Loss of tryptophan associated with photo-polymerization and yellowing of proteins exposed to light over 300nm.

S-Carboxymethyl-lysozyme and S-carboxymethyl-ribonuclease A were irradiated with light of wavelength greater than 300nm. Photo-oxidative loss of tryptophan from the S-carboxymethyl-lysozyme was accompanied by yellowing of the protein and the formation of covalently cross-linked polymers. S-Carboxymethyl-ribonuclease, which contains no tryptophan, showed little yellowing and no polymer formation. The irradiated S-carboxymethyl-lysozyme was similar to the proteins of the brown cataractous human lens nucleus and to bovine lens proteins exposed to sunlight in vitro in that it (1) was insoluble in non-denaturing solvents, (2) contained a new fluorescence, (3) was brown in colour, and (4) contained covalent cross-links that are not disulphide bonds.