Toward an "omic" physiopathology of reactive chemicals: thirty years of mass spectrometric study of the protein adducts with endogenous and xenobiotic compounds

Strategy for identifying unknown hemoglobin adducts using adductome LC-MS/MS data: Identification of adducts corresponding to acrylic acid, glyoxal, methylglyoxal, and 1-octen-3-one

Electrophilic compounds have the ability to form adducts with nucleophilic sites in proteins and DNA in tissues, and thereby constitute risks for toxic effects. Adductomic approaches are developed for systematic screening of adducts to DNA and blood proteins, with the aim to detect unknown internal exposures to electrophiles. In a previous adductomic screening of adducts to N-terminals in hemoglobin, using LC-MS/MS, 19 unknown adducts were detected in addition to seven previously identified adducts. The present paper describes the identification of four of these unknown adducts, as well as the strategy used to identify them. Using LC-MS data from the screening, hypotheses about adduct identities were formulated: probable precursor electrophiles with matching molecular weights were suggested based on the molecular weights of the modifications and the retention times of the analytes, in combination with comparisons of theoretical Log P calculations and databases. Reference adducts were generated by incubation of blood samples with the hypothesized precursor electrophiles. The four identified precursor electrophiles, corresponding to the observed unknown adducts, were glyoxal, methylglyoxal, acrylic acid and 1-octen-3-one. Possible origins/exposure sources and toxicological information concerning the electrophilic precursors are discussed. The identified adducts could be explored as possible biomarkers for exposure.

Covalent adducts of melphalan with free amino acids and a model peptide studied by liquid chromatography/tandem mass spectrometry

RATIONALE

Melphalan is a frequently used chemotherapeutical agent for the treatment of myeloma, breast cancer, ovarian cancer and sarcoma of soft tissue. A good knowledge of the reactivity of the drug toward the different amino acids, e.g. covalent adduct formation, is crucial for the understanding of its activity and side effects during cancer treatment.

METHODS

The reactivity of melphalan and sites of adduct formation were studied by in vitro incubation of melphalan with free amino acids and glutathione as a model peptide. The formed covalent adducts were investigated using ultra-performance liquid chromatography tandem mass spectrometry (UPLC/MS/MS) using a triple-quadrupole instrument. Accurate mass measurements for the confirmation of characteristic product ions were performed on a quadrupole time-of-flight (QTOF) mass spectrometer.

RESULTS

The incubation of melphalan with different classes of amino acids resulted in the formation of adducts on the amino and carboxyl termini, as well as adduct formation in the reactive side chains of Cys, Met, Tyr, His, Lys, Asp and Glu. All these melphalan adducts could be identified by their characteristic collision-induced dissociation (CID) product ion patterns.

CONCLUSIONS

The present study demonstrates the reactivity of melphalan towards the functional groups of amino acids. The different alkylation site products show distinctive fragmentation patterns, which enable a fast identification of the different melphalan adducts. This study is a first important step towards a better understanding of the adduct formation in more complex molecules, e.g. peptides and proteins.

Methemoglobin Formation and Characterization of Hemoglobin Adducts of Carcinogenic Aromatic Amines and Heterocyclic Aromatic Amines

Arylamines (AAs) and heterocyclic aromatic amines (HAAs) are structurally related carcinogens formed during the combustion of tobacco or cooking of meat. They undergo cytochrome P450 mediated N-hydroxylation to form metabolites which bind to DNA and lead to mutations. The N-hydroxylated metabolites of many AAs also can undergo a co-oxidation reaction with oxy-hemolgobin (HbO2) to form methemoglobin (met-Hb) and the arylnitroso intermediates, which react with the β-Cys(93) chain of Hb to form Hb-arylsulfinamide adducts. The biochemistry of arylamine metabolism has been exploited to biomonitor certain AAs through their Hb arylsulfinamide adducts in humans. We examined the reactivity of HbO2 with the N-hydroxylated metabolites of 4-aminobiphenyl (ABP, HONH-ABP), aniline (ANL, HONH-ANL), and the HAAs 2-amino-9H-pyrido[2,3-b]indole (AαC, HONH-AαC), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP, HONH-PhIP), and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx, HONH-MeIQx). HONH-ABP, HO-ANL, and HONH-AαC induced methemoglobinemia and formed Hb sulfinamide adducts. However, HONH-MeIQx and HONH-PhIP did not react with the oxy-heme complex, and met-Hb formation and chemical modification of the β-Cys(93) residue were negligible. Molecular modeling studies showed that the distances between the H-ON-AA or H-ON-HAA substrates and the oxy-heme complex of HbO2 were too far away to induce methemoglobinemia. Different conformational changes in flexible helical and loop regions around the heme pocket induced by the H-ON-AA or H-ON-HAAs may explain the different proclivities of these chemicals to induce methemoglobinemia. Hb-Cys(93β) sulfinamide and sulfonamide adducts of ABP, ANL, and AαC were identified, by Orbitrap MS, following the proteolysis of Hb with trypsin, Glu-C, or Lys-C. Hb sulfinamide and sulfonamide adducts of ABP were identified in the blood of mice exposed to ABP, by Orbitrap MS. This is the first report of the identification of intact Hb sulfinamide adducts of carcinogenic AAs in vivo. The high reactivity of HONH-AαC with HbO2 suggests that the Hb sulfinamide adduct of AαC may be a promising biomarker of exposure to this HAA in humans.

Data-independent-acquisition mass spectrometry for identification of targeted-peptide site-specific modifications

We present a novel strategy based on data-independent acquisition coupled to targeted data extraction for the detection and identification of site-specific modifications of targeted peptides in a completely unbiased manner. This method requires prior knowledge of the site of the modification along the peptide backbone from the protein of interest, but not the mass of the modification. The procedure, named multiplex adduct peptide profiling (MAPP), consists of three steps: 1) A fragment-ion tag is extracted from the data, consisting of the b-type and y-type ion series from the N and C-terminus, respectively, up to the amino-acid position that is believed to be modified; 2) MS1 features are matched to the fragment-ion tag in retention-time space, using the isolation window as a pre-filter to enable calculation of the mass of the modification; and 3) modified fragment ions are overlaid with the unmodified fragment ions to verify the mass calculated in step 2. We discuss the development, applications, and limitations of this new method for detection of unknown peptide modifications. We present an application of the method in profiling adducted peptides derived from abundant proteins in biological fluids with the ultimate objective of detecting biomarkers of exposure to reactive species.

