Oxidation of
mucus proteinase inhibitor (MPI) transforms Met73, the P'1 residue of its active center into
methionine sulfoxide and lowers its affinity for
neutrophil elastase [Boudier, C., and Bieth, J. G. (1994) Biochem. J. 303, 61-68]. Here, we show that the oxidized inhibitor has also a decreased affinity for neutrophil
cathepsin G and pancreatic
chymotrypsin. The Ki of the oxidized MPI-
cathepsin G complex (1.2 microM) is probably too high to be compatible with significant inhibition of
cathepsin G in inflammatory lung secretions. Stopped-flow kinetics shows that, within the inhibitor concentration range used, the mechanism of inhibition of
cathepsin G and
chymotrypsin by oxidized MPI is consistent with a one-step reaction, [equation in text] whereas the inhibition of
elastase takes place in two steps, [equation in text].
Heparin, which accelerates the inhibition of the three
proteinases by native MPI, also favors their interaction with oxidized MPI. Flow calorimetry shows that
heparin binds oxidized MPI with Kd, Delta H degrees, and Delta S degrees values close to those reported for native MPI. In the presence of
heparin, oxidized MPI inhibits
cathepsin G via a two-step reaction characterized by Ki = 0.22 microM, k2 = 0.1 s-1, k-2 = 0.023 s-1, and Ki = 42 nM. Under these conditions, in vivo inhibition of
cathepsin G is again possible.
Heparin also improves the inhibition of
chymotrypsin and
elastase by oxidized MPI by increasing their kass or k2/Ki and decreasing their Ki. Our data suggest that oxidation of MPI during
chronic bronchitis may lead to
cathepsin G-mediated lung tissue degradation and that
heparin may be a useful adjuvant of MPI-based
therapy of acute
lung inflammation in
cystic fibrosis.