Mechanism of Activation of Human Factor IX by Activated Human Factor XI

Human Factor IX (F. IX) has been highly purified using: BaSO₄ adsorption and elution, DEAE-cellulose chromatography, preparative polyacrylamide gel electrophoresis, and chromatography on heparin-agarose. The purified F. IX appeared homogeneous on SDS-gels and revealed a single polypeptide chain of 55,000 MW in the Weber-Osborn system. Human Factor XI was purified by ion exchange chromatography to greater than 95% homogeneity as judged on polyacrylamide gels. Following activation of factor XI by purified human factor XII_a, the factor XI_a was a potent activator of F. IX in a Ca⁺⁺-dependent reaction. As judged on SDS gels, activated F. IX at 43,000 mw contains two disulfide-linked polypeptide chains of 27,000 and 16,000 mw. These mw values for the zymogen and activated form of F. IX are identical to those reported for bovine F. IX. And the amino acid compositions of human F. IX and of the isolated heavy and light chains of F. IX_a are quite similar to those reported for bovine F. IX and F. IX_a. Based on these observations, it is suggested that activated human F. IX contains its active site in a C-terminal 27,000 mw polypeptide which is linked by disulfide bonds to a 16,000 mw N-termina| polypeptide containing gamma carboxyglutamic acid.