2-Amino-9H-pyrido[2,3-b]indole (AαC) Adducts and Thiol Oxidation of Serum Albumin as Potential Biomarkers of Tobacco Smoke

2-Amino-9H-pyrido[2,3-b]indole (AαC) is a carcinogenic heterocyclic aromatic amine formed during the combustion of tobacco. AαC undergoes bioactivation to form electrophilic N-oxidized metabolites that react with DNA to form adducts, which can lead to mutations. Many genotoxicants and toxic electrophiles react with human serum albumin (albumin); however, the chemistry of reactivity of AαC with proteins has not been studied. The genotoxic metabolites, 2-hydroxyamino-9H-pyrido[2,3-b]indole (HONH-AαC), 2-nitroso-9H-pyrido[2,3-b]indole (NO-AαC), N-acetyloxy-2-amino-9H-pyrido[2,3-b]indole (N-acetoxy-AαC), and their [(13)C6]AαC-labeled homologues were reacted with albumin. Sites of adduction of AαC to albumin were identified by data-dependent scanning and targeted bottom-up proteomics approaches employing ion trap and Orbitrap MS. AαC-albumin adducts were formed at Cys(34), Tyr(140), and Tyr(150) residues when albumin was reacted with HONH-AαC or NO-AαC. Sulfenamide, sulfinamide, and sulfonamide adduct formation occurred at Cys(34) (AαC-Cys(34)). N-Acetoxy-AαC also formed an adduct at Tyr(332). Albumin-AαC adducts were characterized in human plasma treated with N-oxidized metabolites of AαC and human hepatocytes exposed to AαC. High levels of N-(deoxyguanosin-8-yl)-AαC (dG-C8-AαC) DNA adducts were formed in hepatocytes. The Cys(34) was the sole amino acid of albumin to form adducts with AαC. Albumin also served as an antioxidant and scavenged reactive oxygen species generated by metabolites of AαC in hepatocytes; there was a strong decrease in reduced Cys(34), whereas the levels of Cys(34) sulfinic acid (Cys-SO2H), Cys(34)-sulfonic acid (Cys-SO3H), and Met(329) sulfoxide were greatly increased. Cys(34) adduction products and Cys-SO2H, Cys-SO3H, and Met(329) sulfoxide may be potential biomarkers to assess exposure and oxidative stress associated with AαC and other arylamine toxicants present in tobacco smoke.

Characterization of quinone derived protein adducts and their selective identification using redox cycling based chemiluminescence assay

The cytotoxic mechanism of many quinones has been correlated to covalent modification of cellular proteins. However, the identification of relevant proteins targets is essential but challenging goals. To better understand the quinones cytotoxic mechanism, human serum albumin (HSA) was incubated in vitro with different concentration of menadione (MQ). In this respect, the initial nucleophilic addition of proteins to quinone converts the conjugates to redox-cycling quinoproteins with altered conformation and secondary structure and extended life span than the short lived, free quinones. The conjugation of MQ with nucleophilic sites likewise, free cysteine as well as ɛ-amino group of lysine residue of HSA has been found to be in concentration dependent manner. The conventional methods for modified proteins identification in complex mixtures are complicated and time consuming. Herein, we describe a highly selective, sensitive, simple, and fast strategy for quinoproteins identification. The suggested strategy exploited the unique redox-cycling capability of quinoproteins in presence of a reductant, dithiothreitol (DTT), to generate reactive oxygen species (ROS) that gave sufficient chemiluminescence (CL) when mixed with luminol. The CL approach is highly selective and sensitive to detect the quinoproteins in ten-fold molar excess of native proteins without adduct enrichment. The approach was also coupled with gel filtration chromatography (GFC) and used to identify adducts in complex mixture of proteins in vitro as well as in rat plasma after MQ administration. Albumin was identified as the main protein in human and rat plasma forming adduct with MQ. Overall, the identification of quinoproteins will encourage further studies of toxicological impact of quinones on human health.

Irreversible binding of an anticancer compound (BI-94) to plasma proteins

1. We investigated the mechanisms responsible for the in vivo instability of a benzofurazan compound BI-94 (NSC228148) with potent anti-cancer activity. 2. BI-94 was stable in MeOH, water, and in various buffers at pHs 2.5-5, regardless of the buffer composition. In contrast, BI-94 was unstable in NaOH and at pHs 7-9, regardless of the buffer composition. BI-94 disappeared immediately after spiking into mice, rat, monkey, and human plasma. BI-94 stability in plasma can be only partially restored by acidifying it, which indicated other mechanisms in addition to pH for BI-94 instability in plasma. 3. BI-94 formed adducts with the trapping agents, glutathione (GSH) and N-acetylcysteine (NAC), in vivo and in vitro via nucleophilic aromatic substitution reaction. The kinetics of adduct formation showed that neutral or physiological pHs enhanced and accelerated GSH and NAC adduct formation with BI-94, whereas acidic pHs prevented it. Therefore, physiological pHs not only altered BI-94 chemical stability but also enhanced adduct formation with endogenous nucleophiles. In addition, adduct formation with human serum albumin-peptide 3 (HSA-T3) at the Cys34 position was demonstrated. 4. In conclusion, BI-94 was unstable at physiological conditions due to chemical instability and irreversible binding to plasma proteins.

Mass Spectrometric Characterization of Human Serum Albumin Adducts Formed with N-Oxidized Metabolites of 2-Amino-1-methylphenylimidazo[4,5-b]pyridine in Human Plasma and Hepatocytes

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a carcinogenic heterocyclic aromatic amine formed in cooked meats, is metabolically activated to electrophilic intermediates that form covalent adducts with DNA and protein. We previously identified an adduct of PhIP formed at the Cys(34) residue of human serum albumin following reaction of albumin with the genotoxic metabolite 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP). The major adducted peptide recovered from a tryptic/chymotryptic digest was identified as the missed-cleavage peptide LQQC*([SO2PhIP])PFEDHVK, a [cysteine-S-yl-PhIP]-S-dioxide linked adduct. In this investigation, we have characterized the albumin adduction products of N-sulfooxy-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-sulfooxy-PhIP), which is thought to be a major genotoxic metabolite of PhIP formed in vivo. Targeted and data-dependent scanning methods showed that N-sulfooxy-PhIP adducted to the Cys(34) of albumin in human plasma to form LQQC*([SO2PhIP])PFEDHVK at levels that were 8-10-fold greater than the adduct levels formed with N-(acetyloxy)-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-acetoxy-PhIP) or HONH-PhIP. We also discovered that N-sulfooxy-PhIP forms an adduct at the sole tryptophan (Trp(214)) residue of albumin in the sequence AW*([PhIP])AVAR. However, stable adducts of PhIP with albumin were not detected in human hepatocytes. Instead, PhIP and 2-amino-1-methyl-6-(5-hydroxy)phenylimidazo[4,5-b]pyridine (5-HO-PhIP), a solvolysis product of the proposed nitrenium ion of PhIP, were recovered during the proteolysis, suggesting a labile sulfenamide linkage had formed between an N-oxidized intermediate of PhIP and Cys(34) of albumin. A stable adduct was formed at the Tyr(411) residue of albumin in hepatocytes and identified as a deaminated product of PhIP, Y(*[desaminoPhIP])TK, where the 4-HO-tyrosine group bound to the C-2 imidazole atom of PhIP.

