The regulation of human factor XIIa by plasma proteinase inhibitors

Abstract

Studies of the inactivation of factor XIIa by plasma protease inhibitors in purified systems and in plasma were initiated to determine the relative importance of these inhibitors to the neutralization of factor XIIa. Factor XIIa was measured by the amidolysis of H-D-prolyl-L-phenylalanyl-L-arginine-p-nitroanilide dihydrochloride or by coagulant activity. C1̄ inhibitor (C1INH), α2-antiplasmin (α2AP), α2-macroglobulin (α2M), and antithrombin III (ATIII) inhibited factor XIIa with second-order rate constants of 2.2 x 105, 1.1 x 104, 5.0 x 103, and 1.3 x 103 M-1 min-1. Factor XIIa activity was not affected by α1-proteinase inhibitor. Incubation of 125I-radiolabeled factor XIIa resulted in 1:1 stoichiometric complexes with C1INH (M(r) 190,000), ATIII (M(r) 125,000), and α2-AP (M(r) 150,000 and 125,000) using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Incubation of 125I-Factor XIIa with α2M resulted in a component of M(r) 85,000 on a reduced sodium dodecyl sulfate-polyacrylamide gel, indicating that a subunit of factor XIIa was covalently bound to a proteolyzed portion of α2M. The relative effectiveness of each inhibitor at plasma concentrations was 61:2:3:1 for C1INH, α2AP, α2M, and ATIII, respectively. Kinetic studies of the inactivation of purified factor XIIa added to various plasma containing different concentrations of C1INH verified the predictions from the purified systems. Gel filtration of radiolabeled factor XIIa incubated with plasma confirmed that factor XIIa-C1INH was the major complex. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the complexes in plasma had the same molecular size as those with purified inhibitors. C1INH functions as the predominant inhibitor of factor XIIa in plasma.