Origin of the 'inactivation' of ribonuclease A at low salt concentration

The effect of salt concentration on catalysis by ribonuclease A (RNase A) h as been reexamined. At low salt concentration, the enzyme is inhibited by l ow-level contaminants in common buffers. When an uncontaminated buffer syst em is used or H12A RNase A, an inactive variant, is added to absorb inhibit ory contaminants, enzymatic activity is manifested fully at low salt concen tration. Catalysis by RNase A does not have an optimal salt concentration. Instead, k(cat)/K-M was > 10(9) M-1 s(-1) for RNA cleavage at low salt conc entration. These findings highlight the care that must accompany the determ ination of meaningful salt-rate profiles for enzymatic catalysis. (C) 2000 Federation of European Biochemical Societies.