CHLORINATION OF TYROSYL RESIDUES IN PEPTIDES BY MYELOPEROXIDASE AND HUMAN NEUTROPHILS

Hypochlorous acid is the major strong oxidant generated by human neutr ophils, and it has the potential to cause much of the tissue damage th at these inflammatory cells promote, It is produced from hydrogen pero xide and chloride by the heme enzyme myeloperoxidase. To unequivocally establish that hypochlorous acid contributes to inflammation, a stabl e and unique marker for its reaction with biomolecules needs to be ide ntified, In this investigation we have found that reagent hypochlorous acid reacts with tyrosyl residues in small peptides and converts them to chlorotyrosine. Purified myeloperoxidase in combination with hydro gen peroxide and chloride, as well as stimulated human neutrophils, ch lorinated tyrosine in the peptide Gly-Gly-Tyr-Arg. Rather than reactin g directly with the aromatic ring of tyrosine, hypochlorous acid initi ally reacted with an amine group of the peptide to form a chloramine. The chloramine then underwent an intramolecular reaction with the tyro syl residue to convert it to chlorotyrosine. This indicates that tyros yl residues in proteins that are close to amine groups will be suscept ible to chlorination. Peroxidases are the only enzymes capable of chlo rinating an aromatic ring. Furthermore, myeloperoxidase isthe only hum an enzyme that produces hypochlorous acid under physiological conditio ns. Therefore, chlorotyrosine will be a specific marker for the produc tion of hypochlorous acid in vivo and for the involvement of myelopero xidase in inflammatory tissue damage.