Photochemical reactivity of the homologous proteins alpha-lactalbumin and lysozyme.

The fluorescent behaviour and the photodynamic effect was studied in native and structurally modified lysozyme and alpha-lactalbumin. The Tyr residues in lysozyme and alpha-lactalbumin show different sensitivities to the photodynamic effect. The effect is zero in the case of Tyr from native lysozyme. In contrast, the Tyr residues in alpha-lactalbumin are susceptible to photooxidation, which indicates a greater degree of exposure to the solvent. The three His residues of alpha-lactalbumin have different degrees of exposure and show two different kinetics of photooxidation whereas the His residue of lysozyme is photooxidized with a single kinetic. Two photooxidation kinetics were obtained for the Trp residues of both native proteins, an indication that in both cases there are Trp residues that are differently exposed to the solvent. The wavelengths of maximum fluorescent emission of the Trp residues were different for the two proteins, an effect which can also be explained in terms of a difference in the environment of these residues. The modified form of these proteins emit at wavelengths longer than those of the native forms. When modified the proteins photooxidize with noticeably greater quantum yields.