Plasma kallikrein (KAL) is inhibited by several plasma protease inhibitors, including C1-inhibitor (C1-INH), antithrombin III (ATIII), α1-antitrypsin (α1AT), and α2-macroglobulin (α2M). To assess the mechanism of action and the relative importance of these inhibitors, we have undertaken inhibition studies with purified proteins, using H-D-Pro- Phe-Arg-Nan as KAL substrate. Inhibition was competitive with C1INH, ATIII, and α1AT and noncompetitive with α2M. KAL retained 14% of its catalytic efficiency when complexed to α2M. The rate constants for inhibition by C1INH, ATIII, α1AT, and α2M were 28, 0.18, 0.003, and 6.9 M-ls-1(10-3) respectively. Michaelis-Menten kinetics was observed for the inhibition by ATIII, αlAT, and α2M. The constants for the rate-limiting formation of the irreversible complexes were 16, 0.27 and 2.0 s-1(xl02), while the KI’s for the reversible complex were 86, 63, and 0.29 γM, respectively for ATIII, α1AT and α2M. In_contrast, no Michaelis-Menten complex was observed when C1INH inhibited KAL. These results indicate that (a) C1INH is the most efficient inhibitor of KAL, (b) α2M is a significant inhibitor of KAL, (c) both ATIII and αlAT are probably not significant inhibitors of KAL. We have shown that high molecular weight kininogen (HMWK) decreases the inactivation rate of KAL by C1INH by forming a reversible complex with KAL. We now report that the reaction rates of KAL with ATIII and α1AT, which are competitive inhibitors, were decreased by 50%, when HMWK was 1 U/ml or 0.73 γM. When KAL was inhibited by α2M, a noncompetitive inhibitor, the inactivation rates were identical in the presence or absence of HMWK. Since HMWK protects KAL from being inhibited by competitive inhibitors but not by a noncompetitive one, these results confirm our previous observation indicating that the binding site for IMWK on KAL is closely linked to its catalytic site.
Inactivation of α2-Macroglobulin by Activated Human Polymorphonuclear Leukocytes
