Identification of free radicals of glycerophosphatidylcholines containing ω-6 fatty acids using spin trapping coupled with tandem mass spectrometry

Coupling capillary electrophoresis with mass spectrometry for the analysis of oxidized phospholipids in human high-density lipoproteins

The oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (ox-PAPC) products in human high-density lipoproteins (HDLs) were investigated by low-flow capillary electrophoresis-mass spectrometry (low-flow CE-MS). To accelerate the optimization, native PAPC (n-PAPC) standard was first analyzed by a commercial CE instrument with a photodiode array detector. The optimal separation buffer contained 60% (v/v) acetonitrile, 40% (v/v) methanol, 20 mM ammonium acetate, 0.5% (v/v) formic acid, and 0.1% (v/v) water. The selected separation voltage and capillary temperature were 20 kV and 23°C. The optimal CE separation buffer was then used for the low-flow CE-MS analysis. The selected MS conditions contained heated capillary temperature (250°C), capillary voltage (10 V), and injection time (1 s). No sheath gas was used for MS. The linear range for n-PAPC was 2.5-100.0 µg/mL. The coefficient of determination (R2 ) was 0.9918. The concentration limit of detection was 1.52 µg/mL, and the concentration limit of quantitation was 4.60 µg/mL. The optimal low-flow CE-MS method showed good repeatability and sensitivity. The ox-PAPC products in human HDLs were determined based on the in vitro ox-PAPC products of n-PAPC standard. Twenty-one ox-PAPC products have been analyzed in human HDLs. Uremic patients showed significantly higher levels of 15 ox-PAPC products than healthy subjects.

Determination of oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine products in human very low-density lipoproteins by nonaqueous low-flow capillary electrophoresis-mass spectrometry

A simple and fast low-flow capillary electrophoresis-mass spectrometry (low-flow CE-MS) method has been developed to analyze oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (ox-PAPC) products in human very low-density lipoproteins (VLDLs). Native PAPC standard was analyzed to optimize the low-flow CE-MS method. The optimal CE conditions included separation buffer (60% (v/v) acetonitrile, 40% (v/v) methanol, 0.1% (v/v) water, 0.5% (v/v) formic acid, 20 mM ammonium acetate), sheath liquid (60% (v/v) acetonitrile, 40% (v/v) methanol, 0.1% (v/v) water, 20 mM ammonium acetate), separation voltage (20 kV), separation capillary internal diameter (i.d.) (75 µm), separation capillary temperature (23˚C) and sample injection time (6 s). The selected MS conditions included heated capillary temperature (250°C), capillary voltage (10 V), and injection time (1 s). Sheath gas was not used in this study. The total ion chromatograms (TICs), extracted ion chromatograms (EICs) and MS spectra of native PAPC standard and its in vitro oxidation products showed good repeatability and sensitivity. To determine the ox-PAPC products in human VLDLs, the EICs and MS spectra of VLDLs were compared with the in vitro oxidation products of native PAPC standard. For native PAPC standard, the measured linear range was 2.5 - 100.0 µg/mL, and the coefficients of determination (R²) was 0.9994. The concentration limit of detection (LOD) was 0.44 µg/mL, and the concentration limit of quantitation (LOQ) was 1.34 µg/mL. A total of 21 ox-PAPC products were analyzed for the VLDLs of healthy and uremic subjects. The levels of 7 short-chain and 5 long-chain ox-PAPC products on uremic VLDLs were significantly higher than healthy VLDLs. This simple low-flow CE-MS method might be a good alternative for LC-MS for the analysis of ox-PAPC products. Furthermore, it might also help scientists to expedite the search for uremic biomarkers.

Analysis of Oxidized 1-Palmitoyl-2-Arachidonoyl-Sn-Glycero-3 Phosphocholine Products in Uremic Patients by LC-ESI/MS

A simple liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI/MS) method has been developed to analyze oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (ox-PAPC) products on the lipoproteins of uremic patients. The native PAPC standard was in vitro oxidized by the Fenton reaction, and the ox-PAPC products were analyzed by LC- ESI/MS. For LC, a C8 column and a mobile phase (acetonitrile-isopropanol containing 0.1% formic acid (70:30, v/v)) were selected. For ESI/MS, the optimal conditions included sheath gas pressure (10 psi), capillary temperature (270 °C), and injection time (1000 ms). The identification of ox-PAPC products on human lipoproteins was based on the extracted ion chromatograms (EICs) and the ESI-MS spectra of the in vitro oxidation products of PAPC standard. The EICs and ESI-MS spectra showed good repeatability and sensitivity. A total of 21 ox-PAPC products was determined. Linear analysis has been performed for the phospholipid standard, 1, 2-Di-O-hexadecyl-sn-glycero-3-phosphocholine (PC(O-16:0/O-16:0)). The linear range was 5.0–100.0 µg/mL, and the coefficient of determination (R2) was 0.989. The concentration limit of detection (LOD) was 1.50 µg/mL, and the concentration limit of quantitation (LOQ) was 4.54 µg/mL. The selected 21 ox-PAPC products have been identified and quantified in very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) of uremic and healthy subjects. Interestingly, the results showed that the levels of 18 products in VLDL, one product in LDL, and 19 products in HDL were significantly higher for uremic patients than healthy controls. This simple LC-ESI/MS method might accelerate the searching for biomarkers of uremia in the future.

