Kinetics and stoichiometry of inactivation of some pancreatic and leucocyte serine proteinases by rabbit alpha 1-proteinase inhibitors F and S.

Purified rabbit alpha 1-proteinase inhibitors F and S were incubated with bovine trypsin, chymotrypsin or horse leucocyte neutral proteinases in order to determine the stoichiometry of inhibition, inactivation rates of the enzymes and dissociation constants of the complexes. Trypsin reacted with the two forms of alpha 1-PI with different velocities but in the molar ratio of 1:1 and yielding stable complexes. Chymotrypsin reacted very fast with the two forms of alpha 1-PI but required a three-fold molar excess of alpha 1-PI-S for almost complete inhibition, and again the dissociation constants were low. Leucocyte proteinases also needed certain molar excess of the inhibitor but the reaction was complicated by instability of the complexes, especially with Z-Ala-ONp as substrate. We conclude that the interaction of rabbit alpha 1-PI with leucocyte proteinases, and particularly alpha 1-PI-S with chymotrypsin, includes not only complex formation but also some inactivation of the inhibitor.