Characterization of bovine factor XIIa (activated Hageman factor)

Factor XIIa (activated Hageman factor) was isolated from bovine plasma by ammonium fractionation followed by heparin-agarose, carboxymethylcellulose, and arginine-agarose column chromatography. It was separated from factor XII in the final step by chromatography on benzamidine-agarose. Factor XIIa has a molecular weight of approximately 74 000 and is composed of a heavy and light chain held together by a disulfide bond(s). The amino-terminal sequence of the heavy chain is Thr-Pro-Pro-Trp--Lys-Gly-Pro-Lys-Lys-His-Lys-Leu- which is the same as the precursor protein. The carobyl-terminal residue in this polypeptide chain is arginine. The amino-terminal sequence of the light chain is Val-Val-Gly-Gly-Leu-Val-Ala-Leu-Pro-Gly-Ala-?-Pro-Tyr-Ile-. This latter sequence is homologous with the amino-terminal sequence of a number of plasma serine proteases when compared with the chain containing the active-site serine residue. These data suggest that factor XII is converted to factor XIIa by the cleavage of a specific internal arginyl-valine peptide bond. Factor XIIa, in contrast to factor XII, has hydrolase activity toward arginine-containing substrates and is readily inhibited by antithrombinIII and diisopropyl phosphorofluoridate. The inhibitors, in each case, are bound to the light chain of factor XIIa which contains the active-site serine residue.