title: An antibody specific for coagulation factor IX enhances the activity of the intrinsic factor X-activating complex.
abstract: 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.
authors.affiliation: Pre-Clinical Product Development, Baxter BioScience, Biomedical Research Center, A-2304 Orth/Donau, Austria.
authors.forename: Randolf J
authors.initials: RJ
authors.lastname: Kerschbaumer
authors.forename: Klaudia
authors.initials: K
authors.lastname: Riedrich
authors.forename: Martina
authors.initials: M
authors.lastname: Kral
authors.forename: Katalin
authors.initials: K
authors.lastname: Varadi
authors.forename: Friedrich
authors.initials: F
authors.lastname: Dorner
authors.forename: Jan
authors.initials: J
authors.lastname: Rosing
authors.forename: Friedrich
authors.initials: F
authors.lastname: Scheiflinger
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000737:chemistry; D002384:Catalysis; D004305:Dose-Response Relationship, Drug; D005164:Factor IX / Q000737:chemistry* / Q000276:immunology; D015944:Factor VIIIa / Q000737:chemistry; D005170:Factor X / Q000737:chemistry*; D006801:Humans; D006825:Hybridomas / Q000378:metabolism; D007700:Kinetics; D051379:Mice; D008807:Mice, Inbred BALB C; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry; D013917:Thrombin / Q000737:chemistry / Q000378:metabolism; D013997:Time Factors
chemical_list: D000911:Antibodies, Monoclonal; D011994:Recombinant Proteins; D015944:Factor VIIIa; D005164:Factor IX; D005170:Factor X; D013917:Thrombin