INACTIVATION DURING DENATURATION OF RIBONUCLEASE-A BY GUANIDINIUM CHLORIDE IS ACCOMPANIED BY UNFOLDING AT THE ACTIVE-SITE

Inactivation of pancreatic RNAase A occurs in guanidinium chloride (Gd mCl) at low concentrations before the unfolding of the molecule as a w hole can be detected [Liu and Tsou (1987) Biochim. Biophys. Acta 916, 455-464]. We have now shown that the rate of digestion of the RNAase m olecule by either trypsin or proteinase K increases significantly at l ow concentrations of GdmCl where the enzyme is largely inactivated, bu t fluorescence and absorption measurements reveal no conformational ch anges. N-Terminal sequence analysis of the peptide fragments generated shows that proteolysis occurs primarily at or near the active site. T he decrease in activity of RNAase at low concentrations of GdmCl is th erefore due to partial unfolding of the molecule, particularly at the active site and not to an inhibition by the denaturant.