Toxicity of Glutathione-Binding Metals: A Review of Targets and Mechanisms

Mercury, cadmium, arsenic and lead are among priority metals for toxicological studies due to the frequent human exposure and to the significant burden of disease following acute and chronic intoxication. Among their common characteristics is chemical affinity to proteins and non-protein thiols and their ability to generate cellular oxidative stress by the best-known Fenton mechanism. Their health effects are however diverse: kidney and liver damage, cancer at specific sites, irreversible neurological damages with metal-specific features. Mechanisms for the induction of oxidative stress by interaction with the cell thiolome will be presented, based on literature evidence and of experimental findings.

Considerations for Using Genetic and Epigenetic Information in Occupational Health Risk Assessment and Standard Setting

Risk assessment forms the basis for both occupational health decision-making and the development of occupational exposure limits (OELs). Although genetic and epigenetic data have not been widely used in risk assessment and ultimately, standard setting, it is possible to envision such uses. A growing body of literature demonstrates that genetic and epigenetic factors condition biological responses to occupational and environmental hazards or serve as targets of them. This presentation addresses the considerations for using genetic and epigenetic information in risk assessments, provides guidance on using this information within the classic risk assessment paradigm, and describes a framework to organize thinking about such uses. The framework is a 4 × 4 matrix involving the risk assessment functions (hazard identification, dose-response modeling, exposure assessment, and risk characterization) on one axis and inherited and acquired genetic and epigenetic data on the other axis. The cells in the matrix identify how genetic and epigenetic data can be used for each risk assessment function. Generally, genetic and epigenetic data might be used as endpoints in hazard identification, as indicators of exposure, as effect modifiers in exposure assessment and dose-response modeling, as descriptors of mode of action, and to characterize toxicity pathways. Vast amounts of genetic and epigenetic data may be generated by high-throughput technologies. These data can be useful for assessing variability and reducing uncertainty in extrapolations, and they may serve as the foundation upon which identification of biological perturbations would lead to a new paradigm of toxicity pathway-based risk assessments.

Considerations for Using Genetic and Epigenetic Information in Occupational Health Risk Assessment and Standard Setting

Risk assessment forms the basis for both occupational health decision-making and the development of occupational exposure limits (OELs). Although genetic and epigenetic data have not been widely used in risk assessment and ultimately, standard setting, it is possible to envision such uses. A growing body of literature demonstrates that genetic and epigenetic factors condition biological responses to occupational and environmental hazards or serve as targets of them. This presentation addresses the considerations for using genetic and epigenetic information in risk assessments, provides guidance on using this information within the classic risk assessment paradigm, and describes a framework to organize thinking about such uses. The framework is a 4 × 4 matrix involving the risk assessment functions (hazard identification, dose-response modeling, exposure assessment, and risk characterization) on one axis and inherited and acquired genetic and epigenetic data on the other axis. The cells in the matrix identify how genetic and epigenetic data can be used for each risk assessment function. Generally, genetic and epigenetic data might be used as endpoints in hazard identification, as indicators of exposure, as effect modifiers in exposure assessment and dose-response modeling, as descriptors of mode of action, and to characterize toxicity pathways. Vast amounts of genetic and epigenetic data may be generated by high-throughput technologies. These data can be useful for assessing variability and reducing uncertainty in extrapolations, and they may serve as the foundation upon which identification of biological perturbations would lead to a new paradigm of toxicity pathway-based risk assessments.

LC-MS/MS screening strategy for unknown adducts to N-terminal valine in hemoglobin applied to smokers and nonsmokers

Electrophilically reactive compounds have the ability to form adducts with nucleophilic sites in DNA and proteins, constituting a risk for toxic effects. Mass spectrometric detection of adducts to N-terminal valine in hemoglobin (Hb) after detachment by modified Edman degradation procedures is one approach for in vivo monitoring of exposure to electrophilic compounds/metabolites. So far, applications have been limited to one or a few selected reactive species, such as acrylamide and its metabolite glycidamide. This article presents a novel screening strategy for unknown Hb adducts to be used as a basis for an adductomic approach. The method is based on a modified Edman procedure, FIRE, specifically developed for LC-MS/MS analysis of N-terminal valine adducts in Hb detached as fluorescein thiohydantoin (FTH) derivatives. The aim is to detect and identify a priori unknown Hb adducts in human blood samples. Screening of valine adducts was performed by stepwise scanning of precursor ions in small mass increments, monitoring four fragments common for the FTH derivative of valine with different N-substitutions in the multiple-reaction mode, covering a mass range of 135 Da (m/z 503-638). Samples from six smokers and six nonsmokers were analyzed. Control experiments were performed to compare these results with known adducts and to check for artifactual formation of adducts. In all samples of smokers and nonsmokers, seven adducts were identified, of which six have previously been studied. Nineteen unknown adducts were observed, and 14 of those exhibited fragmentation patterns similar to earlier studied FTH derivatives of adducts to valine. Identification of the unknown adducts will be the focus of future work. The presented methodology is a promising screening tool using Hb adducts to indicate exposure to potentially toxic electrophilic compounds and metabolites.

Combining bioinformatics and MS-based proteomics: clinical implications

Clinical proteomics research aims at i) discovery of protein biomarkers for screening, diagnosis and prognosis of disease, ii) discovery of protein therapeutic targets for improvement of disease prevention, treatment and follow-up, and iii) development of mass spectrometry (MS)-based assays that could be implemented in clinical chemistry, microbiology or hematology laboratories. MS has been increasingly applied in clinical proteomics studies for the identification and quantification of proteins. Bioinformatics plays a key role in the exploitation of MS data in several aspects such as the generation and curation of protein sequence databases, the development of appropriate software for MS data treatment and integration with other omics data and the establishment of adequate standard files for data sharing. In this article, we discuss the main MS approaches and bioinformatics solutions that are currently applied to accomplish the objectives of clinical proteomic research.

Pooling samples for "top-down" molecular exposomics research: the methodology

BACKGROUND

Exposomics is the cutting-edge concept of screening the environmental risk factors for disease. In the novel "top-down" approach, we estimate the molecular exposome by measuring all body fluid analytes in a case-controlled study. However, to detect diverse pollutants, a sufficient sample size and multiple analytical methods are required. This may lead to dramatically increased costs and research workload.

METHODS

To help reduce complexity, we suggest a sample pooling strategy along with a scheme for combining both general unknown or multi-targeted screening with targeted analysis. The sample pooling method was tested using computer simulations.

RESULTS

By comprehensively analysis of pooled samples, it is possible to identify environmental risk factors. Factors are initially screened in the pooled case and control population samples, then in the randomized grouped and pooled case and control subpopulation samples. In the sample grouping, five or more pools were suggested for groups having 30 individuals per pool.

