Kinetics of the inactivation of human and bovine trypsins and chymotrypsins by α1-proteinase inhibitor and of their reactivation by α2-macroglobulin

Abstract

The time dependency of inactivation of human cationic trypsin and chymotrypsin II and of bovine trypsin and α-chymotrypsin by human serum has been investigated. Since the molar concentration of serum α 1-proteinase inhibitor is much higher than that of other inhibitors, this time dependence could be used to calculate the rate constants k ass for the association of α 1-proteinase inhibitor with the four proteases. The association process was found to be second order, with k ass ranging from 1 × 10 4 s −1 (human trypsin) to 2.6 × 10 6 s −1 (bovine chymotrypsin). The human proteases react much more slowly with human α 1-proteinase inhibitor than the bovine ones. But, whatever the species, chymotrypsin is inhibited more quickly than trypsin. Addition of α 2-macroglobulin to the inactive complexes resulted in a time-dependent regeneration of enzymic activity due to the formation of α 2-macroglobulin-protease complexes. The reactivation (i.e. dissociation) process was first order and extremely slow: the half-life time of the α 1-proteinase inhibitor-proteinase complexes ranged from 8 days (bovine chymotrypsin) to 9 months (human chymotrypsin). The human proteases formed the most stable complexes with α 1-proteinase inhibitor. The pathological implications of these findings are discussed.