Activation of bovine factor IX (Christmas factor) by factor XIa (activated plasma thromboplastin antecedent) and a protease from Russell's viper venom.

Bovine factor IX (Christmas factor) is a coagulation protein present in plasma in a precursor or inactive form. It is a glycoprotein (M, = 55,000) composed of a single polypeptide chain. Factor IX in the presence of calcium ions is converted to factor IX, by factor XI, (activated plasma thromboplastin antecedent) or a protease present in Russell’s viper venom. The activation of factor IX by the protease from Russell’s viper venom is due to the cleavage of a single internal Arg-Val peptide bond in factor IX. This leads to the formation of factor IX,,, a protein with the same molecular weight as the precursor. Factor IX,, is composed of two polypeptide chains held together by a disulfide bond(s), and these two chains have an NH,-terminal sequence of Tyr-Asn-Ser-Glyand Val-Val-Gly-Gly-. The mechanism of activation of factor IX catalyzed by the protease from Russell’s viper venom differs from that catalyzed by factor XI,. The latter enzyme activates factor IX in a two-step reaction. In the first step, an internal Arg-Ala bond is cleaved, and in the second step, an Arg-Val bond is cleaved. The second cleavage leads to the liberation of an activation peptide from the NH,-terminal portion of the heavy chain to produce factor IX,. The critical step in the activation of factor IX by both pathways is the cleavage of the same Arg-Val peptide bond and the formation of a new NH,-terminal sequence of Val-Val-Gly-Glyin the heavy chain of the enzyme which contains the active site serine residue. Factor I&w has approximately 50% of the specific activity of factor IX, in a coagulant or esterase assay, and both enzymes are inhibited by antithrombin III. A number of other purified plasma proteases were also examined for their ability to activate factor IX. Factor &,, (activated Stuart factor) catalyzed a slow but definite activation of factor IX, while the activation of factor IX by bovine kallikrein, thrombin, or factor XII, (activated Hageman factor) was negligible.