CONCLUSIONS

This study suggests that sample pooling is a useful strategy for exposomics research, which provides a hypothesis-free method for pollutant risk screening.

Conditions for sample preparation and quantitative HPLC/MS-MS analysis of bulky adducts to serum albumin with diolepoxides of polycyclic aromatic hydrocarbons as models

Stable adducts to serum albumin (SA) from electrophilic and genotoxic compounds/metabolites can be used as biomarkers for quantification of the corresponding in vivo dose. In the present study, conditions for specific analysis of stable adducts to SA formed from carcinogenic polycyclic aromatic hydrocarbons (PAH) were evaluated in order to achieve a sensitive and reproducible quantitative method. Bulky adducts from diolepoxides (DE) of PAH, primarily DE of benzo[a]pyrene (BPDE) and also DE of dibenzo[a,l]pyrene (DBPDE) and dibenzo[a,h]anthracene (DBADE), were used as model compounds. The alkylated peptides obtained after enzymatic hydrolysis of human SA modified with the different PAHDE were principally PAHDE-His-Pro, PAHDE-His-Pro-Tyr and PAHDE-Lys. Alkaline hydrolysis under optimised conditions gave the BPDE-His as the single analyte of alkylated His, but also indicated degradation of this adduct. It was not possible to obtain the BPDE-His as one analyte from BPDE-alkylated SA through modifications of the enzymatic hydrolysis. The BPDE-His adduct was shown to be stable during the weak acidic conditions used in the isolation of SA. Enrichment by HPLC or SPE, but not butanol extraction, gave good recovery, using Protein LoBind tubes. A simple internal standard (IS) approach using SA modified with other PAHDE as IS was shown to be applicable. A robust analytical procedure based on digestion with pronase, enrichment by HPLC or SPE, and analysis with HPLC/MS-MS electrospray ionisation was achieved. A good reproducibility (coefficient of variation (CV) 11 %) was obtained, and the achieved limit of detection for the studied PAHDE, using standard instrumentation, was approximately 1 fmol adduct/mg SA analysing extract from 5 mg SA.

An enhanced throughput method for quantification of sulfur mustard adducts to human serum albumin via isotope dilution tandem mass spectrometry

Here, we report an enhanced throughput method for the diagnosis of human exposure to sulfur mustard. A hydroxyethylthioethyl (HETE) ester-adducted tripeptide, produced by pronase digestion of human serum albumin, was selected as the quantitative exposure biomarker. Cibacron Blue enrichment was developed from an established cartridge method into a 96-well plate format, increasing throughput and ruggedness. This new method decreased sample volume 2.5-fold. Addition of a precipitation and solid-phase extraction concentration step increased the sensitivity of the method. With the conversion to a 96-well plate and optimization of chromatography, the method resulted in a 3-fold decrease in analysis time. Inclusion of a confirmation ion has increased specificity. The method was found to be linear between 0.050 and 50 µM sulfur mustard exposure with a precision for both quality control samples of ≤6.5% relative standard deviation and an accuracy of >96%. The limit of detection (3So) was calculated to be ∼0.0048 µM, an exposure value similar to that of the HETE-albumin adduct method first described by Noort and co-workers (Noort et al., 1999; Noort el al., 2004) which used protein precipitation to isolate albumin. A convenience set of 124 plasma samples from healthy unexposed individuals was analyzed using this method to assess background levels of exposure to sulfur mustard; no positive results were detected.

Cross-linking of thioredoxin reductase by the sulfur mustard analogue mechlorethamine (methylbis(2-chloroethyl)amine) in human lung epithelial cells and rat lung: selective inhibition of disulfide reduction but not redox cycling

Oxidative stress plays a key role in mechlorethamine (methylbis(2-chloroethyl)amine, HN2) toxicity. The thioredoxin system, consisting of thioredoxin reductase (TrxR), thioredoxin, and NADPH, is important in redox regulation and protection against oxidative stress. HN2 contains two electrophilic side chains that can react with nucleophilic sites in proteins, leading to changes in their structure and function. We report that HN2 inhibits the cytosolic (TrxR1) and mitochondrial (TrxR2) forms of TrxR in A549 lung epithelial cells. TrxR exists as homodimers under native conditions; monomers can be detected by denaturing and reducing SDS-PAGE followed by western blotting. HN2 treatment caused marked decreases in TrxR1 and TrxR2 monomers along with increases in dimers and oligomers under reducing conditions, indicating that HN2 cross-links TrxR. Cross-links were also observed in rat lung after HN2 treatment. Using purified TrxR1, NADPH reduced, but not oxidized, enzyme was inhibited and cross-linked by HN2. LC-MS/MS analysis of TrxR1 demonstrated that HN2 adducted cysteine- and selenocysteine-containing redox centers forming monoadducts, intramolecule and intermolecule cross-links, resulting in enzyme inhibition. HN2 cross-links two dimeric subunits through intermolecular binding to cysteine 59 in one subunit of the dimer and selenocysteine 498 in the other subunit, confirming the close proximity of the N- and C-terminal redox centers of adjacent subunits. Despite cross-linking and inhibition of TrxR activity by HN2, TrxR continued to mediate menadione redox cycling and generated reactive oxygen species. These data suggest that disruption of the thioredoxin system contributes to oxidative stress and tissue injury induced by HN2.

Exposure profiling of reactive compounds in complex mixtures

Humans are constantly exposed to mixtures, such as tobacco smoke, exhaust from diesel, gasoline or new bio-fuels, containing several 1000 compounds, including many known human carcinogens. Covalent binding of reactive compounds or their metabolites to DNA and formation of stable adducts is believed to be the causal link between exposure and carcinogenesis. DNA and protein adducts are well established biomarkers for the internal dose of reactive compounds or their metabolites and are an integral part of science-based risk assessment. However, technical limitations have prevented comprehensive detection of a broad spectrum of adducts simultaneously. Therefore, most studies have focused on measurement of abundant individual adducts. These studies have produced valuable insight into the metabolism of individual carcinogens, but they are insufficient for risk assessment of exposure to complex mixtures. To overcome this limitation, we present herein proof-of-principle for comprehensive exposure assessment, using N-terminal valine adduct profiles as a biomarker. The reported method is based on our previously established immunoaffinity liquid chromatography-tandem mass spectrometry (LC-MS/MS) method with modification to enrich all N-terminal valine alkylated peptides. The method was evaluated using alkylated peptide standards and globin reacted in vitro with alkylating agents (1,2-epoxy-3-butene, 1,2:3,4-diepoxybutane, propylene oxide, styrene oxide, N-ethyl-N-nitrosourea and methyl methanesulfonate), known to form N-terminal valine adducts. To demonstrate proof-of-principle, the method was successfully applied to globin from mice treated with four model compounds. The results suggest that this novel approach might be suitable for in vivo biomonitoring.

