Identification of 4-hydroxy-2-nonenal-modified peptides within unfractionated digests using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Low Levels of IgM Recognizing 4-Hydroxy-2-Nonenal-Modified Apolipoprotein A-I Peptide and Its Association with the Severity of Coronary Artery Disease in Taiwanese Patients

Autoantibodies against apolipoprotein A-I (ApoA-I) are associated with cardiovascular disease risks. We aimed to examine the 4-hydroxy-2-nonenal (HNE) modification of ApoA-I in coronary artery disease (CAD) and evaluate the potential risk of autoantibodies against their unmodified and HNE-modified peptides. We assessed plasma levels of ApoA-I, HNE-protein adducts, and autoantibodies against unmodified and HNE-peptide adducts, and significant correlations and odds ratios (ORs) were examined. Two novel CAD-specific HNE-peptide adducts, ApoA-I251-262 and ApoA-I70-83, were identified. Notably, immunoglobulin G (IgG) anti-ApoA-I251-262 HNE, IgM anti-ApoA-I70-83 HNE, IgG anti-ApoA-I251-262, IgG anti-ApoA-I70-83, and HNE-protein adducts were significantly correlated with triglycerides, creatinine, or high-density lipoprotein in CAD with various degrees of stenosis (<30% or >70%). The HNE-protein adduct (OR = 2.208-fold, p = 0.020) and IgM anti-ApoA-I251-262 HNE (2.046-fold, p = 0.035) showed an increased risk of progression from >30% stenosis in CAD. HNE-protein adducts and IgM anti-ApoA-I251-262 HNE may increase the severity of CAD at high and low levels, respectively.

Reactive Matrices for Analytical Matrix-Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry

In recent years, a special focus is placed on the usage of reactive matrices for analytical matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS). Since 2003, when the term "reactive matrices" was suggested and the dignity of compounds, possessing dualistic properties as matrices and derivatization agents was demonstrated, corresponding approach has found application in various fields and, in particular, in bioanalysis (metabolomics, lipidomics, etc.). The main advantage of this methodology is that it reduces sample treatment time, simplifies the procedure of sample handling, improves the sensitivity of analysis, enhances the molecular identification and profiling. Within the framework of this review, the main attention is paid to "true" reactive matrices that interact with analyte molecules through an exchange or addition reactions. A special section discusses practical application of reactive matrices in the determination of the distribution of targeted and non-targeted organic substances on the surface of biological tissue sections by MALDI-MS imaging. In this critical review, a controversial proposal is made to consider protonating and deprotonating matrices as reactive, because they can undergo a chemical reaction such as proton transfer that occurs in both target solution and MALDI plume. In this respect, special attention is paid to "proton sponge" matrices that have found a wide application in the analysis of various acidic compounds by MALDI-MS in the negative mode. Historical data on the formation of ions and the fate of matrices in MALDI are considered at the beginning of this article.

MALDI Matrices for the Analysis of Low Molecular Weight Compounds: Rational Design, Challenges and Perspectives

The analysis of low molecular weight (LMW) compounds is of great interest to detect small pharmaceutical drugs rapidly and sensitively, or to trace and understand metabolic pathways. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) plays a central role in the analysis of high molecular weight (bio)molecules. However, its application for LMW compounds is restricted by spectral interferences in the low m/z region, which are produced by conventional organic matrices. Several strategies regarding sample preparation have been investigated to overcome this problem. A different rationale is centred on developing new matrices which not only meet the fundamental requirements of good absorption and high ionization efficiency, but are also vacuum stable and "MALDI silent", i. e., do not give matrix-related signals in the LMW area. This review gives an overview on the rational design strategies used to develop matrix systems for the analysis of LMW compounds, focusing on (i) the modification of well-known matrices, (ii) the search for high molecular weight matrices, (iii) the development of binary, hybrid and nanomaterial-based matrices, (iv) the advance of reactive matrices and (v) the progress made regarding matrices for negative or dual polarity mode.

Isotypes of autoantibodies against novel differential 4-hydroxy-2-nonenal-modified peptide adducts in serum is associated with rheumatoid arthritis in Taiwanese women

Background

Rheumatoid arthritis (RA) is an autoimmune disorder with systemic inflammation and may be induced by oxidative stress that affects an inflamed joint. Our objectives were to examine isotypes of autoantibodies against 4-hydroxy-2-nonenal (HNE) modifications in RA and associate them with increased levels of autoantibodies in RA patients.

Methods

Serum samples from 155 female patients [60 with RA, 35 with osteoarthritis (OA), and 60 healthy controls (HCs)] were obtained. Four novel differential HNE-modified peptide adducts, complement factor H (CFAH)^1211–1230, haptoglobin (HPT)^78–108, immunoglobulin (Ig) kappa chain C region (IGKC)^2–19, and prothrombin (THRB)^328–345, were re-analyzed using tandem mass spectrometric (MS/MS) spectra (ProteomeXchange: PXD004546) from RA patients vs. HCs. Further, we determined serum protein levels of CFAH, HPT, IGKC and THRB, HNE-protein adducts, and autoantibodies against unmodified and HNE-modified peptides. Significant correlations and odds ratios (ORs) were calculated.

