Peroxidation of lysozyme treated with Cu(II) and hydrogen peroxide

Abstract

When lysozyme was treated with Cu(II) and H 2O 2 at pH 7.4, the protein underwent polymerization as well as changes in its fluorescent characteristics. Upon prolonged incubation, most of the protein aggregates were degraded into smaller peptides. Amino acid analysis indicated that the basic amino acid residues were most susceptible to the oxidation. Tryptophan residues were converted to N-formylkynurenine and kynurenine, and lysine residues were deaminated to form α-aminoadipic acid δ-semi- aldehyde. During Cu(II)H 2O 2 treatment, the formation of carbonyl groups was accompanied by the loss of free amino groups in the protein. Succinylation of free amino groups protected lysine residues from oxidation by Cu(II)H 2O 2, but failed to prevent polymerization. The studies with the modified lysozyme suggest that Cu(II)H 2O 2 can oxidize various amino acid residues in addition to lysine to generate different types of carbonyl compounds and these carbonyl compounds may be responsible for the formation of crosslinks in the polymerization process.