Cross-linking of protein by peroxidase

The reaction products of peroxidase, a hydrogen donor and hydrogen peroxide decreased the amount of lysine recovered from proteins after acid hydrolysis. Oxidation of peroxidase treated proteins with performic acid prior to hydrolysis formed alpha-amino adipic acid indicating that the peroxidase or the quinones formed by peroxidase had oxidatively deaminated some lysyl residues of the protein to form lysyl aldehyde. Gel filtration and polyacrylamide gel electrophoresis revealed dimers, trimers and higher protein polymers that were not detected when peroxidase was omitted. Since some of the protein polymers were not dissociated by gel electrophoresis in the presence of dodecyl sulfate, urea and mercaptoethanol, it suggests that the free radicals or quinones formed by peroxidase had interacted with or cross-linked protein molecules by the formation of covalent bonds. Oxidative enzymes like peroxidase and polyphenol oxidase may lower the nutritive value of proteins by the oxidative deamination of lysine, reaction with cysteine and methionine and by cross-linking protein molecules to reduce their susceptibility to enzymatic hydrolysis.