Results

Levels of HPT in RA patients were greatly higher than the levels in HCs. Levels of HNE-protein adducts and autoantibodies in RA patients were significantly greater than those of HCs. IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE, IgM anti-IGKC²⁻¹⁹, and IgM anti-IGKC²⁻¹⁹ HNE may be considered as diagnostic biomarkers for RA. Importantly, elevated levels of IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE, IgM anti-IGKC²⁻¹⁹, and IgG anti-THRB^328−345 were positively correlated with the disease activity score in 28 joints for C-reactive protein (DAS28-CRP). Further, the ORs of RA development through IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE (OR 5.235, p < 0.001), IgM anti-IGKC²⁻¹⁹ (OR 12.655, p < 0.001), and IgG anti-THRB^328−345 (OR 5.761, p < 0.001) showed an increased risk. Lastly, we incorporated three machine learning models to differentiate RA from HC and OA, and performed feature selection to determine discriminative features. Experimental results showed that our proposed method achieved an area under the receiver operating characteristic curve of 0.92, which demonstrated that our selected autoantibodies combined with machine learning can efficiently detect RA.

Conclusions

This study discovered that some IgG- and IgM-NAAs and anti-HNE M-NAAs may be correlated with inflammation and disease activity in RA. Moreover, our findings suggested that IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE, IgM anti-IGKC²⁻¹⁹, and IgG anti-THRB^328−345 may play heavy roles in RA development.

Separation and characterization of oxidized isomeric lipid-peptide adducts by ion mobility mass spectrometry

Phospholipids are major components of cell membranes and lipoprotein complexes. They are prone to oxidation by endogenous and exogenous reactive oxygen species yielding a large variety of modified lipids including small aliphatic and phospholipid bound aldehydes and ketones. These carbonyls are strong electrophiles that can modify proteins and, thereby, alter their structures and functions triggering various pathophysiological conditions. The analysis of lipid-protein adducts by liquid chromatography-MS is challenged by their mixed chemical nature (polar peptide and hydrophobic lipid), low abundance in biological samples, and formation of multiple isomers. Thus, we investigated traveling wave ion mobility mass spectrometry (TWIMS) to analyze lipid-peptide adducts generated by incubating model peptides corresponding to the amphipathic β1 sheet sequence of apolipoprotein B-100 with 1-palmitoyl-2-(oxo-nonanoyl)-sn-glycerophosphatidylcholine (PONPC). The complex mixture of peptides, lipids, and peptide-lipid adducts was separated by TWIMS, which was especially important for the identification of two mono-PONPC-peptide isomers containing Schiff bases at different lysine residues. Moreover, TWIMS separated structural conformers of one peptide-lipid adduct possessing most likely different orientations of the hydrophobic sn-1 fatty acyl residue and head group of PONPC, relative to the peptide backbone.

Gel-free proteomic methodologies to study reversible cysteine oxidation and irreversible protein carbonyl formation

Oxidative modifications in proteins have been traditionally considered as hallmarks of damage by oxidative stress and aging. However, oxidants can generate a huge variety of reversible and irreversible modifications in amino acid side chains as well as in the protein backbones, and these post-translational modifications can contribute to the activation of signal transduction pathways, and also mediate the toxicity of oxidants. Among the reversible modifications, the most relevant ones are those arising from cysteine oxidation. Thus, formation of sulfenic acid or disulfide bonds is known to occur in many enzymes as part of their catalytic cycles, and it also participates in the activation of signaling cascades. Furthermore, these reversible modifications have been usually attributed with a protective role, since they may prevent the formation of irreversible damage by scavenging reactive oxygen species. Among irreversible modifications, protein carbonyl formation has been linked to damage and death, since it cannot be repaired and can lead to protein loss-of-function and to the formation of protein aggregates. This review is aimed at researchers interested on the biological consequences of oxidative stress, both at the level of signaling and toxicity. Here we are providing a concise overview on current mass-spectrometry-based methodologies to detect reversible cysteine oxidation and irreversible protein carbonyl formation in proteomes. We do not pretend to impose any of the different methodologies, but rather to provide an objective catwalk on published gel-free approaches to detect those two types of modifications, from a biologist's point of view.

