Protein tyrosine adduct in humans self-poisoned by chlorpyrifos

Dual-mode supramolecular fluorescent probe for rapid and on-site detection of chlorpyrifos in the environment

Chlorpyrifos (CPF) as a classic organophosphorus pesticide has been widely used in agricultural applications to control insects and worms. CPF in the environment can cause deaths of diverse kinds of aquatic organism and bring a high risk to human health. Therefore, the development of effective analytical method for CPF is of great importance. In this work, a novel dual-mode albumin (ALB)-based supramolecular probe FD@ALB was designed and prepared for rapid detection of CPF in the environment. The limit of detection is 0.57 μM (∼ 0.2 ppm) with a wider detection range up to 200 μM, which is satisfactory for application. The sensing mechanism can be ascribed to CPF-induced phosphorylation of ALB, thus leading to a change in the binding microenvironment of FD dye. Moreover, the paper-based test strips were used in conjunction with the FD@ALB, realizing the portable detection of CPF. This method was demonstrated to be suitable for on-site detection of CPF in various kinds of environmental samples, including water, soil, and food samples, with the aid of a smartphone. To the best of our knowledge, this is the first analytical method achieving a combination of the rapid and ratiometric detection of CPF in the environment.

Review: Organophosphorus toxicants, in addition to inhibiting acetylcholinesterase activity, make covalent adducts on multiple proteins and promote protein crosslinking into high molecular weight aggregates

The acute effects of exposure to organophosphorus toxicants are explained by inhibition of acetylcholinesterase activity. However, the mechanisms that explain long term illness associated with organophosphorus exposure are still under investigation. We find that organophosphorus nerve agents and organophosphorus pesticides make covalent adducts not only on the serine from acetylcholinesterase, but also on tyrosine, lysine, glutamate, serine and threonine from a variety of proteins. Almost any protein can be modified by a high dose of organophosphorus toxicant. A low dose of 10 μM chlorpyrifos oxon added to the serum-free culture medium of human neuroblastoma SH-SY5Y cells resulted in tyrosine adducts on 48 proteins immunopurified from the cell lysate. We identified the adducted proteins by mass spectrometry after immunopurifying modified proteins with a rabbit anti-diethoxyphospho-tyrosine monoclonal antibody which biased this study for tyrosine adducts. In cultured cells, the primary organophosphate targets are abundant proteins. Organophosphate-modified proteins may disrupt physiological processes. In separate experiments we identified organophosphate adducts on lysine. Organophosphylation activates the lysine for protein crosslinking. The activated lysine reacts with glutamic acid or aspartic acid protein side chains to form an isopeptide bond between proteins, resulting in high molecular weight crosslinked proteins. Crosslinked proteins form insoluble aggregates that may lead to neurogenerative disease.

Phosphonylated tyrosine and lysine residues as biomarkers of local exposure of human hair to the organophosphorus nerve agents sarin and VX

We herein present for the first time a micro liquid chromatography-electrospray ionization high-resolution tandem-mass spectrometry (μLC-ESI MS/HR MS) procedure to detect phosphonylated tyrosine (Tyr) and lysine (Lys) residues obtained from human hair exposed to organophosphorus nerve agents (OPNA). In general, toxic OPNA react with endogenous blood proteins causing the formation of adducts representing well-known targets for biomedical analysis to prove exposure. In contrast, no protein-derived biomarker has been introduced so far to document local exposure of hair. Accordingly, we developed and characterized a μLC-ESI MS/HR MS method for the analysis of scalp hair exposed to OPNA in vitro. Type I and Type II keratin from hair was dissolved during lysis, precipitated and subjected to pronase-catalyzed hydrolysis yielding single adducted Lys and in a much higher amount Tyr residues. Exposure to sarin caused the adduction of an isopropyl methylphosphonic acid moiety and exposure to VX yielded adducts of ethyl methylphosphonic acid, well suited as biomarkers of exposure. These were of appropriate stability in the autosampler for 24 h. The biomarker yield obtained from hair of six individuals as well as from hair of six different parts of the body of one individual (armpit, beard, leg, arm, scalp, and pubic) differed reasonably indicating the variable individual protein composition and structure of hair. Exposed hair stored at ambient temperature for 9 weeks with contact to air and daylight showed stability of all adducts and therefore their suitability for verification of exposure.

