MYELOPEROXIDASE-INDUCED OXIDATION OF ALBUMIN AND CERULOPLASMIN: ROLE OF TYROSINES
Neutrophil myeloperoxidase (MPO) plays an important role in protecting the body against infections. MPO products - hypohalous acids and phenoxyl radicals - are strong oxidants, which can damage not only foreign intruders but also host tissues including blood plasma proteins. Here, we compared the MPO-induced oxidation of two plasma proteins with antioxidant properties - human serum albumin (HSA) and ceruloplasmin (CP). Incubation of both proteins with hypochlorite (NaOCl) or with catalytically active MPO (MPO + H2O2), which in the presence of chloride ions synthesizes hypochlorous acid (HOCl), resulted in quenching of tryptophan fluorescence of the proteins. Oxidationinduced changes in the structures of HSA and CP were different: HSA due to its high antioxidant capacity efficiently neutralized the MPO-generated oxidants without formation of protein aggregates, while oxidation of CP resulted in formation of large aggregates stabilized by strong covalent bonds between aromatic amino acid residues of tyrosine (which is present in plasma as free amino acid and also as an amino acid residue of polypeptide chains) and tryptophan. The number of tyrosine residues in a protein is not a determining factor for the formation of aggregates. In the case of CP only, the formation of aggregates is primarily due to the high content of tryptophan residues in its polypeptide chain. MPO-dependent oxidation of free amino acid tyrosine results in formation of tyrosyl radicals, which do not oxidize aromatic amino acid residues in proteins because of their high propensity to recombine with formation of dityrosines. At the same time, free tyrosine can modulate MPO-induced protein oxidation due to its ability to modulate the HOCl synthesis in the active center of MPO.