Detection of electrophile-sensitive proteins

BACKGROUND

Redox signaling is an important emerging mechanism of cellular function. Dysfunctional redox signaling is increasingly implicated in numerous pathologies, including atherosclerosis, diabetes, and cancer. The molecular messengers in this type of signaling are reactive species which can mediate the post-translational modification of specific groups of proteins, thereby effecting functional changes in the modified proteins. Electrophilic compounds comprise one class of reactive species which can participate in redox signaling. Electrophiles modulate cell function via formation of covalent adducts with proteins, particularly cysteine residues.

SCOPE OF REVIEW

This review will discuss the commonly used methods of detection for electrophile-sensitive proteins, and will highlight the importance of identifying these proteins for studying redox signaling and developing novel therapeutics.

MAJOR CONCLUSIONS

There are several methods which can be used to detect electrophile-sensitive proteins. These include the use of tagged model electrophiles, as well as derivatization of endogenous electrophile-protein adducts.

GENERAL SIGNIFICANCE

In order to understand the mechanisms by which electrophiles mediate redox signaling, it is necessary to identify electrophile-sensitive proteins and quantitatively assess adduct formation. Strengths and limitations of these methods will be discussed. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Bifunctional electrophiles cross-link thioredoxins with redox relay partners in cells

Thioredoxin protects cells against oxidative damage by reducing disulfide bonds in improperly oxidized proteins. Previously, we found that the baker's yeast cytosolic thioredoxin Trx2 undergoes cross-linking to form several protein-protein complexes in cells treated with the bifunctional electrophile divinyl sulfone (DVSF). Here, we report that the peroxiredoxin Tsa1 and the thioredoxin reductase Trr1, both of which function in a redox relay network with thioredoxin, become cross-linked in complexes with Trx2 upon DVSF treatment. Treatment of yeast with other bifunctional electrophiles, including diethyl acetylenedicarboxylate (DAD), mechlorethamine (HN2), and 1,2,3,4-diepoxybutane (DEB), resulted in the formation of similar cross-linked complexes. Cross-linking of Trx2 and Tsa1 to other proteins by DVSF and DAD is dependent on modification of the active site Cys residues within these proteins. In addition, the human cytosolic thioredoxin, cytosolic thioredoxin reductase, and peroxiredoxin 2 form cross-linked complexes to other proteins in the presence of DVSF, although each protein shows different susceptibilities to modification by DAD, HN2, and DEB. Taken together, our results indicate that bifunctional electrophiles potentially disrupt redox homeostasis in yeast and human cells by forming cross-linked complexes between thioredoxins and their redox partners.

Capturing labile sulfenamide and sulfinamide serum albumin adducts of carcinogenic arylamines by chemical oxidation

Aromatic amines and heterocyclic aromatic amines (HAAs) are a class of structurally related carcinogens that are formed during the combustion of tobacco or during the high temperature cooking of meats. These procarcinogens undergo metabolic activation by N-oxidation of the exocyclic amine group to produce N-hydroxylated metabolites, which are critical intermediates implicated in toxicity and DNA damage. The arylhydroxylamines and their oxidized arylnitroso derivatives can also react with cysteine (Cys) residues of glutathione or proteins to form, respectively, sulfenamide and sulfinamide adducts. However, sulfur-nitrogen linked adducted proteins are often difficult to detect because they are unstable and undergo hydrolysis during proteolytic digestion. Synthetic N-oxidized intermediates of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a carcinogenic HAA produced in cooked meats, and 4-aminobiphenyl, a carcinogenic aromatic amine present in tobacco smoke, were reacted with human serum albumin (SA) and formed labile sulfenamide or sulfinamide adducts at the Cys(34) residue. Oxidation of the carcinogen-modified SA with m-chloroperoxybenzoic acid (m-CPBA) produced the arylsulfonamide adducts, which were stable to heat and the chemical reduction conditions employed to denature SA. The sulfonamide adducts of PhIP and 4-ABP were identified, by liquid chromatography/mass spectrometry, in proteolytic digests of denatured SA. Thus, selective oxidation of arylamine-modified SA produces stable arylsulfonamide-SA adducts, which may serve as biomarkers of these tobacco and dietary carcinogens.

Mapping serum albumin adducts of the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by data-dependent tandem mass spectrometry

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a heterocyclic aromatic amine that is formed during the cooking of meats. PhIP is a potential human carcinogen: it undergoes metabolic activation to form electrophilic metabolites that bind to DNA and proteins, including serum albumin (SA). The structures of PhIP-SA adducts formed in vivo are unknown and require elucidation before PhIP protein adducts can be implemented as biomarkers in human studies. We previously examined the reaction of genotoxic N-oxidized metabolites of PhIP with human SA in vitro and identified covalent adducts formed at cysteine³⁴ (Cys³⁴); however, other adduction products were thought to occur. We have now identified adducts of PhIP formed at multiple sites of SA reacted with isotopic mixtures of electrophilic metabolites of PhIP and 2-amino-1-methyl-6-[²H₅]-phenylimidazo[4,5-b]pyridine ([²H₅]-PhIP). The metabolites used for study were 2-nitro-1-methyl-6-phenylimidazo[4,5-b]pyridine (NO₂-PhIP), 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), or N-acetyloxy-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-acetoxy-PhIP). Following proteolytic digestion, PhIP-adducted peptides were separated by ultra performance liquid chromatography and characterized by ion trap mass spectrometry, employing isotopic data-dependent scanning. Analysis of the tryptic or tryptic/chymotryptic digests of SA modified with NO₂-PhIP revealed that adduction occurred at Cys³⁴, Lys¹⁹⁵, Lys¹⁹⁹, Lys³⁵¹, Lys⁵⁴¹, Tyr¹³⁸, Tyr¹⁵⁰, Tyr⁴⁰¹, and Tyr⁴¹¹, whereas the only site of HONH-PhIP adduction was detected at Cys³⁴. N-Acetoxy-PhIP, a penultimate metabolite of PhIP that reacts with DNA to form covalent adducts, did not appear to form stable adducts with SA; instead, PhIP and 2-amino-1-methyl-6-(5-hydroxy)-phenylimidazo[4,5-b]pyridine, an aqueous reaction product of the proposed nitrenium ion of PhIP, were recovered during the proteolysis of N-acetoxy-PhIP-modified SA. Some of these SA adduction products of PhIP may be implemented in molecular epidemiology studies to assess the role of well-done cooked meat, PhIP, and the risk of cancer.