Chlorpyrifos Oxon Activates Glutamate and Lysine for Protein Cross-linking

Chronic low-dose exposure to organophosphorus (OP) toxicants is correlated with an increase in the risk of impaired cognition and neurodegenerative diseases. A mechanism to explain this relationship is needed. We suggest that the formation of organophosphate-induced high-molecular-weight protein aggregates that disrupt cell function may be the missing link. It has been demonstrated that such aggregation can be promoted by OP-labeled lysine. Alternatively, OP-labeled glutamate may be the initiator. To test this hypothesis, we treated MAP-rich tubulin Sus scrofa and human transglutaminase with chlorpyrifos oxon. Trypsin-digested proteins were subjected to liquid chromatography-tandem mass spectrometry followed by Protein Prospector searches to identify diethyl phosphate adducts and cross-linked peptides. We report the presence of diethyl phosphate adducts on the side chains of glutamate, lysine, and tyrosine, as well as cross-links between glutamate and lysine. Glutamate-lysine cross-linking could be initiated either by diethyl phosphate-activated glutamate or by diethyl phosphate-activated lysine to form stable isopeptide bonds between and within proteins. It was concluded that organophosphate-induced high-molecular-weight protein aggregates could promote brain dysfunction.

Quo vadis blood protein adductomics?

Chemicals are measured regularly in air, food, the environment, and the workplace. Biomonitoring of chemicals in biological fluids is a tool to determine the individual exposure. Blood protein adducts of xenobiotics are a marker of both exposure and the biologically effective dose. Urinary metabolites and blood metabolites are short term exposure markers. Stable hemoglobin adducts are exposure markers of up to 120 days. Blood protein adducts are formed with many xenobiotics at different sites of the blood proteins. Newer methods apply the techniques developed in the field of proteomics. Larger adducted peptides with 20 amino acids are used for quantitation. Unfortunately, at present the methods do not reach the limits of detection obtained with the methods looking at single amino acid adducts or at chemically cleaved adducts. Therefore, to progress in the field new approaches are needed.

Rabbit Antidiethoxyphosphotyrosine Antibody, Made by Single B Cell Cloning, Detects Chlorpyrifos Oxon-Modified Proteins in Cultured Cells and Immunopurifies Modified Peptides for Mass Spectrometry

Chronic low-dose exposure to organophosphorus pesticides is associated with the risk of neurodegenerative disease. The mechanism of neurotoxicity is independent of acetylcholinesterase inhibition. Adducts on tyrosine, lysine, threonine, and serine can occur after exposure to organophosphorus pesticides, the most stable being adducts on tyrosine. Rabbit monoclonal 1C6 to diethoxyphosphate-modified tyrosine (depY) was created by single B cell cloning. The amino acid sequence and binding constant (K_d 3.2 × 10^–8 M) were determined. Cultured human neuroblastoma SH-SY5Y and mouse neuroblastoma N2a cells incubated with a subcytotoxic dose of 10 μM chlorpyrifos oxon contained depY-modified proteins detected by monoclonal 1C6 on Western blots. depY-labeled peptides from tryptic digests of cell lysates were immunopurified by binding to immobilized 1C6. Peptides released with 50% acetonitrile and 1% formic acid were analyzed by liquid chromatography tandem mass spectrometry (LC-MS/MS) on an Orbitrap Fusion Lumos mass spectrometer. Protein Prospector database searches identified 51 peptides modified on tyrosine by diethoxyphosphate in SH-SY5Y cell lysate and 73 diethoxyphosphate-modified peptides in N2a cell lysate. Adducts appeared most frequently on the cytoskeleton proteins tubulin, actin, and vimentin. It was concluded that rabbit monoclonal 1C6 can be useful for studies that aim to understand the mechanism of neurotoxicity resulting from low-dose exposure to organophosphorus pesticides.

