Inhibition of proteinase 3 by [alpha]1-antitrypsin in vitro predicts very fast inhibition in vivo.

Neutrophil proteinase 3 (Pr3) cleaves elastin and other matrix proteins, and is thought to cause lung tissue destruction in emphysema and cystic fibrosis. Its deleterious action is theoretically prevented by alpha1-antitrypsin, a serpin present in lung secretions. We have evaluated the anti-Pr3 activity of this inhibitor to decide whether it may play a physiologic proteolysis-preventing function in vivo. We show that (i). the oxidized inhibitor does not inhibit Pr3; (ii). the inhibitor competes favorably with elastin for the binding of Pr3, but is less efficient for inhibiting elastin-bound proteinase than for complexing free enzyme; and (iii). the inhibition takes place in at least two steps: the enzyme and the inhibitor first form a high-affinity reversible inhibitory complex EI* with an equilibrium dissociation constant K*i of 38 nM; EI* subsequently transforms into an irreversible complex EI with a first-order rate constant k2 of 0.04 s-1. Because the alpha1-antitrypsin concentration in the epithelial lining fluid is much higher than K*i, any Pr3 molecule released from neutrophils will be taken up as an EI* complex within much less than 1 s, indicating very efficient inhibition in vivo.