Selective and Reversible Photo-oxidation of the Methionyl Residues in Lysozyme

Abstract The photo-oxidation of lysozyme (hen's egg white), sensitized by methylene blue in 84% acetic acid solution, causes the selective modification of the methionyl residues, which are converted to the sulfoxide. The photo-oxidized protein shows a drastically reduced enzymic activity. The data obtained by comparing the sensitivity to proteolytic enzymes, as well as the accessibility of disulfide bonds to reducing agents and the optical parameters, for the native and the photo-oxidized lysozyme, allow one to correlate the loss of catalytic efficiency attending photo-oxidation with a conformational change of the protein. The segments of the polypeptide chain containing some tryptophyl residues play a major role in such rearrangement. Chemical reduction of the photo-oxidized lysozyme by aqueous 2-mercaptoethanol results in the restoration of the primary and tertiary structure, characteristic of the native lysozyme. A high recovery of enzymic activity is also obtained