CHARACTERIZATION OF A MONOCLONAL-ANTIBODY DIRECTED AGAINST THE CARBOXYL-TERMINUS OF HUMAN FACTOR-XIII - AN EPITOPE EXPOSED UPON DENATURATION AND CONSERVED ACROSS SPECIES LINES

By deriving an anti-peptide monoclonal antibody, mAb 7A4, we character ized the relatively unstudied carboxyl-terminal end of the a-chain of human factor XIII, the plasma transglutaminase. MAb 7A4 was directed a gainst the last eight amino acids (Gln-Ile-Gln-Arg-Arg-Pro-Ser-Met) an d bound with a dissociation constant of 3.4 x 10(-8) M. In a solid ass ay format, mAb 7A4 bound equally well to factor XIII obtained from hum an plasma, platelets or placenta. However, in a solution-phase assay f ormat, the epitope was largely unavailable but could be readily expose d by heat denaturation. Immunoblotting showed that this epitope is con served among all species of plasma factor XIII tested except rabbit su ggesting that the carboxyl-terminus might be an important structural e lement. Other competitive binding experiments with synthetic peptides as inhibitors pointed toward the final carboxyl-terminal amino acid, M et-731, as an immunochemically important determinant. This was used ad vantageously to confirm the finding that the carboxyl-terminal Mct-731 is largely absent from placental factor XIII (1) as compared to plate let or plasma factor XIII.