Nontargeted High-Resolution Mass Spectrometric Workflow for the Detection of Butyrylcholinesterase-Derived Adducts with Organophosphorus Toxicants and Structural Characterization of Their Phosphyl Moiety after In-Source Fragmentation

The phosphylated butyrylcholinesterase-derived tetrapeptide GlyGluSerAla proves exposure to organophosphorus agents with enantioselectivity

We herein present for the first time the phosphylated (*) tetrapeptide (TP)-adduct GlyGluSer198*Ala generated from butyrylcholinesterase (BChE) with proteinase K excellently suited for the verification of exposure to toxic organophosphorus nerve agents (OPNA). Verification requires bioanalytical methods mandatory for toxicological and legal reasons. OPNA react with BChE by phosphonylation of the active site serine residue (Ser198) forming one of the major target protein adducts for verification. After its enzymatic cleavage with pepsin, the nonapeptide (NP) PheGlyGluSer*AlaGlyAlaAlaSer is typically produced as biomarker. Usually OPNA occur as racemic mixtures of phosphonic acid derivatives with the stereocenter at the phosphorus atom, e.g. (±)-VX. Both enantiomers react with BChE, but the adducted NP does not allow their chromatographic distinction. In contrast, the herein introduced TP-adducts appeared as two peaks when using a stationary reversed phase (1.8 µm) in micro-liquid chromatography-electrospray ionisation tandem-mass spectrometry (µLC-ESI MS/MS) analysis. These two peaks represent diastereomers of the (+)- and (-)-OPNA adducted to the peptide that comprises chiral L-amino acids exclusively. Concentration- and time-dependent effects of adduct formation with (±)-VX and its pure enantiomers (+)- and (-)-VX as well as with (±)-cyclosarin (GF) were investigated in detail characterising enantioselective adduct formation, stability, ageing and spontaneous reactivation. The method was also successfully applied to samples from a real case of pesticide poisoning as well as to samples of biomedical proficiency tests provided by the Organisation for the Prohibition of Chemical Weapons.

In vitro comparison of the acetylcholinesterase inhibition caused by V- and A-series nerve agents' surrogates

Nerve agents (NA) pose as a great risk in the modern world. NA from the V-series, such as VX, are currently recognized as the most toxic among those compounds. However, the emergence of new classes of toxicants recently included in the Chemical Weapons Convention (CWC), such as the A-series NA, a class of organophosphorus compounds related to phosphoramidates, pose a new source of concern due to the lack of information. In order advance in the investigation on the toxicity of such toxic chemicals, we performed in vitro studies to compare representatives of the V- and A-series using affordable surrogates. Results suggest a similar inhibition potency between both agents.

Isolation of human TRPA1 channel from transfected HEK293 cells and identification of alkylation sites after sulfur mustard exposure

Transient receptor potential (TRP) channels are important in the sensing of pain and other stimuli. They may be triggered by electrophilic agonists after covalent modification of certain cysteine residues. Sulfur mustard (SM) is a banned chemical warfare agent and its reactivity is also based on an electrophilic intermediate. The activation of human TRP ankyrin 1 (hTRPA1) channels by SM has already been documented, however, the mechanism of action is not known in detail. The aim of this work was to purify hTRPA1 channel from overexpressing HEK293 cells for identification of SM-induced alkylation sites. To confirm hTRPA1 isolation, Western blot analysis was performed showing a characteristic double band at 125 kDa. Immunomagnetic separation was carried out using either an anti-His-tag or an anti-hTRPA1 antibody to isolate hTRPA1 from lysates of transfected HEK293 cells. The identity of the channel was confirmed by micro liquid chromatography-electrospray ionization high-resolution tandem-mass spectrometry. Following SM exposure, hTRPA1 channel modifications were found at Cys⁴⁶² and Cys⁶⁶⁵, as well as at Asp³³⁹ and Glu³⁴¹ described herein for the first time. Since Cys⁶⁶⁵ is a well-known target of hTRPA1 agonists and is involved in hTRPA1 activation, SM-induced modifications of cysteine, as well as aspartic acid and glutamic acid residues may play a role in hTRPA1 activation. Considering hTRPA1 as a target of other SM-related chemical warfare agents, analogous adducts may be predicted and identified applying the analytical approach described herein.