In vitro interactions of biological nucleophiles with fluorotelomer unsaturated acids and aldehydes: fate and consequences

Fluorotelomer unsaturated aldehydes and acids (FTUALs and FTUCAs) are intermediate metabolites that form from the biotransformation of fluorotelomer-based chemicals. FTUALs and FTUCAs have been previously suggested to contribute to the toxicity associated with human exposure to fluorotelomer compounds by covalently binding to biological nucleophiles. However, the extent of their reactivity has only been assessed with glutathione. The purpose of the present study was to assess the reactivity of these intermediate metabolites with a series of nucleophilic amino acids and model proteins. In vitro experiments were carried out in an aqueous buffer system to determine the reactivity of nucleophilic amino acids with FTUCAs and FTUALs having varying fluorinated chain lengths. Using (19)F NMR spectroscopy to monitor the disappearance of the FTUCAs and FTUAL signals and the production of a fluoride signal, reaction rate constants were determined under pseudo-first-order conditions. The FTUCAs reacted only with cysteine with the following second order rate constants: 3.63 (± 1.37) × 10(-5) min(-1) mM(-1) (4:2 FTUCA), 1.19 (± 0.91) × 10(-5) min(-1) mM(-1) (6:2 FTUCA), and 4.56 (± 0.94) × 10(-5) min(-1) mM(-1) (8:2 FTUCA). The FTUALs were significantly more reactive than any of the FTUCAs with reactivity decreasing in the following order: cysteine >> histidine > lysine >> arginine. The following second-order rate constants were obtained: 5.7 (± 4.2) × 10(-4) min(-1) mM(-1) (histidine), 4.3 (± 1.4) × 10(-4) min(-1) mM(-1) (lysine), and 1.4 (± 0.73) × 10(-4) min(-1) mM(-1) (arginine). FTUCAs and FTUALs were also reacted with model proteins to assess their potential for forming covalent adducts. Electrospray ionization mass spectrometry (ESI-MS) was used to investigate the stoichiometry of FTUCAs and FTUALs covalently bound to apomyoglobin (ApoMg) and human serum albumin (HSA). FTUCAs were not reactive, whereas two measurable FTUAL adducts were formed with both ApoMg and HSA at each of the FTUAL chain lengths (6:2, 8:2, and 10:2). This is the first study to probe the reactivity of FTUALs and FTUCAs with nucleophiles other than glutathione, further elucidating possible FTUAL and FTUCA fate within biological systems.

Molecular mechanisms underlying chemical liver injury

The liver is necessary for survival. Its strategic localisation, blood flow and prominent role in the metabolism of xenobiotics render this organ particularly susceptible to injury by chemicals to which we are ubiquitously exposed. The pathogenesis of most chemical-induced liver injuries is initiated by the metabolic conversion of chemicals into reactive intermediate species, such as electrophilic compounds or free radicals, which can potentially alter the structure and function of cellular macromolecules. Many reactive intermediate species can produce oxidative stress, which can be equally detrimental to the cell. When protective defences are overwhelmed by excess toxicant insult, the effects of reactive intermediate species lead to deregulation of cell signalling pathways and dysfunction of biomolecules, leading to failure of target organelles and eventual cell death. A myriad of genetic factors determine the susceptibility of specific individuals to chemical-induced liver injury. Environmental factors, lifestyle choices and pre-existing pathological conditions also have roles in the pathogenesis of chemical liver injury. Research aimed at elucidating the molecular mechanism of the pathogenesis of chemical-induced liver diseases is fundamental for preventing or devising new modalities of treatment for liver injury by chemicals.

MALDI in-source decay, from sequencing to imaging

Matrix-assisted laser desorption/ionization (MALDI) is now a mature method allowing the identification and, more challenging, the quantification of biopolymers (proteins, nucleic acids, glycans, etc). MALDI spectra show mostly intact singly charged ions. To obtain fragments, the activation of singly charged precursors is necessary, but not efficient above 3.5 kDa, thus making MALDI MS/MS difficult for large species. In-source decay (ISD) is a prompt fragmentation reaction that can be induced thermally or by radicals. As fragments are formed in the source, precursor ions cannot be selected; however, the technique is not limited by the mass of the analyzed compounds and pseudo MS3 can be performed on intense fragments. The discovery of new matrices that enhance the ISD yield, combined with the high sensitivity of MALDI mass spectrometers, and software development, opens new perspectives. We first review the mechanisms involved in the ISD processes, then discuss ISD applications like top-down sequencing and post-translational modifications (PTMs) studies, and finally review MALDI-ISD tissue imaging applications.

Mass spectrometric characterization of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine N-oxidized metabolites bound at Cys34 of human serum albumin

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a heterocyclic aromatic amine that is formed during the cooking of meats and poultry. PhIP is a carcinogen in rodents and a potential human carcinogen. Several short-term biomarkers of PhIP have been established for human biomonitoring, but validated long-term biomarkers of the biologically effective dose of PhIP remain to be developed. Metabolites of PhIP have been reported to covalently bind to human serum albumin (SA), which is the most abundant protein in plasma; however, the chemical structures of PhIP-SA adducts are unknown. Cysteine(34) is one of 35 conserved Cys residues in SA across species. Thirty-four of these Cys are involved in 17 disulfide bonds. The single unpaired Cys(34) residue in SA is well-known to react with carcinogenic metabolites and toxic electrophiles. 2-Nitro-1-methyl-6-phenylimidazo[4,5-b]pyridine (NO(2)-PhIP), 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), and 2-nitroso-1-methyl-6-phenylimidazo[4,5-b]pyridine (NO-PhIP), three genotoxic metabolites of PhIP, were reacted with purified human SA or human plasma, and the SA adduction products, following enzymatic digestion, were separated by ultra performance liquid chromatography and characterized with a linear quadrupole ion trap mass spectrometer. The major adduct of NO(2)-PhIP was formed at the Cys(34) of SA with bond formation occurring between the sulfhydryl group of Cys and the C-2 imidazole atom of PhIP. The major adducts formed between SA and HNOH-PhIP or NO-PhIP were identified as acid-labile sulfinamide linkages at Cys(34). These PhIP-SA adducts represent a measure of bioactivation of PhIP and may serve as long-term biomarkers of the biologically effective dose of PhIP.

