Cross-linking methionine and amine residues with reactive halogen species

Irreversible cross-links are increasingly being recognized as important posttranslational oxidative protein modifications that contribute to tissue injury during oxidative stress and inflammation. They also have a structural function in extracellular matrix proteins such as collagen IV. Likely contenders for forming such cross-links are the reactive halogen species that are generated by neutrophils and eosinophils, including hypochlorous acid, hypobromous acid, and their related haloamines. Methionine residues are kinetically preferred targets for these oxidants and oxidation can potentially result in sulfilimine (>S=N-) bonds with amines. Therefore, we investigated whether oxidation of methionine in the model peptide formyl-Met-Leu-Phe-Lys (fMLFK) produces cross-links with lysine residues, using mass spectrometry to characterize the products. As expected, the sulfoxide was the major product with each reactive halogen species. However, intra- and intermolecular cross-linked products were also formed. Isomers of an intramolecular sulfilimine were readily produced by hypobromous acid and bromamines, with hypochlorous acid forming lesser amounts. The predominant cross-link with chloramines was an intermolecular bond between the sulfur of fMLFK and the amine derived from the chloramine. Reactive halogen species also formed these sulfilimine cross-links in other peptides that contain methionine. We propose that protein cross-links involving methionine and amine residues will form via this mechanism when granulocytes are activated at sites of inflammation. Our results also support the proposal that reactive halogen species generated by the peroxidase peroxidasin could be responsible for the sulfilimine bonds that are integral to the structure of collagen IV.