Adaptive Immune Responses Associated with the Central Nervous System Pathology of Gulf War Illness

Gulf War Illness is a multisymptomatic condition which affects 30% of veterans from the 1991 Gulf War. While there is evidence for a role of peripheral cellular and humoral adaptive immune responses in Gulf War Illness, a potential role of the adaptive immune system in the central nervous system pathology of this condition remains unknown. Furthermore, many of the clinical features of Gulf War Illness resembles those of autoimmune diseases, but the biological processes are likely different as the etiology of Gulf War Illness is linked to hazardous chemical exposures specific to the Gulf War theatre. This review discusses Gulf War chemical–induced maladaptive immune responses and a potential role of cellular and humoral immune responses that may be relevant to the central nervous system symptoms and pathology of Gulf War Illness. The discussion may stimulate investigations into adaptive immunity for developing novel therapies for Gulf War Illness.

Identification of 2-(diethylamino)ethylthiol dipeptide (Cys-Pro) adduct as biomarker of nerve agents VR and CVX in human plasma using liquid chromatography-high-resolution tandem mass spectrometry

Organophosphorus nerve agents pose a significant threat to human health. The most toxic compounds in this class include V-type poisonous substances such as VX, CVX, and VR. Although all stockpiles of this type of substance are subject to destruction under the Chemical Weapons Convention (CWC), there is still a risk that they could be used for criminal and terrorist purposes. The latter determines the relevance of studies aimed at identification of biomarkers that may indicate the exposure of these group substances to the organism. A liquid chromatography mass spectrometry/high-resolution mass spectrometry (LC-MS/HR MS) method for determination of trace amounts of nerve agents such as VR and CVX in human plasma was proposed. The method is based on enzymatic plasma hydrolysis with the use of pronase to form a stable adduct of 2-(diethylamino)ethylthiol with dipeptide cysteine-proline (DEAET-CP) with its subsequent determination by LC-MS/HR MS. Synthesis of DEAET-CP as reference compound was conducted using non-toxic precursors. Sample preparation of human blood plasma samples exposed to VR was carried out with the use of solid-phase extraction (SPE). Liquid chromatography (LC) separation on the reversed-phase column and mass spectrometric detection (selection of optimal transitions and detection modes) were performed. The achieved limit of detection (LOD) of VR (in the form of DEAET-CP) in human blood plasma was 0.05 ng mL^-1. The proposed approach was developed using plasma samples exposed to VR and CVX obtained in the frame of the Fifth Official Biomedical Test of the Organization for the Prohibition of Chemical Weapons (OPCW) and showed good specificity of detection.

Mass Spectral Detection of Diethoxyphospho-Tyrosine Adducts on Proteins from HEK293 Cells Using Monoclonal Antibody depY for Enrichment

Chronic illness from exposure to organophosphorus toxicants is hypothesized to involve modification of unknown proteins. Tyrosine in proteins that have no active site serine readily reacts with organophosphorus toxicants. We developed a monoclonal antibody, depY, that specifically recognizes diethoxyphospho-tyrosine in proteins and peptides, independent of the surrounding amino acid sequence. Our goal in the current study was to identify diethoxyphosphorylated proteins in human HEK293 cell lysate treated with chlorpyrifos oxon. Cell lysates treated with chlorpyrifos oxon were recognized by depY antibody in ELISA and capillary electrophoresis based Western blot. Tryptic peptides were analyzed by liquid chromatography tandem mass spectrometry. Liquid chromatography tandem mass spectrometry identified 116 diethoxyphospho-tyrosine peptides from 73 proteins in immunopurified samples, but found only 15 diethoxyphospho-tyrosine peptides from 12 proteins when the same sample was not immunopurified on depY. The most abundant proteins in the cell lysate, histone H4, heat shock 70 kDa protein 1A/1B, heat shock protein HSP 90 β, and α-enolase, were represented by several diethoxyphospho-tyrosine peptides. It was concluded that use of immobilized depY improved the number of diethoxyphospho-tyrosine peptides identified in a complex mixture. The mass spectrometry results confirmed the specificity of depY for diethoxyphospho-tyrosine peptides independent of the context of the modified tyrosine, which means depY could be used to analyze modified proteins in any species. Use of the depY antibody could lead to an understanding of chronic illness from organophosphorus pesticide exposure.

