Photodynamic Action of Porphyrins on Amino Acids and Proteins. Spectral Studies on Mono and Di-Sulphoxide Derivatives of Lysozyme

Absorption spectroscopy and circular dichroism measurements have been used to elucidate the mechanism of lysozyme inactivation on photo-oxidation of the methionyl residues to methionine sulphoxide. The monosulphoxide derivative, which is formed by irradiation of the protein in aqueous solution, displayed spectral properties very similar to those of native lysozyme and retained about 54% enzymic activity. This reduced catalytic efficiency appeared to be due to limited conformational changes of the protein molecule. Striking differences were found, on the contrary, between the optical parameters of native lysozyme and of the di-sulphoxide derivative, which is prepared by irradiation in acetic acid solution and is almost devoid of enzymic activity. Studies on the unirradiated lysozyme demonstrated that this extensive denaturation was caused by the action of the protic solvent used. Whereas the acid-induced denaturation was reversible in the case of unmodified lysozyme, the photo-oxidation of the two methionyl residues irreversibly blocked the molecule in a largely disordered form. These results are discussed with respect to the location of the two methionines in the known atomic model of lysozyme.