Analysis of protein carbonylation--pitfalls and promise in commonly used methods

Oxidation of proteins has received a lot of attention in the last decades due to the fact that they have been shown to accumulate and to be implicated in the progression and the pathophysiology of several diseases such as Alzheimer, coronary heart diseases, etc. This has also resulted in the fact that research scientists are becoming more eager to be able to measure accurately the level of oxidized protein in biological materials, and to determine the precise site of the oxidative attack on the protein, in order to get insights into the molecular mechanisms involved in the progression of diseases. Several methods for measuring protein carbonylation have been implemented in different laboratories around the world. However, to date no methods prevail as the most accurate, reliable, and robust. The present paper aims at giving an overview of the common methods used to determine protein carbonylation in biological material as well as to highlight the limitations and the potential. The ultimate goal is to give quick tips for a rapid decision making when a method has to be selected and taking into consideration the advantage and drawback of the methods.

Protein carbonylation as a major hallmark of oxidative damage: update of analytical strategies

Protein carbonylation, one of the most harmful irreversible oxidative protein modifications, is considered as a major hallmark of oxidative stress-related disorders. Protein carbonyl measurements are often performed to assess the extent of oxidative stress in the context of cellular damage, aging and several age-related disorders. A wide variety of analytical techniques are available to detect and quantify protein-bound carbonyls generated by metal-catalyzed oxidation, lipid peroxidation or glycation/glycoxidation. Here we review current analytical approaches for protein carbonyl detection with a special focus on mass spectrometry-based techniques. The utility of several carbonyl-derivatization reagents, enrichment protocols and especially advanced mass spectrometry techniques are compared and discussed in detail. Furthermore, the mechanisms and biology of protein carbonylation are summarized based on recent high-throughput proteomics data.

Lipoxidation adducts with peptides and proteins: deleterious modifications or signaling mechanisms?

Protein lipoxidation refers to the modification by electrophilic lipid oxidation products to form covalent adducts, which for many years has been considered as a deleterious consequence of oxidative stress. Oxidized lipids or phospholipids containing carbonyl moieties react readily with lysine to form Schiff bases; alternatively, oxidation products containing α,β-unsaturated moieties are susceptible to nucleophilic attack by cysteine, histidine or lysine residues to yield Michael adducts, overall corresponding to a large number of possible protein adducts. The most common detection methods for lipoxidized proteins take advantage of the presence of reactive carbonyl groups to add labels, or use antibodies. These methods have limitations in terms of specificity and identification of the modification site. The latter question is satisfactorily addressed by mass spectrometry, which enables the characterization of the adduct structure. This has allowed the identification of lipoxidized proteins in physiological and pathological situations. While in many cases lipoxidation interferes with protein function, causing inhibition of enzymatic activity and increased immunogenicity, there are a small number of cases where lipoxidation results in gain of function or activity. For certain proteins lipoxidation may represent a form of redox signaling, although more work is required to confirm the physiological relevance and mechanisms of such processes. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Possible evidence of amide bond formation between sinapinic acid and lysine-containing bacterial proteins by matrix-assisted laser desorption/ionization (MALDI) at 355 nm

We previously reported the apparent formation of matrix adducts of 3,5-dimethoxy-4-hydroxy-cinnamic acid (sinapinic acid or SA) via covalent attachment to disulfide bond-containing proteins (HdeA, Hde, and YbgS) from bacterial cell lysates ionized by matrix-assisted laser desorption/ionization (MALDI) time-of-flight-time-of-flight tandem mass spectrometry (TOF-TOF-MS/MS) and post-source decay (PSD). We also reported the absence of adduct formation when using α-cyano-4-hydroxycinnamic acid (CHCA) matrix. Further mass spectrometric analysis of disulfide-intact and disulfide-reduced over-expressed HdeA and HdeB proteins from lysates of gene-inserted E. coli plasmids suggests covalent attachment of SA occurs not at cysteine residues but at lysine residues. In this revised hypothesis, the attachment of SA is preceded by formation of a solid phase ammonium carboxylate salt between SA and accessible lysine residues of the protein during sample preparation under acidic conditions. Laser irradiation at 355 nm of the dried sample spot results in equilibrium retrogradation followed by nucleophilic attack by the amine group of lysine at the carbonyl group of SA and subsequent amide bond formation and loss of water. The absence of CHCA adducts suggests that the electron-withdrawing effect of the α-cyano group of this matrix may inhibit salt formation and/or amide bond formation. This revised hypothesis is supported by dissociative loss of SA (-224 Da) and the amide-bound SA (-206 Da) from SA-adducted HdeA and HdeB ions by MS/MS (PSD). It is proposed that cleavage of the amide-bound SA from the lysine side-chain occurs via rearrangement involving a pentacyclic transition state followed by hydrogen abstraction/migration and loss of 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-ynal (-206 Da).

