Study of a photo-induced lysozyme-riboflavin bond.

Irradiation of lysozyme in the presence of riboflavin results in the formation of a lysozyme-riboflavin adduct. Reduction and carboxymethylation of the four disulfide bonds as well as the chemical modification of the Tyr residues and the photochemical alteration of the His residue in lysozyme, do not affect the formation of the photo-induced lysozyme-riboflavin bond. When the lysozyme-riboflavin adduct was subjected to mild acid hydrolysis and ion exchange chromatography, the retention of a compound containing 14C-riboflavin was observed. Free 14C-riboflavin, on the contrary is not retained by the column. The photo-oxidation of free Trp in the presence of 14C-riboflavin, gave a compound which bound to the ion exchange resin like the above-mentioned derivative. The photo-oxidation of the Trp residues in lysozyme and in peptides obtained from lysozyme showed very high quantum yields, and these values were directly related to the incorporation of 14C-riboflavin in these samples.