Catalysis by zymogens: increased reactivity at high ionic strength

Kinetic analysis of the intrinsic activity of trypsinogen and chymotrypsinogen has shown that a major cause for the diminished activity of these zymogens, as compared to the corresponding enzymes, is a deformation of the primary binding site [l-6] . The binding of inhibitors by trypsinogen is -104times less effective than by trypsin [2] while the maximum velocity of catalysis is only 1 02-times lower [6] . Attempts to increase the intrinsic catalytic activity of these zymogens without proteolytic activation have met with limited success. The addition of dipeptides corresponding to the amino-terminus of trypsin resulted in conformational changes of the p-guanidinobenzoyl derivative of trypsinogen similar to that obtained by tryptic activation [7,8] . However, this change requires the presence of the guanidinobenzoyl moiety and hence does not manifest itself directly as an increase in the catalytic activity of the zymogen ([7] and J. D. L.-E., unpublished observations). On the other hand the oxidation of Met-192 [3] results in a small but significant increase in the intrinsic activity of chymotrypsinogen by improving the binding of the substrate to the zymogen. In this