A mass spectrometric analysis of 4-hydroxy-2-(E)-nonenal modification of cytochrome c

Cytochrome c is a key mitochondrial respiratory protein that is particularly susceptible to modification during oxidative stress. The nature of this susceptibility is linked to the mitochondrial membrane being rich in esterified linoleic acid, which predisposes this organelle to the formation of lipid peroxidation products such as 4-hydroxy-2-(E)-nonenal (4-HNE). To better understand the nature of cytochrome c modification by 4-HNE, we initiated an in vitro study utilizing a combination of MALDI-TOF mass spectrometry, LC-ESI-MS/MS and isotope labeling to monitor 4-HNE modification of cytochrome c under various conditions. The overwhelming reaction observed is Michael addition by Lys side-chains in addition to the modification of His 33. While the Lys-4-HNE adducts were generally observed to be reversible, the 4-HNE-His 33 was observed to be stable with half of the formed adduct surviving the denaturation and proteolysis protocols used to generate proteolytic peptides for LC-ESI-MS/MS.

Inhibition by 4-hydroxynonenal (HNE) of Ca2+ transport by SERCA1a: low concentrations of HNE open protein-mediated leaks in the membrane

Exposure of sarcoplasmic reticulum membranes to 4-hydroxy-2-nonenal (HNE) resulted in inhibition of the maximal ATPase activity and Ca(2+) transport ability of SERCA1a, the Ca(2+) pump in these membranes. The concomitant presence of ATP significantly protected SERCA1a ATPase activity from inhibition. ATP binding and phosphoenzyme formation from ATP were reduced after treatment with HNE, whereas Ca(2+) binding to the high-affinity sites was altered to a lower extent. HNE reacted with SH groups, some of which were identified by MALDI-TOF mass spectrometry, and competition studies with FITC indicated that HNE also reacted with Lys(515) within the nucleotide binding pocket of SERCA1a. A remarkable fact was that both the steady-state ability of SR vesicles to sequester Ca(2+) and the ATPase activity of SR membranes in the absence of added ionophore or detergent were sensitive to concentrations of HNE much smaller than those that affected the maximal ATPase activity of SERCA1a. This was due to an increase in the passive permeability of HNE-treated SR vesicles to Ca(2+), an increase in permeability that did not arise from alteration of the lipid component of these vesicles. Judging from immunodetection with an anti-HNE antibody, this HNE-dependent increase in permeability probably arose from modification of proteins of about 150-160kDa, present in very low abundance in longitudinal SR membranes (and in slightly larger abundance in SR terminal cisternae). HNE-induced promotion, via these proteins, of Ca(2+) leakage pathways might be involved in the general toxic effects of HNE.

Markers of oxidative and nitrosative stress in systemic lupus erythematosus: correlation with disease activity

OBJECTIVE

Free radical-mediated reactions have been implicated as contributors in a number of autoimmune diseases, including systemic lupus erythematosus (SLE). However, the potential for oxidative/nitrosative stress to elicit an autoimmune response or to contribute to disease pathogenesis, and thus be useful when determining a prognosis, remains largely unexplored in humans. This study was undertaken to investigate the status and contribution of oxidative/nitrosative stress in patients with SLE.

METHODS

Sera from 72 SLE patients with varying levels of disease activity according to the SLE Disease Activity Index (SLEDAI) and 36 age- and sex-matched healthy controls were evaluated for serum levels of oxidative/nitrosative stress markers, including antibodies to malondialdehyde (anti-MDA) protein adducts and to 4-hydroxynonenal (anti-HNE) protein adducts, MDA/HNE protein adducts, superoxide dismutase (SOD), nitrotyrosine (NT), and inducible nitric oxide synthase (iNOS).

RESULTS

Serum analysis showed significantly higher levels of both anti-MDA/anti-HNE protein adduct antibodies and MDA/HNE protein adducts in SLE patients compared with healthy controls. Interestingly, not only was there an increased number of subjects positive for anti-MDA or anti-HNE antibodies, but also the levels of both of these antibodies were statistically significantly higher among SLE patients whose SLEDAI scores were > or = 6 as compared with SLE patients with lower SLEDAI scores (SLEDAI score <6). In addition, a significant correlation was observed between the levels of anti-MDA or anti-HNE antibodies and the SLEDAI score (r = 0.734 and r = 0.647, respectively), suggesting a possible causal relationship between these antibodies and SLE. Furthermore, sera from SLE patients had lower levels of SOD and higher levels of iNOS and NT compared with healthy control sera.

CONCLUSION

These findings support an association between oxidative/nitrosative stress and SLE. The stronger response observed in serum samples from patients with higher SLEDAI scores suggests that markers of oxidative/nitrosative stress may be useful in evaluating the progression of SLE and in elucidating the mechanisms of disease pathogenesis.

The specific isolation of C-terminal peptides of proteins through a transamination reaction and its advantage for introducing functional groups into the peptide

A novel method for isolating C-terminal peptides from proteolytic digests of proteins was developed. Proteins were digested with lysyl endopeptidase (LysC) and applied to metal-ion-catalyzed transamination reactions. This reaction enabled the selective conversion of an Nalpha-amino group to a carbonyl group. Subsequent incubation with p-phenylenediisothiocyanate (DITC) glass effectively scavenged the lysine-containing N-terminus and internal peptides. The obtained C-terminal peptide is open to modification with reagents having virtually any type of functionality owing to the reactive alpha-ketocarbonyl group. In this report, 2,4-dinitrophenylhydrazine (DNPH) was used as an example of a nucleophile to the carbonyl group. The isolated C-terminal peptide was modified with DNPH, which exhibited signal enhancement, and was sequenced by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).

