Cross-linking between cysteine and lysine, tryptophan or tyrosine in peptides and proteins treated with hypochlorous acid and other reactive halogens

Cysteine residues are the most favored targets for oxidation by hypochlorous acid and other reactive halogen species. The end-products of cysteine oxidation are usually considered to be reversibly formed disulfides and the more highly oxidized sulfinic and sulfonic acids. However, reactive halogen species are capable of generating additional products in which cysteine is cross-linked to other amino acids. Here we have treated a range of peptides with hypochlorous acid (HOCl) and hypobromous acid (HOBr), and used mass spectrometry to demonstrate sulfenamide, sulfinamide and sulfonamide formation with lysine residues, as well as –S(O)- and –S(O₂)- linkages with tyrosine, tryptophan and arginine residues. The -(SO₂)- products were more prevalent with HOCl than HOBr, reflecting its higher oxidizing ability. There was also considerable variation between peptides in efficiency of cross-linking compared with other modifications. The –S(O)- and –S(O₂)- forms were much more resistant than the disulfide to reduction by dithiothreitol. Using calprotectin as a representative cysteine-containing protein, we show that a range of products containing each of these cross-links is formed when the protein is treated with HOCl. Two of the identified cysteine-lysine calprotectin cross-links were also detected in bronchoalveolar lavage fluid from children with cystic fibrosis. Our results imply that cross-linked species would be formed when cysteine-containing proteins are exposed to reactive halogen species, with the nature of the specific products depending on structural features around the cysteine residue. Cross-linking could have a modulatory effect on protein function or be detrimental in causing oligomerization and aggregation.