This paper examines in detail the activation of bovine and porcine trypsinogens and of bovine chymotrypsinogens A and B by trypsin and aspergillopeptidase A. Kinetic data have also been obtained (Km and kcat) for the hydrolysis catalyzed by these proteases of several model peptides with sequences related to the N‐terminal sequence of bovine trypsinogen. The N‐terminal sequence of (aspartyl)4 residues is not necessary for the recognition of the strategic Lys‐Ile bond of trypsinogen. We have shown previously that there are two binding sites for Ca2+ on trypsinogen. One of these sites is constituted by the 2 aspartyl residues which are the nearest neighbours of the important Lys‐Ile bond. The saturation of the site by Ca2+ improves the formation of the trypsinogen‐trypsin complex; Ca2+ has no effect on the rate of decomposition of this complex. The comparison of the kinetic data for the activation of trypsinogens and chymotrypsinogens A and B on one hand and the comparison of kcat and Km of different model peptides, among them the synthetic nonapeptide Val‐(Asp)4‐Lys‐Ile‐Val‐Gly, on the other hand, implies that the 4 aspartyl residues confer no special reactivity to the Lys‐Ile bond but conversely have a very negative effect. For example kcat= 1.6 sec−1 for chymotrypsinogen B but only 2.5 and 5.5 × 10−3 sec−1 for bovine and porcine trypsinogens. The values of Km are similar. For the peptide Val‐(Ala)2‐Lys‐Ile‐Val‐Gly kcat= 7.0 sec−1 but kcat between 10−4 and 10−2 sec−1 has been found for the peptide Val‐(Asp)4‐Lys‐Ile‐Val‐Gly. This very negative effect is observed only with trypsin. In the activations by aspergillopeptidase A, trypsinogen is a much better substrate than chymotrypsinogen. The implications of this exceptionally slow hydrolysis of the Lys‐Ile bond are discussed. The problem of the formation of inert proteins in particular appears to be closely related to the very poor quality of this bond as a substrate for trypsin. A mechanism is given for the formation of inert proteins; a similar mechanism also explains the degradative autolysis of trypsin. Copyright © 1969, Wiley Blackwell. All rights reserved
