Kinetics of the inactivation of human and bovine trypsins and chymotrypsins by alpha1-proteinase inhibitor and of their reactivation by alpha2-macroglobulin.

The time dependency of inactivation of human cationic trypsin and chymotrypsin II and of bovine trypsin and alpha-chymotrypsin by human serum has been investigated. Since the molar concentration of serum alpha1-proteinase inhibitor is much higher than that of other inhibitors, this time dependence could be used to calculate the rate constants kass for the association of alpha1-proteinase inhibitor with the four proteases. The association process was found to be second order, with kass ranging from 1 x10(4) s-1 (human trypsin) to 2.6 x 10(6) s-1 (bovine chymotrypsin). The human proteases react much more slowly with human alpha1-proteinase inhibitor than the bovine ones. But, whatever the species, chymotrypsin is inhibited more quickly than trypsin. Addition of alpha2-macroblobulin to the inactive complexes resulted in a time-dependent regeneration of enzymic activity due to the formation of alpha2-macroglobulin-protease complexes. The reactivation (i.e. dissociation) process was first order and extremely slow: the half-life of the alpha1-proteinase inhibitor-proteinase complexes ranged from 8 days (bovine chymotrypsin) to 9 months (human chymotrypsin). The human proteases formed the most stable complexes with alpha1-proteinase inhibitor. The pathological implications of these findings are discussed.