Monoclonal Antibody That Recognizes Diethoxyphosphotyrosine-Modified Proteins and Peptides Independent of Surrounding Amino Acids

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are irreversibly inhibited by organophosphorus pesticides through formation of a covalent bond with the active site serine. Proteins that have no active site serine, for example albumin, are covalently modified on tyrosine and lysine. Chronic illness from pesticide exposure is not explained by inhibition of AChE and BChE. Our goal was to produce a monoclonal antibody that recognizes proteins diethoxyphosphorylated on tyrosine. Diethoxyphosphate-tyrosine adducts for 13 peptides were synthesized. The diethoxyphosphorylated (OP) peptides cross-linked to four different carrier proteins were used to immunize, boost, and screen mice. Monoclonal antibodies were produced with hybridoma technology. Monoclonal antibody depY was purified and characterized by ELISA, western blotting, Biacore, and Octet technology to determine binding affinity and binding specificity. DepY recognized diethoxyphosphotyrosine independent of the amino acid sequence around the modified tyrosine and independent of the identity of the carrier protein or peptide. It had an IC₅₀ of 3 × 10^-9 M in a competition assay with OP tubulin. K_d values measured by Biacore and OctetRED96 were 10^-8 M for OP-peptides and 1 × 10^-12 M for OP-proteins. The limit of detection measured on western blots hybridized with 0.14 μg/mL of depY was 0.025 μg of human albumin conjugated to YGGFL-OP. DepY was specific for diethoxyphosphotyrosine (chlorpyrifos oxon adduct) as it failed to recognize diethoxyphospholysine, phosphoserine, phosphotyrosine, phosphothreonine, dimethoxyphosphotyrosine (dichlorvos adduct), dimethoxyphosphoserine, monomethoxyphosphotyrosine (aged dichlorvos adduct), and cresylphosphoserine. In conclusion, a monoclonal antibody that specifically recognizes diethoxyphosphotyrosine adducts has been developed. The depY monoclonal antibody could be useful for identifying new biomarkers of OP exposure.

Immunopurification of Acetylcholinesterase from Red Blood Cells for Detection of Nerve Agent Exposure

Nerve agents and organophosphorus pesticides make a covalent bond with the active site serine of acetylcholinesterase (AChE), resulting in inhibition of AChE activity and toxic symptoms. AChE in red blood cells (RBCs) serves as a surrogate for AChE in the nervous system. Mass spectrometry analysis of adducts on RBC AChE could provide evidence of exposure. Our goal was to develop a method of immunopurifying human RBC AChE in quantities adequate for detecting exposure by mass spectrometry. For this purpose, we immobilized 3 commercially available anti-human acetylcholinesterase monoclonal antibodies (AE-1, AE-2, and HR2) plus 3 new monoclonal antibodies. The monoclonal antibodies were characterized for binding affinity, epitope mapping by pairing analysis, and nucleotide and amino acid sequences. AChE was solubilized from frozen RBCs with 1% (v/v) Triton X-100. A 16 mL sample containing 5.8 μg of RBC AChE was treated with a quantity of soman model compound that inhibited 50% of the AChE activity. Native and soman-inhibited RBC AChE samples were immunopurified on antibody–Sepharose beads. The immunopurified RBC AChE was digested with pepsin and analyzed by liquid chromatography tandem mass spectrometry on a 6600 Triple-TOF mass spectrometer. The aged soman-modified PheGlyGluSerAlaGlyAlaAlaSer (FGESAGAAS) peptide was detected using a targeted analysis method. It was concluded that all 6 monoclonal antibodies could be used to immunopurify RBC AChE and that exposure to nerve agents could be detected as adducts on the active site serine of RBC AChE.