Detection and structural analysis of lipid-derived radicals in vitro and in vivo

Lipids can be oxidized by reactive oxygen species, resulting in lipid peroxidation and the formation of reactive metabolites such as lipid-derived electrophiles. These products have been reported to induce inflammation, angiogenesis, and ferroptosis. Lipid peroxidation can produce many different products, each of which performs a different function, and which can be challenging to detect in vivo. The initial products of lipid oxidation are lipid-derived radicals, which can cause extensive chain reactions leading to lipid peroxidation. Hence, the ability to detect lipid radicals may provide information about this important class of molecules and the mechanism by which they cause cellular and tissue damage in a wide range of oxidative conditions. In this review, we report recent scientific advances in the detection of lipid-derived radicals in vitro and in cultured cells. We also introduce the possibility of visualization and structural analysis of lipid-derived radicals generated not only in in cells but also in animal tissue samples from oxidative disease models, using fluorescence-based lipid radicals' detection probes. We anticipate that the various innovative techniques summarized in this paper will be applied and further developed to clarify the role of lipid peroxidation in the pathogenesis of oxidative stress-associated diseases.

Derivatization and detection of small aliphatic and lipid-bound carbonylated lipid peroxidation products by ESI-MS

Double bonds in polyunsaturated fatty acids (PUFA) and lipids are one of the major targets of reactive oxygen species (ROS). The resulting lipid peroxidation products (LPP) represent a group of chemically diverse compounds formed by several consecutive oxidative reactions. Oxidative cleavage leads to the formation of small aliphatic and lipid-bound aldehydes and ketones (oxoLPPs). These strong electrophiles can readily react with nucleophilic substrates, for example, side chains in proteins which can alter structure, function, and cellular distribution of the modified proteins. Despite growing interest in the field of oxidative lipidomics, only a few dominantly formed oxoLPP were identified. Due to the chemical and physical properties, aliphatic oxoLPPs are usually analyzed using gas chromatography-mass spectrometry (GC- MS), while nonvolatile lipid-bound oxoLPPs require liquid chromatography-mass spectrometry (LC-MS). To overcome the need for the two analyses, we have developed a new derivatization strategy to capture all oxoLPP independent to their properties with electrospray ionization (ESI) MS allowing simultaneous detection of aliphatic and lipid-bound oxoLPPs. Thus, the 7-(diethylamino)coumarin-3-carbohydrazide (CHH) derivatization reagent allowed us to identify 122 carbonyl compounds in a mixture of four PUFA and phosphatidylcholines (PC) oxidized in vitro.

In vivo detection of free radicals using molecular MRI and immuno-spin trapping in a mouse model for amyotrophic lateral sclerosis

Free radicals associated with oxidative stress play a major role in amyotrophic lateral sclerosis (ALS). By combining immuno-spin trapping and molecular magnetic resonance imaging, in vivo trapped radical adducts were detected in the spinal cords of SOD1(G93A)-transgenic (Tg) mice, a model for ALS. For this study, the nitrone spin trap DMPO (5,5-dimethyl-1-pyrroline N-oxide) was administered (ip) over 5 days before administration (iv) of an anti-DMPO probe (anti-DMPO antibody covalently bound to an albumin-gadolinium-diethylenetriamine pentaacetic acid-biotin MRI contrast agent) to trap free radicals. MRI was used to detect the presence of the anti-DMPO radical adducts by a significant sustained increase in MR signal intensities (p < 0.05) or anti-DMPO probe concentrations measured from T₁ relaxations (p < 0.01). The biotin moiety of the anti-DMPO probe was targeted with fluorescence-labeled streptavidin to locate the probe in excised tissues. Negative controls included either Tg ALS mice initially administered saline rather than DMPO followed by the anti-DMPO probe or non-Tg mice initially administered DMPO and then the anti-DMPO probe. The anti-DMPO probe was found to bind to neurons via colocalization fluorescence microscopy. DMPO adducts were also confirmed in diseased/nondiseased tissues from animals administered DMPO. Apparent diffusion coefficients from diffusion-weighted images of spinal cords from Tg mice were significantly elevated (p < 0.001) compared to wild-type controls. This is the first report regarding the detection of in vivo trapped radical adducts in an ALS model. This novel, noninvasive, in vivo diagnostic method can be applied to investigate the involvement of free radical mechanisms in ALS rodent models.

