Discrimination of Normal and Abnormal Prothrombin and Protein C in Plasma Using a Calcium ton-Inhibited Monoclonal Antibody to a Common Epitope on Several Vitamin K-Dependent Proteins

Vitamin K deficiency or administration of vitamin K antagonists results in the biosynthesis of abnormal des-’y-carboxy forms of the vitamin K-dependent proteins. Monoclonal antibody H-i 1 binds several vitamin K-dependent proteins at a determinant that includes the first two residues of ‘y-carboxyglutamic acid. Antibody H-i 1 binds fully carboxylated prothrombin and protein C in the presence of EDTA but binding is inhibited by the divalent metal ions. calcium. magnesium. and manganese. By contrast. des-y-carboxy prothrombin and protein C bind antibody H-i i the same in the presence of EDTA or calcium ion. Antibody H-i i thus appears to bind a conserved antigenic site containing y-carboxyglutamic acid that in the presence of divalent metal ion undergoes a conformational transition. This ability of antibody H-i 1 to bind des-’y-carboxy prothrombin