Biomonitoring Human Albumin Adducts: The Past, the Present, and the Future

Serum albumin (Alb) is the most abundant protein in blood plasma. Alb reacts with many carcinogens and/or their electrophilic metabolites. Studies conducted over 20 years ago showed that Alb forms adducts with the human carcinogens aflatoxin B1 and benzene, which were successfully used as biomarkers in molecular epidemiology studies designed to address the role of these chemicals in cancer risk. Alb forms adducts with many therapeutic drugs or their reactive metabolites such as β-lactam antibiotics, acetylsalicylic acid, acetaminophen, nonsteroidal anti-inflammatory drugs, chemotherapeutic agents, and antiretroviral therapy drugs. The identification and characterization of the adduct structures formed with Alb have served to understand the generation of reactive metabolites and to predict idiosyncratic drug reactions and toxicities. The reaction of candidate drugs with Alb is now exploited as part of the battery of screening tools to assess the potential toxicities of drugs. The use of gas chromatography-mass spectrometry, liquid chromatography, or liquid chromatography-mass spectrometry (LC-MS) enabled the identification and quantification of multiple types of Alb xenobiotic adducts in animals and humans during the past three decades. In this perspective, we highlight the history of Alb as a target protein for adduction to environmental and dietary genotoxicants, pesticides, and herbicides, common classes of medicinal drugs, and endogenous electrophiles, and the emerging analytical mass spectrometry technologies to identify Alb-toxicant adducts in humans.

Modification of human serum albumin by the nerve agent VX: microbore liquid chromatography/electrospray ionization high-resolution time-of-flight tandem mass spectrometry method for detection of phosphonylated tyrosine and novel cysteine containing disulfide adducts

RATIONALE

Organophosphorus nerve agents still constitute a considerable threat to the health of military personnel and the civilian population. Long-term biomarkers are crucial for reliable verification of exposure to banned substances. Therefore, current research focuses on identification of endogenous protein targets showing covalent modifications by organophosphorus nerve agents (adducts).

METHODS

Purified human serum albumin and human plasma were incubated with the nerve agent VX followed by enzymatic proteolysis with pronase. Resulting peptide cleavage products were separated by microbore liquid chromatography (μLC) online coupled to positive electrospray ionization (ESI) with subsequent high-resolution time-of-flight tandem mass spectrometry (HR MS/MS) allowing identification of known and novel adducts.

RESULTS

In addition to known phosphonylation of various tyrosine residues, albumin was found to be modified at diverse cysteine residues by covalent attachment of the leaving group of VX. These novel disulfide adducts were cleaved from at least two regions of the intact protein as dipeptides containing cysteine and proline either as CP or PC. A rapid and sensitive method was developed for simultaneous detection of the diverse covalent modifications of human albumin by VX.

CONCLUSIONS

Identification of the novel leaving group adducts with human albumin expands the basic knowledge on molecular toxicology of the nerve agent VX. Furthermore, the presented μLC/ESI HR MS/MS method might be of relevance for verification of VX poisoning. Copyright © 2016 John Wiley & Sons, Ltd.

Detection of Carbofuran-Protein Adducts in Serum of Occupationally Exposed Pesticide Factory Workers in Pakistan

This study was conducted to investigate the protein adducts with pesticides in a cohort of 172 factory workers that were exposed to a mixture of pesticides. The 35 samples showing considerable variation in biochemical parameters, i.e., butyrylcholinestrase (BChE), serum glutamic pyruvic transaminase (SGPT), serum glutamic oxaloacetic transaminase (SGOT), gamma-glutamyl transferase (GGT), serum glutamic pyruvic transaminase (SGPT), alkaline phosphatase (ALP/ALKP), lactate dehydrogenase (LDH), creatine phosphokinase (CPK) enzymes, and controls were analyzed by reversed-phase nanoscale liquid chromatography tandem mass spectrometry (nLC-MS/MS) on an Orbitrap mass spectrometer employing a shotgun proteomics approach. Only protein adducts with carbofuran were found on serum proteins of these workers. These adducts were of carbofuran labeled lysine (Lys-142, Lys-183, Lys-287, and Lys-467), arginine (Arg-210, Arg-242, and Arg-256) from serum albumin, and serine (Ser-07, Ser-54, and Ser-150) from immunoglobulin proteins. The arginine residues (Arg-210, Arg-242, Arg-246, and Arg-434) from albumin were also found to be glycated in serum of workers showing a high level of glucose who also had glycated arginine (Arg-1120) modified with carbofuran in their tankyrase-1-binding protein. The number of tandem mass spectra of modified peptides increased with increasing time of exposure. This is the first report to demonstrate the presence of carbofuran-labeled albumin, immunoglobulin, and glycated arginine, which shows that lysine and arginine of human albumin and serine of immunoglobulin are covalently modified in the serum of workers that were occupationally exposed to carbofuran, and the modification is detectable by tandem mass spectrometry. These peptides modified with carbofuran can potentially be used as a biomarker of carbofuran exposure.