Detection of carbonyl-modified proteins in interfibrillar rat mitochondria using N'-aminooxymethylcarbonylhydrazino-D-biotin as an aldehyde/keto-reactive probe in combination with Western blot analysis and tandem mass spectrometry

There is now a large body of supporting data available that links oxidative modifications of proteins to a large number of diseases, degenerative disorders and aging. However, the detailed analysis of oxidative protein modifications remains challenging. Here, we report a new efficient method for identification of oxidatively modified proteins in complex biological samples which is based on the use of an aldehyde-reactive probe, N'-aminooxymethylcarbonylhydrazino-D-biotin (ARP), in combination with Western-type analyses and MS. The biotinylated hydroxylamine derivative forms a chemically stable oxime derivative with the aldehyde/keto group found in carbonyl-modified proteins. The biotin tag is detected by avidin affinity staining. ARP-positive proteins are subsequently subjected to in-gel trypsinization and MS/MS for protein identification. We demonstrate the usefulness of the method for the analysis of protein extracts obtained from interfibrillar heart mitochondria (IFM) from young and old rats. In this study, we identified as putative major protein targets of oxidative modifications the mitochondrial matrix protein, aconitase, the inner mitochondrial membrane protein, ADP/ATP translocase, and constituents of the electron transport chain complexes IV and V. An age-related increase of carbonyl levels was found for aconitase and ATP synthase.

Oxidative stress and covalent modification of protein with bioactive aldehydes

The term "oxidative stress" links the production of reactive oxygen species to a variety of metabolic outcomes, including insulin resistance, immune dysfunction, and inflammation. Antioxidant defense systems down-regulated due to disease and/or aging result in oxidatively modified DNA, carbohydrates, proteins, and lipids. Increased production of hydroxyl radical leads to the formation of lipid hydroperoxides that produce a family of alpha,beta-unsaturated aldehydes. Such reactive aldehydes are subject to Michael addition reactions with the side chains of lysine, histidine, and cysteine residues, referred to as "protein carbonylation." Although not widely appreciated, reactive lipids can accumulate to high levels in cells, resulting in extensive protein modification leading to either loss or gain of function. The use of mass spectrometric methods to identify the site and extent of protein carbonylation on a proteome-wide scale has expanded our view of how oxidative stress can regulate cellular processes.

Rapid analysis of alpha-dicarbonyl compounds by laser desorption/ionization mass spectrometry using 9-(3,4-diaminophenyl)acridine (DAA) as a reactive matrix

A rapid, sensitive and selective method has been developed for the analysis of alpha-dicarbonyls using a readily ionizable compound, 9-(3,4-diaminophenyl)acridine (DAA), as a reactive matrix (derivatizing agent and ionization efficiency enhancer), by reactive matrix laser desorption/ionization time-of-flight mass spectrometry (RM-LDI-TOF MS). The reaction between the DAA and alpha-dicarbonyls resulted exclusively in formation of vacuum-stable dicarbonyl-quinoxaline acridine derivatives that were found to possess excellent ionization efficiency in positive ion mode, without the need to use an additional matrix. The alpha-dicarbonyls used as test compounds included methylglyoxal, dimethylglyoxal, and diphenylglyoxal. Both one-pot and rapid on-plate chemical modification approaches were employed with no extraction or purification necessary. The approach is particularly suitable for high-throughput analysis. The method was found to be selective and specific, with alpha-dicarbonyls unequivocally identified, even in complex matrices, e.g. beer. The figures of merit: relative standard deviation (RSD) 6.9-17%, (n = 4); limit of detection (LOD) < or =0.3 ng mL(-1) for the three standards tested using the one-pot derivatization method; and a good linear calibration curve using an internal standard derivatized in situ (R(2) > or = 0.979), demonstrate the applicability of the technique and its utility in improving the sensitivity and precision of the LDI analysis of small molecules.