Analysis of hemoglobin adducts from acrylamide, glycidamide, and ethylene oxide in paired mother/cord blood samples from Denmark

The knowledge about fetal exposure to acrylamide/glycidamide from the maternal exposure through food is limited. Acrylamide, glycidamide, and ethylene oxide are electrophiles and form adducts with hemoglobin (Hb), which could be used for in vivo dose measurement. In this study, a method for analysis of Hb adducts by liquid chromatography-mass spectrometry, the adduct FIRE procedure, was applied to measurements of adducts from these compounds in maternal blood samples (n = 87) and umbilical cord blood samples (n = 219). The adduct levels from the three compounds, acrylamide, glycidamide, and ethylene oxide, were increased in tobacco smokers. Highly significant correlations were found between cord and maternal blood with regard to measured adduct levels of the three compounds. The mean cord/maternal hemoglobin adduct level ratios were 0.48 (range 0.27-0.86) for acrylamide, 0.38 (range 0.20-0.73) for glycidamide, and 0.43 (range 0.17-1.34) for ethylene oxide. In vitro studies with acrylamide and glycidamide showed a lower (0.38-0.48) rate of adduct formation with Hb in cord blood than with Hb in maternal blood, which is compatible with the structural differences in fetal and adult Hb. Together, these results indicate a similar life span of fetal and maternal erythrocytes. The results showed that the in vivo dose in fetal and maternal blood is about the same and that the placenta gives negligible protection of the fetus to exposure from the investigated compounds. A trend of higher levels of the measured adducts in cord blood with gestational age was observed, which may reflect the gestational age-related change of the cord blood Hb composition toward a higher content of adult Hb. The results suggest that the Hb adduct levels measured in cord blood reflect the exposure to the fetus during the third trimester. The evaluation of the new analytical method showed that it is suitable for monitoring of background exposures of the investigated electrophilic compounds in large population studies.

Adductomics: characterizing exposures to reactive electrophiles

To understand environmental causes of disease, unbiased methods are needed to characterize the human exposome, which represents all toxicants to which people are exposed from both exogenous and endogenous sources. Because they directly modify DNA and important proteins, reactive electrophiles are probably the most important constituents of the exposome. Exposures to reactive electrophiles can be characterized by measuring adducts from reactions between circulating electrophiles and blood nucleophiles. We define an 'adductome' as the totality of such adducts with a given nucleophilic target. Because of their greater abundance and residence times in human blood, adducts of hemoglobin (Hb) and human serum albumin (HSA) are preferable to those of DNA and glutathione for characterizing adductomes. In fact, the nucleophilic hotspot represented by the only free sulfhydryl group in HSA (HSA-Cys(34)) offers particular advantages for adductomic experiments. Although targeted adducts of HSA-Cys(34) have been monitored for decades, an unbiased method has only recently been reported for visualizing the HSA-Cys(34) 'subadductome'. The method relies upon a novel mass spectrometry application, termed fixed-step selected reaction monitoring (FS-SRM), to profile Cys(34) adducts in tryptic digests of HSA. Here, we selectively review the literature regarding the potential of adductomics to partially elucidate the human exposome, with particular attention to the HSA-Cys(34) subadductome.

Strategy for identification and detection of multiple oxidative modifications within proteins applied on persulfate-oxidized hemoglobin and human serum albumin

Oxidative stress has been suggested as an underlying mechanism of many human diseases. However, definitive evidence for this association has not been presented due to different shortcomings of the methods used to measure biomarkers of oxidative stress. Persulfates are oxidizing agents known to elicit hypersensitive reactions from the airways and skin. Despite a frequent use of persulfates at many work places, no biomarkers for persulfate exposure are available. The aim of this study was to develop a strategy for the identification and detection of multiple oxidative modifications within proteins. This strategy was applied on persulfate-oxidized proteins to identify oxidized peptides suitable for further investigation as biomarkers of persulfate exposure or oxidative stress. A strategy for the identification and the relative quantification of multiple oxidative modifications within proteins was developed. The usage of two software packages facilitated the search for modified peptides to a great extent. Oxidized peptides were relatively quantified using liquid chromatography/tandem mass spectrometry in selected reaction monitoring mode. The result showed that persulfates oxidize tryptophans and methionines resulting in mass shifts of 16 and/or 32 Da. Also, oxidized albumin peptides in nasal lavage fluid samples from subjects challenged with persulfate were detected. The oxidation degree before and after challenge remained constant for peptides containing methionine sulfoxide. For peptides containing oxidized tryptophan the oxidation degree increased after exposure. Some of these oxidized peptides may be suitable as biomarkers; however, further evaluation is required.

Implications of the exposome for exposure science

During the 1920s, the forerunners of exposure science collaborated with health professionals to investigate the causes of occupational diseases. With the birth of U.S. regulatory agencies in the 1970s, interest in the environmental origins of human diseases waned, and exposure scientists focused instead upon levels of selected contaminants in air and water. In fact, toxic chemicals enter the body not only from exogenous sources (air, water, diet, drugs, and radiation) but also from endogenous processes, including inflammation, lipid peroxidation, oxidative stress, existing diseases, infections, and gut flora. Thus, even though current evidence suggests that non-genetic factors contribute about 90% of the risks of chronic diseases, we have not explored the vast majority of human exposures that might initiate disease processes. The concept of the exposome, representing the totality of exposures received by a person during life, encompasses all sources of toxicants and, therefore, offers scientists an agnostic approach for investigating the environmental causes of chronic diseases. In this context, it is appropriate to regard the "environment" as the body's internal chemical environment and to define "exposures" as levels of biologically active chemicals in this internal environment. To explore the exposome, it makes sense to employ a top-down approach based upon biomonitoring (e.g. blood sampling) rather than a bottom-up approach that samples air, water, food, and so on. Because sources and levels of exposure change over time, exposomes can be constructed by analyzing toxicants in blood specimens obtained during critical stages of life. Initial investigations could use archived blood from prospective cohort studies to measure important classes of toxic chemicals, notably, reactive electrophiles, metals, metabolic products, hormone-like substances, and persistent organic compounds. The exposome offers health scientists an avenue for integrating research that is currently fractured along lines related to particular diseases and risk factors, and can thereby promote discovery of the key exposures responsible for chronic diseases. By embracing the exposome as its operational paradigm, exposure science can play a major role in discovering and mitigating these exposures.

Profiling Cys34 adducts of human serum albumin by fixed-step selected reaction monitoring

A method is described for profiling putative adducts (or other unknown covalent modifications) at the Cys34 locus of human serum albumin (HSA), which represents the preferred reaction site for small electrophilic species in human serum. By comparing profiles of putative HSA-Cys34 adducts across populations of interest it is theoretically possible to explore environmental causes of degenerative diseases and cancer caused by both exogenous and endogenous chemicals. We report a novel application of selected-reaction-monitoring (SRM) mass spectrometry, termed fixed-step SRM (FS-SRM), that allows detection of essentially all HSA-Cys34 modifications over a specified range of mass increases (added masses). After tryptic digestion, HSA-Cys34 adducts are contained in the third largest peptide (T3), which contains 21 amino acids and an average mass of 2433.87 Da. The FS-SRM method does not require that exact masses of T3 adducts be known in advance but rather uses a theoretical list of T3-adduct m/z values separated by a fixed increment of 1.5. In terms of added masses, each triply charged parent ion represents a bin of ±2.3 Da between 9.1 Da and 351.1 Da. Synthetic T3 adducts were used to optimize FS-SRM and to establish screening rules based upon selected b- and y-series fragment ions. An isotopically labeled T3 adduct is added to protein digests to facilitate quantification of putative adducts. We used FS-SRM to generate putative adduct profiles from six archived specimens of HSA that had been pooled by gender, race, and smoking status. An average of 66 putative adduct hits (out of a possible 77) were detected in these samples. Putative adducts covered a wide range of concentrations, were most abundant in the mass range below 100 Da, and were more abundant in smokers than in nonsmokers. With minor modifications, the FS-SRM methodology can be applied to other nucleophilic sites and proteins.