Relationships of human α/β hydrolase fold proteins and other organophosphate-interacting proteins

Organophosphates (OPs) are either found in nature or synthetized for use as pesticides, flame retardants, neurotoxic warfare agents or drugs (cholinergic enhancers in Alzheimer's disease and myasthenia gravis, or inhibitors of lipases in metabolic diseases). Because of the central role of acetylcholinesterase cholinergic neurotransmission in humans, one of the main purposes for using OPs is inactivation of the enzyme by phosphorylation of the nucleophilic serine residue in the active center. However, hundreds of serine hydrolases are expressed in the human proteome, and many of them are potential targets for OP adduction. In this review, we first situate the α/β hydrolase fold proteins among the distinctively folded proteins known to interact with OPs, in particular the different lipases, peptidases, and enzymes hydrolyzing OPs. Second, we compile the human α/β hydrolases and review those that have been experimentally shown to interact with OPs. Among the 120 human α/β hydrolase fold proteins, 102 have a serine in the consensus GXSXG pentapeptide compatible with an active site, 6 have an aspartate or a cysteine as the active site nucleophile residue, and 12 evidently lack an active site. 76 of the 120 have been experimentally shown to bind an OP.

Identification of protein adduction using mass spectrometry: Protein adducts as biomarkers and predictors of toxicity mechanisms

The determination of protein-xenobiotic adducts using mass spectrometry is an emerging area which allows detailed understanding of the underlying mechanisms involved in toxicity. These approaches can also be used to reveal potential biomarkers of exposure or toxic response. The following review covers studies of protein adducts resulting from exposure to a wide variety of xenobiotics including organophosphates, polycyclic aromatic hydrocarbons, acetaminophen, alkylating agents and other related compounds.

PHOS-select iron affinity beads enrich peptides for the detection of organophosphorus adducts on albumin

Albumin is covalently modified by organophosphorus toxicants (OP) on tyrosine 411, but less than 1% of albumin is modified in humans by lethal OP doses that inhibit 95% of plasma butyrylcholinesterase. A method that enriches OP-modified albumin peptides could aid analysis of low dose exposures. Soman or chlorpyrifos oxon treated human plasma was digested with pepsin. Albumin peptides were enriched by binding to Fe(3+) beads at pH 11 and eluted with pH 2.6 buffer. Similarly, mouse and guinea pig albumin modified by chlorpyrifos oxon were digested with pepsin and enriched by binding to Fe(3+) beads. Peptides were identified by MALDI-TOF/TOF mass spectrometry. PHOS-select iron affinity beads specifically enriched albumin peptides VRY411TKKVPQVST and LVRY411TKKVPQVST in a pepsin digest of human plasma. The unmodified as well as OP-modified peptides bound to the beads. The binding capacity of 500 μL of beads was the pepsin digest of 2.1 μL of human plasma. The limit of detection was 0.2% of OP-modified albumin peptide in 0.43 μL of plasma. Enrichment of OP-modified albumin peptides by binding to Fe(3+) beads is a method with potential application to diagnosis of OP pesticide and nerve agent exposure in humans, mice, and guinea pigs.

Laboratory examination in nerve agent intoxication

Diagnosis of nerve agent intoxication is based on anamnestic data, clinical signs and laboratory examination. For acute poisoning, cholinesterase activity in the blood (erythrocyte AChE, plasma/serum BuChE) is sensitive, simple and most frequent laboratory examination performed in biochemical laboratories. Specialized examinations to precise treatment (reactivation test) or to make retrospective diagnosis (fluoride induced reactivation etc.) can be conducted. Other sophisticated methods are available, too.