-layer chromatography (HPTLC). Phospholipids were separated on HPTLC by chloroform:ethyl acetate:acetone:isopropanol:ethanol:methanol:water:acetic acid (30:6:6:6:16:28:6:2); visualized by charring with 7.5% Cu-acetate (w/v), 2.5% CuSO(4) (w/v), and 8% H(3)PO(4) (v/v) in water; and quantified by photodensitometric scanning. Erythrocyte membranes were used to validate the HPTLC system. Microparticles were isolated from plasma of healthy individuals (n = 10). On HPTLC, mixtures of (purified) phospholipids, i.e., lysophosphatidylcholine, phosphatidylcholine (PC), sphingomyelin (SM), lysophosphatidylserine, phosphatidylserine, lysophosphatidylethanolamine, phosphatidylethanolamine (PE), and phosphatidylinositol, could be separated and quantified. All phospholipids were detectable in erythrocyte ghosts, and their quantities fell within ranges reported earlier. Quantitation of phospholipids, including extraction, was highly reproducible (CV < 10%). Microparticles contained PC (59%), SM (20.6%), and PE (9.4%), with relatively minor (<5%) quantities of other phospholipids. HPTLC can be used to study the phospholipid composition of cell-derived microparticles and may also be a useful technique for the analysis of other samples that are available only in minor quantities.

Preservation of cardiolipin content during aging in rat heart interfibrillar mitochondria

Aging selectively decreases the rate of oxidative phosphorylation in the interfibrillar population of cardiac mitochondria (IFM) located between the myofibers. In contrast, subsarcolemmal mitochondria (SSM), located below the plasma membrane, remain unaffected. IFM from elderly (24-month-old) Fischer 344 rats have a decreased specific activity of complexes III and IV. Complexes III and IV require an inner mitochondrial membrane lipid environment enriched in the oxidatively sensitive phospholipid cardiolipin for maximal activity. We asked if aging decreases the content or alters the composition of cardiolipin as a potential mechanism of the aging defect in IFM. The content and composition of mitochondrial phospholipids were measured in SSM and IFM from adult and aging rat hearts. Aging did not alter the content of mitochondrial phospholipids, including cardiolipin, in either population of mitochondria. The composition of cardiolipin based on characterization of both acyl group and the individual molecular species of cardiolipin was also unaltered by aging. Lipid-mediated oxidative modification of complex III subunits was not detected, making cardiolipin-derived oxidative damage to complex III unlikely. Thus, alterations in cardiolipin are not the mechanism for the aging defect in IFM in Fischer 344 rats.

Myocardial ischemia selectively depletes cardiolipin in rabbit heart subsarcolemmal mitochondria

Mitochondria contribute to myocyte injury during ischemia. After 30 and 45 min of ischemia in the isolated perfused rabbit heart, subsarcolemmal mitochondria (SSM), located beneath the plasma membrane, sustain a decrease in oxidative phosphorylation through cytochrome oxidase. In contrast, oxidation through cytochrome oxidase in interfibrillar mitochondria (IFM), located between the myofibrils, remains unaffected. Cytochrome oxidase activity in the intact membrane requires an inner mitochondrial membrane lipid environment enriched in cardiolipin. During ischemia, the content of cardiolipin decreased only in SSM, whereas the content of other phospholipids was preserved. Ischemia did not alter the composition of the cardiolipin that remained in SSM. Cardiolipin content was preserved in IFM during ischemia. Thus cardiolipin is a relatively early target of ischemic mitochondrial damage, leading to loss of oxidative phosphorylation through cytochrome oxidase in SSM.