Chemical modification of human lysozyme. Acetylation and nitration of tyrosine.

Abstract

When human lysozyme is reacted with a 60 M excess of N-acetylimidazole, only one of six tyrosine residues and two amino groups are acetylated. The acetylated lysozyme is 1.2 times as active towards M. lysodeikticus as the unmodified enzyme. When human lysozyme is reacted with a 4 M excess of tetranitromethane, approximately one tyrosine is nitrated. The tetranitromethane also simultaneously induces a high degree of polymerization of the lysozyme. In a typical experiment, nitration leads to a polymerized product that has only 25% of the activity of unmodified enzyme towards M. lysodeikticus. The polymerized lysozyme can be separated into several components by gel filtration on Sephadex G-75. Enzyme activity analyses of the chromatographed lysozyme oligomers indicate that tetranitromethane reduces the activity of human lysozyme primarily by polymerization, since the lysozyme monomer, which contains one nitrotyrosine per molecule, has 65% activity while the trimer has only 5% activity. N-Acetylglucosamine, N,N′-diacetylchitobiose, and N,N',N″-triacetylchitotriose, all inhibitors or substrates of human lysozyme, prevent neither the nitration of the single tyrosine residue nor the polymerization due to tetranitromethane action.