Highly stable peptide adducts from hard keratins as biomarkers to verify local sulfur mustard exposure of hair by high-resolution mass spectrometry

In the recent past, the blister agent sulfur mustard (SM) deployed by the terroristic group Islamic State has caused a huge number of civilian and military casualties in armed conflicts in the Middle East. The vaporized or aerolized agent might be inhaled and have direct contact to skin and hair. Reaction products of SM with plasma proteins (adducts) represent well-established systemic targets for the bioanalytical verification of exposure. The SM-derived hydroxyethylthioethyl (HETE)-moiety is attached to nucleophilic amino acid side chains and allows unambiguous adduct detection. For shipping of common blood and plasma samples, extensive packaging rules are to be followed as these matrices are considered as potentially infectious material. In contrast, hair is considered as non-infectious thus making its handling and transportation much less complicated. Therefore, we addressed this matrix to develop a procedure for bioanalytical verification. Following optimized lysis of SM-treated human scalp hair and pepsin-catalyzed proteolysis of adducts of keratin type I and II, microbore liquid chromatography-electrospray ionization high-resolution tandem-mass spectrometry (µLC-ESI MS/HR MS) was used to detect three alkylated keratin-derived biomarker peptides: AE(-HETE)IRSDL, FKTIE(-HETE)EL, and LE(-HETE)TKLQF simultaneously. All bear the HETE-moiety bound to a glutamic acid residue. Protein adducts were stable for at least 14 weeks at ambient temperature and contact to air, and were not affected by washing the hair with shampoo. The biomarker peptides were also obtained from beard, armpit, abdominal, and pubic hair. This is the first report introducing stable local peptide adduct biomarkers from hair, that is easily accessible by a non-invasive sampling process.

Alkylated glutamic acid and histidine derived from protein-adducts indicate exposure to sulfur mustard in avian serum

Sulfur mustard (SM, bis(2-chloroethyl)-sulfide) is a banned chemical warfare agent deployed in the violent conflict in the Middle East poisoning humans and animals. For legal reasons bioanalytical methods are mandatory proving exposure to SM. Reaction products (adducts) of SM with endogenous proteins e.g., serum albumin (SA) are valuable long-lived targets for analysis. Whereas nearly all methods known so far focus on human proteins, we address for the first time neat chicken SA and avian serum from chicken, duck and ostrich. After proteolysis, protein precipitation, evaporation of the supernatant and re-dissolution analysis was performed by micro liquid chromatography-electrospray ionization tandem-mass spectrometry in the selected reaction monitoring mode, μLC-ESI MS/MS (SRM), for detection of the hydroxyethylthioethyl product ion [HETE]⁺ at m/z 105.0. After in vitro incubation with SM and pronase-catalyzed proteolysis the alkylated amino acids Glu(-HETE) and His(-HETE) were detected. Both borne the SM-characteristic HETE-moiety bound to their side chain. The 8-fold deuterated SM analogue (d8-SM) was also applied to support adduct identification. Proteolysis conditions were optimized with respect to pH (8.0), temperature (50°C) and time to maximize the yield of Glu(-HETE) (30 min) and His(-HETE) (180 min). Amino acid adducts were stable in the autosampler for at least 24 h. Protein-adducts were stable in serum at -30°C for at least 33 d and for three freeze-and-thaw cycles. At the body temperature of chicken (+40°C) Glu(-HETE) was degraded in serum (period of half-change 3 d) whereas His(-HETE) remained stable. The presented method broadens the toolbox of procedures to document poisoning with SM.