Involvement of lipid peroxidation-derived aldehyde-protein adducts in autoimmunity mediated by trichloroethene

Lipid peroxidation, a major contributor to cellular damage, is also implicated in the pathogenesis of autoimmune diseases (AD). The focus of this study was to elucidate the role of lipid peroxidation-derived aldehydes in autoimmunity induced and/or exacerbated by chemical exposure. Previous studies showed that trichloroethene (TCE) is capable of inducing/accelerating autoimmunity. To test whether TCE-induced lipid peroxidation might be involved in the induction/exacerbation of autoimmune responses, groups of autoimmune-prone female MRL +/+ mice were treated with TCE (10 mmol/kg, i.p., every 4th day) for 6 or 12 wk. Significant increases of the formation of malondialdehyde (MDA)- and 4-hydroxynonenal (HNE)-protein adducts were found in the livers of TCE-treated mice at both 6 and 12 wk, but the response was greater at 12 wk. Further characterization of these adducts in liver microsomes showed increased formation of MDA-protein adducts with molecular masses of 86, 65, 56, 44, and 32 kD, and of HNE-protein adducts with molecular masses of 87, 79, 46, and 17 kD in TCE-treated mice. In addition, significant induction of anti-MDA- and anti-HNE-protein adduct-specific antibodies was observed in the sera of TCE-treated mice, and showed a pattern similar to MDA- or HNE-protein adducts. The increases in anti-MDA- and anti-HNE-protein adduct antibodies were associated with significant elevation in serum anti-nuclear-, anti-ssDNA- and anti-dsDNA-antibodies at 6 wk and, to a greater extent, at 12 wk. These studies suggest that TCE-induced lipid peroxidation is associated with induction/exacerbation of autoimmune response in MRL+/+ mice, and thus may play an important role in disease pathogenesis. Further interventional studies are needed to establish a causal relationship between lipid peroxidation and TCE-induced autoimmune response.

active site modifications of the brain isoform of creatine kinase by 4-hydroxy-2-nonenal correlate with reduced enzyme activity: mapping of modified sites by Fourier transform-ion cyclotron resonance mass spectrometry

Creatine kinase reversibly catalyzes the transfer of the high-energy phosphoryl group from phosphocreatine to MgADP for rapid regeneration of ATP. It is hypothesized that factors which perturb creatine kinase activity, such as reactive oxygen species resulting from oxidative stress, could have a major role in the pathogenesis of diseases, particularly in the brain, where the level of ATP utilization is high. The reactive aldehyde 4-hydroxy-2-nonenal is a major secondary product of lipid peroxidation caused by oxidative stress; the levels of both free and protein-bound 4-hydroxy-2-nonenal are increased in Alzheimer's disease brain. Preliminary reports indicated that creatine kinase had lower activity in Alzheimer's disease brain. In this study, we investigated the structural and functional consequences of reacting the cytosolic brain isoform of creatine kinase with 4-hydroxy-2-nonenal at pathophysiologically relevant concentrations of 4-hydroxy-2-nonenal (10-300 microM). Dose-dependent reduction of enzyme activity was observed and, for the first time, correlated with 4-hydroxy-2-nonenal adduct formation on specific amino acid residues, including the active site residues His66, His191, Cys283, and His296 as determined by Fourier transform-ion cyclotron resonance mass spectrometry.

Proteomic Mapping of 4-Hydroxynonenal Protein Modification Sites by Solid-Phase Hydrazide Chemistry and Mass Spectrometry

The modification of proteins by the cytotoxic, reactive aldehyde 4-hydroxynonenal (HNE) is known to alter protein function and impair cellular mechanisms. In order to identify susceptible amino acid sites of HNE modification within complex biological mixtures by microcapillary liquid chromatography and linear ion trap tandem mass spectrometry, we have developed a solid-phase capture and release strategy that utilizes reversible hydrazide chemistry to enrich HNE-modified peptides. To maximize the detection of fragment ions diagnostic of HNE modification, both neutral loss-dependent acquisition of MS/MS/MS spectra and the pulsed Q dissociation operation mode were employed. When the solid-phase hydrazide enrichment strategy was applied to a yeast lysate treated with HNE, 125 distinct amino acid sites of HNE modification were mapped on 67 different proteins. The endogenous susceptibility of many of these proteins to HNE modification was demonstrated by analyzing HNE-treated yeast cell cultures with a complementary biotin hydrazide enrichment strategy. Further analysis revealed that the majority of amino acid sites susceptible to HNE modification were histidine residues, with most of these sites being flanked by basic amino acid residues, and predicted to be solvent exposed. These results demonstrate the effectiveness of this novel strategy as a general platform for proteome-scale identification of amino acid sites susceptible to HNE modification from within complex mixtures.

