Alkylation of rabbit muscle creatine kinase surface methionine residues inhibits enzyme activity in vitro

Creatine kinase (CK) catalyzes the formation of phosphocreatine from adenosine triphosphate (ATP) and creatine. The highly reactive free cysteine residue in the active site of the enzyme (Cys²⁸³) is considered essential for the enzymatic activity. In previous studies we demonstrated that Cys²⁸³ is targeted by the alkylating chemical warfare agent sulfur mustard (SM) yielding a thioether with a hydroxyethylthioethyl (HETE)-moiety. In the present study, the effect of SM on rabbit muscle CK (rmCK) activity was investigated with special focus on the alkylation of Cys²⁸³ and of reactive methionine (Met) residues. For investigation of SM-alkylated amino acids in rmCK, micro liquid chromatography-electrospray ionization high-resolution tandem-mass spectrometry measurements were performed using the Orbitrap technology. The treatment of rmCK with SM resulted in a decrease of enzyme activity. However, this decrease did only weakly correlate to the modification of Cys²⁸³ but was conclusive for the formation of Met⁷⁰-HETE and Met¹⁷⁹-HETE. In contrast, the activity of mutants of rmCK produced by side-directed mutagenesis that contained substitutions of the respective Met residues (Met⁷⁰Ala, Met¹⁷⁹Leu, and Met⁷⁰Ala/Met¹⁷⁹Leu) was highly resistant against SM. Our results point to a critical role of the surface exposed Met⁷⁰ and Met¹⁷⁹ residues for CK activity.

Transthyretin as a target of alkylation and a potential biomarker for sulfur mustard poisoning: Electrophoretic and mass spectrometric identification and characterization

For the verification of exposure to the banned blister agent sulfur mustard (SM) and the better understanding of its pathophysiology, protein adducts formed with endogenous proteins represent an important field of toxicological research. SM and its analogue 2-chloroethyl ethyl sulfide (CEES) are well known to alkylate nucleophilic amino acid side chains, for example, free-thiol groups of cysteine residues. The specific two-dimensional thiol difference gel electrophoresis (2D-thiol-DIGE) technique making use of maleimide dyes allows the staining of free cysteine residues in proteins. As a consequence of alkylation by, for example, SM or CEES, this staining intensity is reduced. 2D-thiol-DIGE analysis of human plasma incubated with CEES and subsequent matrix-assisted laser desorption/ionization time-of-flight (tandem) mass-spectrometry, MALDI-TOF MS(/MS), revealed transthyretin (TTR) as a target of alkylating agents. TTR was extracted from SM-treated plasma by immunomagnetic separation (IMS) and analyzed after tryptic cleavage by microbore liquid chromatography-electrospray ionization high-resolution tandem-mass spectrometry (μLC-ESI MS/HR MS). It was found that the Cys¹⁰ -residue of TTR present in the hexapeptide C(-HETE)PLMVK was alkylated by the hydroxyethylthioethyl (HETE)-moiety, which is characteristic for SM exposure. It was shown that alkylated TTR is stable in plasma in vitro at 37°C for at least 14 days. In addition, C(-HETE)PLMVK can be selectively detected, is stable in the autosampler over 24 h, and shows linearity in a broad concentration range from 15.63 μM to 2 mM SM in plasma in vitro. Accordingly, TTR might represent a complementary protein marker molecule for the verification of SM exposure.

Identification of creatine kinase and alpha-1 antitrypsin as protein targets of alkylation by sulfur mustard

Sulfur mustard (SM) is a toxic chemical warfare agent deployed in several conflicts within the last 100 years and still represents a threat in terroristic attacks and warfare. SM research focuses on understanding the pathophysiology of SM and identifying novel biomarkers of exposure. SM is known to alkylate nucleophilic moieties of endogenous proteins, for example, free thiol groups of cysteine residues. The two-dimensional-thiol-differences in gel electrophoresis (2D-thiol-DIGE) technique is an initial proteomics approach to detect proteins with free cysteine residues. These amino acids are selectively labeled with infrared-maleimide dyes visualized after GE. Cysteine residues derivatized by alkylating agents are no longer accessible for the maleimide-thiol coupling resulting in the loss of the fluorescent signal of the corresponding protein. To prove the applicability of 2D-thiol-DIGE, this technology was exemplarily applied to neat human serum albumin treated with SM, to lysates from human cell culture exposed to SM as well as to human plasma exposed to CEES (chloroethyl ethyl sulfide, an SM analogue). Exemplarily, the most prominent proteins modified by SM were identified by matrix-assisted laser desorption/ionization time-of-flight (tandem) mass spectrometry, MALDI-TOF MS(/MS), as creatine kinase (CK) from human cells and as alpha-1 antitrypsin (A1AT) from plasma samples. Peptides containing the residue Cys²⁸² of CK and Cys²³² of A1AT were unambiguously identified by micro liquid chromatography-electrospray ionization high-resolution tandem-mass spectrometry (μLC-ESI MS/HR MS) as being alkylated by SM bearing the specific hydroxyethylthioethyl-(HETE)-moiety. Both peptides might represent potential biomarkers of SM exposure. This is the first report introducing these endogenous proteins as targets of SM alkylation.

Transient Receptor Potential Channel A1 (TRPA1) Regulates Sulfur Mustard-Induced Expression of Heat Shock 70 kDa Protein 6 (HSPA6) In Vitro

The chemosensory transient receptor potential ankyrin 1 (TRPA1) ion channel perceives different sensory stimuli. It also interacts with reactive exogenous compounds including the chemical warfare agent sulfur mustard (SM). Activation of TRPA1 by SM results in elevation of intracellular calcium levels but the cellular consequences are not understood so far. In the present study we analyzed SM-induced and TRPA1-mediated effects in human TRPA1-overexpressing HEK cells (HEKA1) and human lung epithelial cells (A549) that endogenously exhibit TRPA1. The specific TRPA1 inhibitor AP18 was used to distinguish between SM-induced and TRPA1-mediated or TRPA1-independent effects. Cells were exposed to 600 µM SM and proteome changes were investigated 24 h afterwards by 2D gel electrophoresis. Protein spots with differential staining levels were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and nano liquid chromatography electrospray ionization tandem mass spectrometry. Results were verified by RT-qPCR experiments in both HEKA1 or A549 cells. Heat shock 70 kDa protein 6 (HSPA6) was identified as an SM-induced and TRPA1-mediated protein. AP18 pre-treatment diminished the up-regulation. RT-qPCR measurements verified these results and further revealed a time-dependent regulation. Our results demonstrate that SM-mediated activation of TRPA1 influences the protein expression and confirm the important role of TRPA1 ion channels in the molecular toxicology of SM.