Potential implication of the chemical properties and bioactivity of nitrone spin traps for therapeutics

Nitrone therapeutics has been employed in the treatment of oxidative stress-related diseases such as neurodegeneration, cardiovascular disease and cancer. The nitrone-based compound NXY-059, which is the first drug to reach clinical trials for the treatment of acute ischemic stroke, has provided promise for the development of more robust pharmacological agents. However, the specific mechanism of nitrone bioactivity remains unclear. In this review, we present a variety of nitrone chemistry and biological activity that could be implicated for the nitrone's pharmacological activity. The chemistries of spin trapping and spin adduct reveal insights on the possible roles of nitrones for altering cellular redox status through radical scavenging or nitric oxide donation, and their biological effects are presented. An interdisciplinary approach towards the development of novel synthetic antioxidants with improved pharmacological properties encompassing theoretical, synthetic, biochemical and in vitro/in vivo studies is covered.

In vivo imaging of immuno-spin trapped radicals with molecular magnetic resonance imaging in a diabetic mouse model

Oxidative stress plays a major role in diabetes. In vivo levels of membrane-bound radicals (MBRs) in a streptozotocin-induced diabetic mouse model were uniquely detected by combining molecular magnetic resonance imaging (mMRI) and immunotrapping techniques. An anti-DMPO (5,5-dimethyl-1-pyrroline N-oxide) antibody (Ab) covalently bound to an albumin (BSA)-Gd (gadolinium)-DTPA (diethylene triamine penta acetic acid)-biotin MRI contrast agent (anti-DMPO probe), and mMRI, were used to detect in vivo levels of DMPO-MBR adducts in kidneys, livers, and lungs of diabetic mice, after DMPO administration. Magnetic resonance signal intensities, which increase in the presence of a Gd-based molecular probe, were significantly higher within the livers, kidneys, and lungs of diabetic animals administered the anti-DMPO probe compared with controls. Fluorescence images validated the location of the anti-DMPO probe in excised tissues via conjugation of streptavidin-Cy3, which targeted the probe biotin moiety, and immunohistochemistry was used to validate the presence of DMPO adducts in diabetic mouse livers. This is the first report of noninvasively imaging in vivo levels of MBRs within any disease model. This method can be specifically applied toward diabetes models for in vivo assessment of free radical levels, providing an avenue to more fully understand the role of free radicals in diabetes.

Identification of free radicals in oxidized and glycoxidized phosphatidylethanolamines by spin trapping combined with tandem mass spectrometry

RATIONALE

Nonenzymatic glycation of phosphatidylethanolamines (PEs) seems to a have a role in angiogenesis and atherosclerosis. Glycated PEs are more easily oxidized, enhancing oxidative stress. This study aims to evaluate the influence of glycation on the formation of intermediate radical species during oxidation of glycated PEs.

METHODS

In the present study, the radical intermediaries formed during the oxidation of palmitoyl-lineoyl phosphatidylethanolamine (PLPE) and glycated PLPE (gPLPE) were trapped using a spin trap (DMPO) and the radical adducts were analyzed by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS). Mass spectra were acquired using a electrospray Q-TOF 2 mass spectrometer.

RESULTS

Several spin adducts of PLPE and gPLPE were identified, corresponding to carbon- and oxygen-centered radicals. Interpretation of the MS/MS spectra showed the existence of different sites where radical formation occurred, at the sn-2 acyl chain, ethanolamine moiety (particularly in C-1) and, in the case of glycated derivatives, also in the glucose moiety (particularly in C-3, C-4 and C-5).

CONCLUSIONS

These results suggested the presence of more sites susceptible to oxidation in glycated PLPE, which may be responsible for the increase in the oxidative reaction rate occurring in glycated compounds.