Proteomics of lipid oxidation-induced oxidation of porcine and bovine oxymyoglobins

Myoglobin (Mb) redox state affects meat color and is destabilized by lipid oxidation products such as 4-hydroxy-2-nonenal (HNE). Our objective was to investigate lipid oxidation-induced oxymyoglobin (OxyMb) oxidation in Mb from two major meat-producing livestock species utilizing MS and proteomics tools. Porcine OxyMb was incubated with HNE and analyzed for metmyoglobin (MetMb) formation. MetMb formation was greater in the presence of HNE than controls at pH 7.4 and 37 degrees C (p <0.05). MALDI-TOF MS was used to identify adduct formation; only mono-adducts of HNE (via Michael addition) with porcine Mb were detected. LC-ESI-MS/MS identified three histidine (HIS) residues in porcine Mb that were readily adducted by HNE (HIS 24, 36 and 119), whereas in bovine Mb seven histidine residues (HIS 24, 36, 81, 88, 93, 119 and 152) were adducted. Quantitation of HNE-adducted peptides using isotope-labeled phenyl isocyanate indicated that, initially, HIS 36 was preferentially adducted in porcine Mb whereas HIS 81, 88 and 93 were the predominant sites of early HNE adduction in bovine Mb. Preferential HNE adduction at the proximal histidine (HIS 93) was observed exclusively in bovine OxyMb and may explain why lipid oxidation-induced OxyMb oxidation appears more extensive in beef, than in pork.

Direct analysis and MALDI imaging of formalin-fixed, paraffin-embedded tissue sections

Formalin fixation, generally followed by paraffin embedding, is the standard and well-established processing method employed by pathologist. This treatment conserves and stabilizes biopsy samples for years. Analysis of FFPE tissues from biopsy libraries has been, so far, a challenge for proteomics biomarker studies. Herein, we present two methods for the direct analysis of formalin-fixed, paraffin-embedded (FFPE) tissues by MALDI-MS. The first is based on the use of a reactive matrix, 2,4-dinitrophenylhydrazine, useful for FFPE tissues stored less than 1 year. The second approach is applicable for all FFPE tissues regardless of conservation time. The strategy is based on in situ enzymatic digestion of the tissue section after paraffin removal. In situ digestion can be performed on a specific area of the tissue as well as on a very small area (microdigestion). Combining automated microdigestion of a predefined tissue array with either in situ extraction prior to classical nanoLC/MS-MS analysis or automated microspotting of MALDI matrix according to the same array allows the identification of both proteins by nanoLC-nanoESI and MALDI imaging. When adjacent tissue sections are used, it is, thus, possible to correlate protein identification and molecular imaging. These combined approaches, along with FFPE tissue analysis provide access to massive amounts of archived samples in the clinical pathology setting.

Chemical Nature of Stochastic Generation of Protein-based Carbonyls: Metal-catalyzed Oxidation versus Modification by Products of Lipid Oxidation†

An assessment of 2,4-dinitrophenylhydrazine (DNPH)-detectable protein-based carbonyls is one of the most common assays used to quantify oxidative stress in vitro and in vivo. In this study, we compared, for the lipid-binding protein beta-lactoglobulin, the extent to which carbonyl reactivity could be introduced by adventitious metal-catalyzed oxidation (MCO) in the absence and presence of a polyunsaturated lipid or by treatment with various individual bifunctional lipid oxidation products capable of introducing carbonyls into proteins by adduction to nucleophilic side chains. With metal ions and either O2/reductant or H2O2 as the terminal oxidant, the maximal level of DNPH-detectable carbonyl generation obtainable in several hours was 0.1-0.2 mol carbonyl per mol protein monomer, with Cu(II) being more effective than Fe(II). Exposure instead to bifunctional lipoxidation-derived aldehydes (1-2 mM) generated in some cases in excess of 1 mol carbonyl per mol protein. The rank order of carbonyl incorporation reactivity was acrolein > 4-oxo-2-nonenal > 4-hydroxy-2-nonenal > 2,4-decadienal > malondialdehyde. Protein cross-linking ability followed a somewhat different rank order. Parallel studies on reductively methylated beta-lactoglobulin revealed that His and Cys residues are intrinsically more responsible than Lys residues for carbonyl appearance and that the availability of Lys residues accounts for the reduction of carbonyl content at later time (presumably reflecting cross-linking chemistry) that occurs for acrolein and 4-oxo-2-nonenal. Overall, these results suggest that DNPH reactivity observed physiologically on nonmetalloproteins may arise more from the attachment of lipid-derived products of oxidative stress than from adventitious MCO of side chains. Additional studies carried out to clarify the potential use of DNPH derivatization to tag peptide-based carbonyls for mass spectrometric analysis revealed that DNPH derivatization can reverse under the conditions used for proteolysis.

New Role for an Old Probe: Affinity Labeling of Oxylipid Protein Conjugates byN‘-Aminooxymethylcarbonylhydrazinod-biotin

