Abstract
Effects of 60Co γ-ray irradiation on subtilisin BPN' solutions have been studied. Kinetic parameters of the enzyme toward N-acetyl-L-tyrosine ethyl ester and N-benzoyl-L-arginine ethyl ester were affected in a different way by the irradiation. Amino acid composition of γ-irradiated diisopropyl phosphoryl-subtilisin, which was separated from low molecular weight fractions by gel-filtration, showed that two tyrosine and about one tryptophan residues were damaged and one methionine residue was oxidized to methionine sulfoxide with dosage of 125 k rads. Conformational changes of the irradiated enzyme were studied by two methods. Modification with 5-diazo-l-H-tetrazole and hydrolysis with achymotrypsin of the irradiated diisopropyl phosphoryl-subtilisin indicated that the tertiary structure of the enzyme was less rigid than the native one. From these results, it is concluded that the radiation damage of a few amino acid residues in the enzyme brought about the conformational changes and the altered kinetic parameters of the irradiated enzyme.
