An Antibody Specific for Coagulation Factor IX Enhances the Activity of the Intrinsic Factor X-activating Complex

• Published in: The Journal of biological chemistry Vol. 279; no. 39; pp. 40445 - 40450
• Main Authors: Kerschbaumer, Randolf J., Riedrich, Klaudia, Kral, Martina, Varadi, Katalin, Dorner, Friedrich, Rosing, Jan, Scheiflinger, Friedrich
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 24.09.2004; American Society for Biochemistry and Molecular Biology
• Subjects: Animals; Antibodies, Monoclonal - chemistry; Catalysis; Dose-Response Relationship, Drug; Factor IX - chemistry; Factor IX - immunology; Factor VIIIa - chemistry; Factor X - chemistry; Humans; Hybridomas - metabolism; Kinetics; Mice; Mice, Inbred BALB C; Protein Binding; Recombinant Proteins - chemistry; Thrombin - chemistry; Thrombin - metabolism; Time Factors
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M405966200

During hemostasis the zymogen factor X (FX) is converted into its enzymatically active form factor Xa by the intrinsic FX-activating complex. This complex consists of the protease factor IXa (FIXa) that assembles, together with its cofactor, factor VIIIa, on a phospholipid surface. We have studied the functional properties of a FIXa-specific monoclonal antibody, 224AE3, which has the potential to enhance intrinsic FX activation. Binding of the antibody to FIXa improved the catalytic properties of the intrinsic FX-activating complex in two ways: (i) factor VIIIa bound to the FIXa-antibody complex with a more than 18-fold higher affinity than to FIXa, and (ii) the turnover number (kcat) of the enzyme complex increased 2- to 3-fold whereas the Km for FX remained unaffected. The ability of 224AE3 to increase the FXa-generation potential (called the “booster effect”) was confirmed in factor VIII (FVIII)-depleted plasma, which was supplemented with different amounts of recombinant FVIII. In the presence of antibody 224AE3 the coagulant activity was increased 2-fold at physiological FVIII concentration and up to 15-fold at low FVIII concentrations. The booster effect that we describe demonstrates the ability of antibodies to function as an additional cofactor in an enzymatic reaction and might open up a new principle for improving the treatment of hemophilia.