Peroxide bond driven dissociation of hydroperoxy-cholesterol esters following collision induced dissociation

Analysis of artificially oxidized cardiolipins and monolyso-cardiolipins via liquid chromatography/high-resolution mass spectrometry and Kendrick mass defect plots after hydrophilic interaction liquid chromatography based sample preparation

RATIONALE

Cardiolipins (CL) are a special lipid class which plays a main role in energy metabolism in mitochondria and is involved in apoptosis. In contrast to other glycerophospholipids, they contain four fatty acyl residues which results in a high structural diversity. Oxidation, for example by reactive oxygen species, or lyso forms such as monolyso-CL (MLCL), increases this diversity. Mass spectrometric analysis and computational identification of CL, MLCL and their oxidation products is therefore a challenging task.

METHODS

In order to distinguish CL, MLCL and their oxidation products, a liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed. A hydrophilic interaction liquid chromatography (HILIC)-based solid-phase extraction (SPE) clean-up approach was developed for CL enrichment. Graphical analysis of CL, MLCL and their oxidation products was carried out by a three-dimensional Kendrick mass defect (3D-KMD) plot module, as well as a refined lipid search module of the open-source metabolomics data mining software MZmine 2.

RESULTS

The HILIC-based SPE clean-up enabled complete separation of polar and nonpolar lipid classes. A yeast (Saccharomyces cerevisiae) lipid extract, which was artificially oxidized by means of the Fenton reaction, was analyzed by the developed LC/MS/MS method. CL species with differences in chain length and degree of unsaturation have been separated by high-performance liquid chromatography (HPLC). In total 66 CL, MLCL and oxidized species have been identified utilizing 3D-KMD plots in combination with database matching using MZmine 2. For further characterization of annotated species, MS/MS experiments have been utilized.

CONCLUSIONS

3D-KMD plots capturing chromatographic and high-resolution mass spectrometry data have been successfully used for graphical identification of CL, MLCL as well as their oxidized species. Therefore, we chose multiple KMD bases such as hydrogen and oxygen to visualize the degree of unsaturation and oxidation capturing chromatographic data by means of a color-coded paint scale as the third dimension. In combination with database matching, the analysis of low concentrated lipid species in complex samples has been significantly improved.

Overview of Lipid Mass Spectrometry and Lipidomics

Mass spectrometry has played a critical role in the identification and quantitation of lipids present in biological extracts. Various strategies have emerged in order to carry out lipidomic studies. These include both shotgun approaches as well as those engaging liquid chromatographic separation of lipid species prior to mass spectrometric analysis. Nonetheless challenges remain at every level of the lipidomic experiment, including extraction of lipids, identification of specific species, and quantitation of the vast array of lipids present in the sample extract. New strategies have emerged to address some of these issues; however, precise quantitation remains a significant challenge. The use of the ratio of the abundance of the molecular ion species to that of an internal standard enables quite accurate assessment of fold changes within complex lipid species without the need for exact quantitation. Challenges continue to remain in terms of availability of reference standard material as well as relevant internal standards.

Detection and characterization of cholesteryl ester hydroperoxides in oxidized LDL and oxidized HDL by use of an Orbitrap mass spectrometer

Oxidation of cholesteryl esters in lipoproteins by reactive oxygen species yields cholesteryl ester hydroperoxides (CEOOH). In this study, we developed a novel method for identification and characterization of CEOOH molecules in human lipoproteins by use of reversed-phase liquid chromatography with an hybrid linear ion trap-Orbitrap mass spectrometer (LC-LTQ Orbitrap). Electrospray ionization tandem mass spectrometric analysis was performed in both positive-ion and negative-ion modes. Identification of CEOOH molecules was completed by use of high-mass-accuracy (MA) mass spectrometric data obtained by using the spectrometer in Fourier-transform (FT) mode. Native low-density lipoproteins (nLDL) and native high-density lipoproteins (nHDL) from a healthy donor were oxidized by CuSO(4), furnishing oxidized LDL (oxLDL) and oxidized HDL (oxHDL). No CEOOH molecules were detected in the nLDL and the nHDL, whereas six CEOOH molecules were detected in the oxLDL and the oxHDL. In positive-ion mode, CEOOH was detected as [M + NH(4)](+) and [M + Na](+) ions. In negative-ion mode, CEOOH was detected as [M + CH(3)COO](-) ions. CEOOH were more easily ionized in positive-ion mode than in negative-ion mode. The LC-LTQ Orbitrap method was applied to human plasma and six species of CEOOH were detected. The limit of detection was 0.1 pmol (S/N = 5:1) for synthesized CEOOH.

Electrospray MS/MS reveals extensive and nonspecific oxidation of cholesterol esters in human peripheral vascular lesions

Although LDL is rendered proatherogenic by various experimental treatments (e.g., acetylation), the exact structural changes that drive LDL transformation in vivo remain enigmatic. Among the many hypothesized targets of oxidative modification are cholesterol esters (CE). This family of neutral lipids, which carries a highly unsaturated pool of fatty acyl groups, is the main component of both LDL particles and atherosclerotic plaques. Tandem mass spectrometry (MS/MS) was employed to reveal abundant and diverse oxidized CEs (oxCE), including novel oxidation products, within human peripheral vascular lesions. These oxCE species composed up to 40% of the total CE pool, with cholesteryl linoleate being oxidized to the greatest extent. Imaging mass spectrometry studies showed that oxCE was entirely confined within the plaque, along with unmodified CE and triacylglyceride (TAG). Interestingly, we found no evidence for TAG oxidation, although polyunsaturated species were abundant. Enzymatic oxidation of cholesteryl linoleate by 15-lipoxygenase (15-LO), an enzyme often invoked in CE oxidation, initially results in a regio- and stereospecific product. Analysis of intact cholesteryl hydroxyoctadecadienoate isomers in human atheromata revealed no regio- or stereospecificity, indicating 15-LO was either not a major source of oxCE or nonenzymatic processes had eroded any product specificity.

Glycerolipid and cholesterol ester analyses in biological samples by mass spectrometry

Neutral lipids are a diverse family of hydrophobic biomolecules that have important roles in cellular biochemistry of all living species but have in common the property of charge neutrality. A large component of neutral lipids is the glycerolipids composed of triacylglycerols, diacylglycerols, and monoacylglycerols that can serve as cellular energy stores as well as signaling molecules. Another abundant lipid class in many cells is the cholesterol esters that are on one hand sterols and the other fatty acyl lipids, but in either case are neutral lipids involved in cholesterol homeostasis and transport in the blood. The analysis of these molecules in the context of lipidomics remains challenging because of their charge neutrality and the complex mixtures of molecular species present in cells. Various techniques have been used to ionize these neutral lipids prior to mass spectrometric analysis including electron ionization, atmospheric chemical ionization, electrospray ionization and matrix assisted laser desorption/ionization. Various approaches to deal with the complex mixture of molecular species have been developed including shotgun lipidomics and chromatographic-based separations such as gas chromatography, reversed phase liquid chromatography, and normal phase liquid chromatography. Several applications of these approaches are discussed. .