Peroxidase-catalyzed cross linking of proteins

Incubation of casein and water-soluble soybean protein, separately or together, at 10 mg/ml with horseradish peroxidase (2.4 or 24 M) and H₂O₂ (1.8 or 18 mM) at pH 9.0 (0.2 M borate buffer) for 24 hr at 37°C in air led to formation of higher molecular weight compounds as determined by sodium dodecyl sulfate polyacrylamide disc gel electrophoresis. Incubation under the same conditions with peroxidase alone (in air) gave a smaller amount of higher molecular weight compounds. Incubation of lysozyme separately or with water-soluble soybean protein did not produce detectable amounts of higher molecular weight compounds. These results are discussed in terms of previously observed di- and tertyrosine isolated from peroxidase/H₂O₂-treated and naturally occurring proteins following acid hydrolysis. Transglutaminase, lipoxygenase, polyphenol oxidase, and lysyl oxidase are examples of other enzymes that can cross link proteins.