TYROSYL RADICAL GENERATED BY MYELOPEROXIDASE CATALYZES THE OXIDATIVE CROSS-LINKING OF PROTEINS

Phagocytes generate H2O2 for use by a secreted heme enzyme, myeloperox idase, to kill invading bacteria, viruses, and fungi. We have explored the possibility that myeloperoxidase might also convert L-tyrosine to a radical catalyst that cross-links proteins. Protein-bound tyrosyl r esidues exposed to myeloperoxidase, H2O2, and L-tyrosine were oxidized to o,o'-dityrosine, a stable product of the tyrosyl radical. The cros s-linking reaction required L-tyrosine but was independent of halide a nd free transition metal ions; the heme poisons azide and aminotriazol e were inhibitory. Activated neutrophils likewise converted polypeptid e tyrosines to dityrosine. The pathway for oxidation of peptide tyrosy l residues was dependent upon L-tyrosine and was inhibited by heme poi sons and catalase. Dityrosine synthesis was little affected by plasma concentrations of Cl- and amino acids, suggesting that the reaction pa thway might be physiologically relevant. The requirement for free L-ty rosine and H2O2 for dityrosine formation and the inhibition by heme po isons support the hypothesis that myeloperoxidase catalyzes the cross- linking of proteins by a peroxidative mechanism involving tyrosyl radi cal. In striking contrast to the pathways generally used to study prot ein oxidation in vitro, the reaction does not require free metal ions. We speculate that protein dityrosine cross-linking by myeloperoxidase may play a role in bacterial killing or injuring normal tissue. The i ntense fluorescence and stability of biphenolic compounds may allow di tyrosine to act as a marker for proteins oxidatively damaged by myelop eroxidase in phagocyte-rich inflammatory lesions.