The Action of U.V. Light of Various Wavelengths on Papain

Papain, prepared according to Kimmel and Smith is activated by irradiation with u.v. light of 254, 280 and 313 nm (φA= 0·022 ± 10 per cent). This activation is caused by the reductive splitting of a mixed disulfide in position 25. This disulfide is split with higher probability than any of the three structural disulfide links. This selectivity is likely caused by specific reduction of the mixed disulfide by thiol groups produced elsewhere in the molecule, although specific reduction by direct photochemical processes cannot be completely excluded on the basis of the data available. The quantum yields for destruction of structural cystine residues at the three wavelengths are comparable with the yields of cystine destruction in other proteins. The data also confirm that quanta absorbed by aromatic amino acid residues contribute to cystine destruction. In contrast to other enzymes with structural disulfide bonds, however, no correlation was found between the destruction of disulfide links and loss in activity. The results suggest that the mechanism of papain inactivation is not only dependent on the wavelength, but also on the dose.