Chemistry of phospholipid oxidation

The oxidation of lipids has long been a topic of interest in biological and food sciences, and the fundamental principles of non-enzymatic free radical attack on phospholipids are well established, although questions about detail of the mechanisms remain. The number of end products that are formed following the initiation of phospholipid peroxidation is large, and is continually growing as new structures of oxidized phospholipids are elucidated. Common products are phospholipids with esterified isoprostane-like structures and chain-shortened products containing hydroxy, carbonyl or carboxylic acid groups; the carbonyl-containing compounds are reactive and readily form adducts with proteins and other biomolecules. Phospholipids can also be attacked by reactive nitrogen and chlorine species, further expanding the range of products to nitrated and chlorinated phospholipids. Key to understanding the mechanisms of oxidation is the development of advanced and sensitive technologies that enable structural elucidation. Tandem mass spectrometry has proved invaluable in this respect and is generally the method of choice for structural work. A number of studies have investigated whether individual oxidized phospholipid products occur in vivo, and mass spectrometry techniques have been instrumental in detecting a variety of oxidation products in biological samples such as atherosclerotic plaque material, brain tissue, intestinal tissue and plasma, although relatively few have achieved an absolute quantitative analysis. The levels of oxidized phospholipids in vivo is a critical question, as there is now substantial evidence that many of these compounds are bioactive and could contribute to pathology. The challenges for the future will be to adopt lipidomic approaches to map the profile of oxidized phospholipid formation in different biological conditions, and relate this to their effects in vivo. This article is part of a Special Issue entitled: Oxidized phospholipids-their properties and interactions with proteins.

Characterization of free radical spin adducts of the DIPPMPO using mass spectrometry and (31)P NMR

5-Diisopropyloxy-phosphoryl-5-methyl-1-pyrroline-N-oxide (DIPPMPO) was used to trap a variety of free radicals and the stable compounds generated by the natural decomposition of the initially formed spin adducts were characterized by (31)P nuclear magnetic resonance (NMR) and mass spectrometry. Initially, the starting spin trap DIPPMPO was completely characterized using GC-MS and its fragmentation pathway was studied in detail. Then, DIPPMPO was used to trap an oxygen-centered free radical (the hydroxyl radical *OH) and two carbon- centered free radicals (methyl *CH(3) and 1-phenyl-ethanol-1-yl *CCH(3)(OH)Ph radicals). The (31)P NMR signals were thus assigned and the structures of adducts were studied and confirmed by mass spectrometry. Overall, the fragmentation pathways of the radical adducts proceed mainly via the loss of the diisopropyloxy(oxido)phosphoranyl radical. For the specific case of trapping *OH radicals, it is possible to visualize the rearrangement of the nitroxide radical adduct to its nitrone form as invoked in the literature. This spin trapping technique, coupled with (31)P NMR and MS, provides a tool for the identification of short-lived and low molecular weight free radicals present in a variety of processes.

Identification of free radicals by spin trapping with DEPMPO and MCPIO using tandem mass spectrometry

This study evaluates the use of a pyrroline (DEPMPO) and an imidazole (MCPIO) spin trap for the detection of hydroxyl and biomolecule (a peptide and a phospholipid) free radical adducts by Electrospray Ionization Mass Spectrometry (ESI-MS). The hydroxyl and biomolecule free radical adducts were detected using a QTOF and a linear ion trap (LIT) mass spectrometers. In the presence of hydroxyl radical, the mass spectrum obtained for each of the spin traps, DEPMPO and MCPIO, showed the presence of ions that could be attributed to hydroxyl and peroxyl radicals. Further characterisation by tandem mass spectrometry (ESI-MS/MS) revealed also the presence of hydroxy-hydroxyl adducts. Based on the results here described, we show that DEPMPO is a better spin trap for free radicals trapping and detection by mass spectrometry mainly because adducts show increased signal intensity. The ESI-MS spectra obtained for DEPMPO and MCPIO in the presence of biomolecule radicals (peptide and phospholipid) show molecular ions of DEPMPO and MCPIO adducts, which were characterised by tandem mass spectrometry. Both carbon centered radicals and oxygen centered radicals were efficiently trapped by the two spin traps and analysis of QTOF-MS/MS mass spectra allowed the location of the radical position in either the peptide or in the phospholipid fatty acyl chain. However, the tandem mass spectra of MCPIO adducts were more informative than DEPMPO adducts. The LIT-MS/MS spectra only shows typical peptide and phospholipid fragmentation, which difficult the structural characterisation of the spin adduct. In this study, the DEPMPO and MCPIO adducts were identified either in the nitrone or in the hydroxylamine form, which are ESR silent forms. The results described here show that both spin traps coupled with detection by mass spectrometry are valuable tools for trapping radicals of biomolecules. Furthermore, the acquired data provide valuable information on the presence of adducts (hydroxyl and biomolecule) that are Electron Spin Resonance (ESR) silent. This is especially important considering the complexity of the radical species in biological environment and the presence of reducing compounds that convert the spin adducts to silent ESR forms.