Conformational stability of ribonuclease T1. II. Salt-induced renaturation

In the presence of high concentrations of the monovalent salts, sodium chloride and potassium fluoride, disulfide-reduced RNase T1 having four cysteinyl residues intact regenerates the spectral properties characteristic of native RNase T1, e.e., the fluorescence spectrum of the aromatic side chains and the ultraviolet circular dichroism spectrum. The folding of the polypeptide chain proceeded without formation of disulfide bonds to yield an enzymatically active species having an activity toward RNA equivalent to 25% of that of the native enzyme at the same salt concentration of 2 m. Unfolding of RNase T1 by a denaturant, urea, was suppressed in the presence of salts, and the salt-induced chain folding was observed spectroscopically even in 6.9 m urea solution. The salts also induced the chain folding of disulfide reduced and modified (carboxymethylated or carboxamidomethylated) RNase T1 into the native conformation, as indicated by its spectroscopic properties, but did not restore the enzymatic activity.