Blood clotting factor IX Nagoya 3: the molecular defect of zymogen activation caused by an arginine-145 to histidine substitution.

Factor IX Nagoya 3 (IX Nagoya 3) is a natural mutant of factor IX recognized in a patient with moderately severe hemophilia B. The patient had 0.60 units/ml of factor IX antigen and 2-5% of clotting activity. IX Nagoya 3 was purified from the patient's plasma by immunoaffinity chromatography with an anti-factor IX monoclonal antibody column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the treatment of IX Nagoya 3 with factor XIa/calcium ions resulted in cleavage only at the Arg180-Val181 bond. The amino acid sequence analysis of one of the lysyl endopeptidase peptides derived from IX Nagoya 3 revealed that Arg-145 is replaced by His. This substitution impairs the cleavage between the light chain and the activation peptide by factor XIa/calcium ions.