Toxicogenomic profiling of chemically exposed humans in risk assessment

Gene-environment interactions contribute to complex disease development. The environmental contribution, in particular low-level and prevalent environmental exposures, may constitute much of the risk and contribute substantially to disease. Systematic risk evaluation of the majority of human chemical exposures, has not been conducted and is a goal of regulatory agencies in the U.S. and worldwide. With the recent recognition that toxicological approaches more predictive of effects in humans are required for risk assessment, in vitro human cell line data as well as animal data are being used to identify toxicity mechanisms that can be translated into biomarkers relevant to human exposure studies. In this review, we discuss how data from toxicogenomic studies of exposed human populations can inform risk assessment, by generating biomarkers of exposure, early effect, and/or susceptibility, elucidating mechanisms of action underlying exposure-related disease, and detecting response at low doses. Good experimental design incorporating precise, individual exposure measurements, phenotypic anchors (pre-disease or traditional toxicological markers), and a range of relevant exposure levels, is necessary. Further, toxicogenomic studies need to be designed with sufficient power to detect true effects of the exposure. As more studies are performed and incorporated into databases such as the Comparative Toxicogenomics Database (CTD) and Chemical Effects in Biological Systems (CEBS), data can be mined for classification of newly tested chemicals (hazard identification), and, for investigating the dose-response, and inter-relationship among genes, environment and disease in a systems biology approach (risk characterization).

Protein adducts as prospective biomarkers of nevirapine toxicity

Nevirapine (NVP) is a non-nucleoside reverse transcriptase inhibitor used against human immunodeficiency virus type-1 (HIV-1), mostly to prevent mother-to-child HIV-1 transmission in developing countries. Despite its clinical efficacy, NVP administration is associated with a variety of toxic responses that include hepatotoxicity and skin rash. Although the reasons for the adverse effects of NVP administration are still unclear, increasing evidence supports the involvement of metabolic activation to reactive electrophiles. In particular, Phase II activation of the NVP metabolite 12-hydroxy-NVP is thought to mediate NVP binding to bionucleophiles, which may be at the onset of toxicity. In the present study, we investigated the nature and specific locations of the covalent adducts produced in human serum albumin and human hemoglobin by reaction in vitro with the synthetic model electrophile 12-mesyloxy-NVP, used as a surrogate for the Phase II metabolite 12-sulfoxy-NVP. Multiple sites of modification were identified by two different mass spectrometry-based methodologies, liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) and matrix-assisted laser desorption ionization tandem mass spectrometry (MALDI-TOF-TOF-MS). These two distinct methodologies, which in some instances afforded complementary information, allowed the identification of multiple adducts involving cysteine, lysine, tryptophan, histidine, serine, and the N-terminal valine of hemoglobin. Tryptophan, which is not a common site of covalent protein modification, was the NVP-modified amino acid residue detected in the two proteins and consistently identified by both LC-ESI-MS/MS and MALDI-TOF-TOF-MS. The propensity of tryptophan to react with the NVP-derived electrophile is further emphasized by the fact that human serum albumin possesses a single tryptophan residue, which suggests a remarkable selectivity that may be useful for biomonitoring purposes. Likewise, the NVP adduct with the terminal valine of hemoglobin, detected by LC-ESI-MS/MS after N-alkyl Edman degradation, appears as an easily assessed marker of NVP binding to proteins. Our results demonstrate the merits and complementarity of the two MS-based methodologies for the characterization of protein binding by NVP and suggest a series of plausible biomarkers of NVP toxicity that should be useful in the monitoring of toxicity effects in patients administered NVP.

Epidemiology. Environment and disease risks

A new paradigm is needed to assess how a lifetime of exposure to environmental factors affects the risk of developing chronic diseases.

Metabolomic profiling can predict which humans will develop liver dysfunction when deprived of dietary choline

Choline is an essential nutrient, and deficiency causes liver and muscle dysfunction. Common genetic variations alter the risk of developing organ dysfunction when choline deficient, probably by causing metabolic inefficiencies that should be detectable even while ingesting a normal choline-adequate diet. We determined whether metabolomic profiling of plasma at baseline could predict whether humans will develop liver dysfunction when deprived of dietary choline. Fifty-three participants were fed a diet containing 550 mg choline/70 kg/d for 10 d and then fed < 50 mg choline/70 kg/d for up to 42 d. Participants who developed organ dysfunction on this diet were repleted with a choline-adequate diet for > or = 3 d. Plasma samples, obtained at baseline, end of depletion, and end of repletion, were used for targeted and nontargeted metabolomic profiling. Liver fat was assessed using magnetic resonance spectroscopy. Metabolomic profiling and targeted biochemical analyses were highly correlated for the analytes assessed by both procedures. In addition, we report relative concentration changes of other small molecules detected by the nontargeted metabolomic analysis after choline depletion. Finally, we show that metabolomic profiles of participants when they were consuming a control baseline diet could predict whether they would develop liver dysfunction when deprived of dietary choline.

Enrichment of cysteinyl adducts of human serum albumin

We report a method to enrich cysteinyl adducts of human serum albumin (HSA), representing biomarkers of exposure to systemic electrophiles. Because the major site of HSA adduction is the single free sulfhydryl group at Cys34, we used thiol-affinity resins to remove mercaptalbumin (i.e., unadducted HSA) from the cysteinyl adducts. Electrospray ionization mass spectrometry was used to detect mercaptalbumin and HSA-Cys34 modifications before and after enrichment of HSA. Differences in adduct content were detected across samples of freshly isolated, archived, and commercial HSA. Cysteinylated and glycosylated adducts were present in all samples, with abundances decreasing in the following order: commercial HSA>archived HSA>fresh HSA. After enrichment of HSA, mercaptalbumin was no longer observed in mass spectra. The ratios of HSA adducts post-/preenrichment, quantified via the Bradford assay and gel electrophoresis, were 0.029 mg adducts/mg HSA in fresh HSA and 0.323 mg adducts/mg HSA in archived HSA. The apparent elevation of adduct levels in archived samples could be due to differences in specimen preparation and storage rather than to differences in circulating HSA adducts. We conclude that thiol-affinity resins can efficiently remove mercaptalbumin from HSA samples prior to characterization and quantitation of protein adducts of reactive systemic electrophiles.