| Abstract: | An inhibitor of factor XIIa has been purified to homogeneity from bovine plasma. The purification steps included precipitation of contaminating proteins with polyethylene glycol and chromatography on DEAE-cellulose, Affi-Gel blue, and immobilized wheat germ lectin. The apparent molecular weight of the XIIa inhibitor (called INH1) was 85,000, reduced, and 92,000, nonreduced, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The extinction coefficient (E0.1%(280)) of INH1 is 1.3, and the protein contains 17.7% carbohydrate. Purified antibody to INH1 raised in either rabbits or chickens formed a precipitin line of identity with purified INH1 and a component of bovine plasma, but there was no reaction with purified human inhibitors or with any component of human plasma. INH1 inhibits bovine and human XIIa, bovine and human C1-esterase, and human kallikrein, but does not inhibit bovine kallikrein, bovine trypsin, human plasmin, or human thrombin. This activity is similar to that of C1-inhibitor but different from antithrombin III, alpha 2-antiplasmin, or alpha 1-protease inhibitor. INH1 at a physiological concentration (0.47 microM) causes rapid inactivation of XIIa. The two molecules react in a 1:1 stoichiometry with a second-order rate constant of 1.23 X 10(6) M-1 min-1. |
