Factor IX (FIX), a vitamin K dependent coagulation protein, is functionally deficient or absent in patients with hemophilia B. Binding of Ca++ by the gammacarboxyglutamic acid residues (Gla) of FIX is necessary for coagulant activity. Antibodies havebeendes-cribed which selectively bind to FIX in the presence of Ca++ and appear to interact with Ca-H- stabilized epitopes of FIX. One IgM, Kappa murine monoclonal antibody (Mab), 129-1, has been found to react preferentially with FIX in the absence of Ca-H- or with FIX having a reduction in gammacarboxylation. 129-1 was characterized by direct binding ELISA assays, the standard assay buffer (.02M Tris - .15M NaCL pH 7.5) was supplemented with Ca-H- or ED TA to final concentrations to 5mM and lOmM respectively. In the presence of Ca-H-, titers ranged from 10 to 100 as compared to 100 K to 500 K in the presence of EDTA. Control Mab's, FIX-30 and 2D521 showed no such effect. Maximum inhibition occurred ≥2.5 mM Ca-H- concentration. Mg++, Sr-H- and Mn-H- were tested with similar results. Chemically degammacarboxylated FIXa was compared to FIX and FIXa controls in the presence of Ca-H- or EDTA, to determine the importance of Gla residues. In the presence of EDTA, Mab 129-1 had essentially equal reactivity with all these proteins (titers 100 K). In the presence of Ca-H-, binding to FIX and FIXa was inhibited (500 and 1 K respectively). However, the binding to Gla modified FIXa in the presence of Ca-H- was only slightly reduced (10 K), suggesting the importance of Gla residues in the Ca-H- mediated inhibition. FIX was purified from concentrate, coumadinized plasma and culture supernatant (produced in the absence of vitamin K) from the cell line PMN45 using HPLC affinity chromatography with Mab 2D521. Mab 129-1 binding to FIX immunopurified from concentrate and a control biochemically purified FIX protein preparation was significantly higher in the presence of EDTA (titers 500 and 5 K) than in Ca-H- (titers 10 and 10). FIX purified from coumadinized plasma or PMN45 cells showed equal low reactivity with 129-1 in the presence or absence of Ca-H- (titer 10 in all cases). FIX-30 and 2D521 controls showed no such reduction in binding. Therefore, Gla residues may play a role in the binding of 129-1 to FIX.
Identification of the Phospholipid Binding Site in the Vitamin K-dependent Blood Coagulation Protein Factor IX