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

The effect of salt concentration on catalysis by ribonuclease A (RNase A) has been reexamined. At low salt concentration, the enzyme is inhibited by low-level contaminants in common buffers. When an uncontaminated buffer system is used or H12A RNase A, an inactive variant, is added to absorb inhibitory contaminants, enzymatic activity is manifested fully at low salt concentration. Catalysis by RNase A does not have an optimal salt concentration. Instead, k(cat)/K(M)10(9) M(-1)s(-1) for RNA cleavage at low salt concentration. These findings highlight the care that must accompany the determination of meaningful salt-rate profiles for enzymatic catalysis.