Free radicals, electrophiles, and endogenous reactive intermediates are generated during normal physiological processes and are capable of modifying DNA, lipids, and proteins. However, elevated levels of oxidative modifications of proteins by reactive species are implicated in the etiology and pathology of oxidative stress-mediated diseases, neurodegeneration, and aging. A mass spectrometry-based approach is reported that aids to the identification and characterization of carbonyl-modified proteins. The method uses N'-aminooxymethylcarbonylhydrazino d-biotin, a biotinylated hydroxylamine derivative that forms an oxime derivative with the aldehyde/keto group found in oxidatively modified proteins. In this paper, the method is demonstrated for one class of carbonyl-modified proteins, namely, oxylipid peptide and protein conjugates formed by Michael addition-type conjugation reactions of alpha,beta-unsaturated aldehydic lipid peroxidation products with nucleophilic peptide side chains. This new application of an "old" probe, which has been used for the detection of abasic sites in DNA strands, introduces a biotin moiety into the oxylipid peptide conjugate. The biotin-modified oxylipid peptide conjugate is then amenable to enrichment using avidin affinity capture. The described method represents an attractive alternative to hydrazine-based derivatization methods for oxidized peptides and proteins because the reduction step necessary for the transformation of the hydrazone bond to the chemically more stable hydrazine bond can be omitted. Tandem mass spectrometry of the labeled oxylipid peptide conjugates indicates that the biotin moiety is at least partially retained on the fragment ion during the collisionally induced dissociation experiments, a prerequisite for the use of automated database searching of uninterpreted tandem mass spectra. The reported approach is outlined for the detection, identification, and characterization of oxylipid peptide conjugates, but the labeling chemistry may also be applicable to other carbonyl-modified proteins.

Contribution of mass spectrometry to assess quality of milk-based products

The vast knowledge of milk chemistry has been extensively used by the dairy manufacturing industry to develop and optimize the modern technology required to produce high-quality milk products to which we are accustomed. A thorough understanding of the chemistry of milk and its numerous components is essential for designing processing equipment and conditions needed for the manufacture and distribution of high-quality dairy products. Knowledge and application of milk chemistry is also indispensable for fractionating milk into its principal components for use as functional and nutritional ingredients by the food industry. For all these reasons, powerful analytical methods are required. Because of the complexity of the milk matrix, mass spectrometry, coupled or not to separation techniques, constitutes a key tool in this area. In the present manuscript, we review the contribution and potentialities of mass spectrometry-based techniques to assess quality of milk-based products.

Application of semiquantitative proteomics techniques to the maillard reaction

Proteomic tools-in particular, mass spectrometry (MS)-have advanced significantly in recent years, and the identification of proteins within complex mixtures is now a routine procedure. Quantitative methods of analysis are less well advanced and continue to develop. These include the use of stable isotope ratio approaches, isotopically labeled peptide standards, and nonlabeling methods. This paper summarizes the use of MS as a proteomics tool to identify and semiquantify proteins and their modified forms by using examples of relevance to the Maillard reaction. Finally, some challenges for the future are presented.

Carbonylation of milk powder proteins as a consequence of processing conditions

During industrial treatments, milk proteins could be oxidatively modified, thus leading to the formation of modified/oxidised amino acid residues. The apparition of such modified residues may contribute to the formation of new immunologically reactive structures. Some of these adducts could, in an advanced stage, lead to cross-linked protein species whose proteolytic susceptibility would be drastically decreased. Such protein species, that are resistant to digestion, could also constitute major food allergens. Therefore, these oxidative protein modifications tend to increase the natural allergenicity of milk proteins. For these reasons, monitoring milk protein oxidative modifications could be very useful regarding both product quality and allergenicity issues. In the present paper, we highlight, using different analytical approaches, the preferential carbonylation of beta-lactoglobulin (beta-Lg) during industrial treatments of milk. This result is particularly interesting since native beta-Lg represents one of the major milk allergens.

4-Oxo-2-nonenal Is Both More Neurotoxic and More Protein Reactive than 4-Hydroxy-2-nonenal

Electrophilic aldehydes, generated from oxidation of polyunsaturated fatty acyl chains under conditions of oxidative stress, bind to proteins and polynucleotides and can lead to cell death. 4-Hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE) have been shown here to be toxic to human neuroblastoma cells in culture at low micromolar concentrations. ONE is 4-5 times more neurotoxic at concentrations near the threshold of lethality. The reactions of these two aldehydes with two model proteins, ribonuclease A and beta-lactoglobulin, and their Lys epsilon-dimethylamino derivatives, have been followed spectrophotometrically. On the basis of t(1/2) measurements for the disappearance of the alpha,beta-unsaturated chromophore, ONE is 6-31 times more reactive with these proteins. The fastest reaction of ONE with proteins involves Schiff base formation at Lys epsilon-amino groups, whereas Schiff base formation is not spectroscopically apparent for HNE. Detailed kinetic studies of the initial reactions of HNE and ONE have been carried out with amino acids and amino acid surrogates. Whereas the reactions with imidazole and thiol nucleophiles involve straightforward Michael adduct formation, kinetics analyses reveal the reversibility of both the HNE Michael adduction of amines and the ONE Schiff base adduction of amines. Although ONE is more reactive than HNE toward conjugate addition of imidazole and thiol nucleophiles, it is less reactive than HNE toward Lys/amine Michael adduction. The greater neurotoxicity of ONE could reflect in part the different reactivity characteristics of ONE as compared to HNE.