THE OXIDATION WITH N‐BROMOSUCCINIMIDE OF THE SINGLE TRYPTOPHAN RESIDUE IN RIBONUCLEASE T₁

Abstract

1. The single tryptophan residue‐59 present in RNase T1 was oxidized almost specifically by NBS in 8 M urea. The enzymatic activity of RNase T1 decreased with the increase in the amount of NBS used, and complete inactivation took place within 10 minutes when 2 equivalents of the reagent were used. 2. During the oxidation with NBS of RNase T1 no cleavage of peptide bonds was found and none of the amino acid residues other than tryptophan was affected except for oxidation of cystine to cysteic acid in a small amount (up to 20 %). The complete inactivation of the enzyme occurred when the tryptophan residue was completely oxidized. 3. The oxidized RNase T1 possessed a lower solubility than the native enzyme. Measurements of the optical rotatory dispersion and the circular dichroism indicated the occurrence, of some but not so great conformational changes in the enzyme by the oxidation. 4. These residts clearly demonstrated the importance of the single tryptophan residue in RNase T1 for the proper function of the enzyme.