Alpha-Chain cross-linking in fibrin(ogen) Marburg

The fibrinogen structural variant, Marburg (A alpha 1–460B beta gamma)2, is comprised of normal B beta and gamma chains but contains severely truncated A alpha chains that are missing approximately one half of their factor XIIIa cross-linking domain. Immunochemical studies of fibrin(ogen) Marburg were conducted to characterize the degree to which deletion of a defined A alpha-chain segment, A alpha 461–610, can affect the process of fibrin stabilization, ie, the factor XIIIa- mediated covalent interaction that occurs between alpha chains of neighboring fibrin molecules and between alpha chains and alpha 2 antiplasmin (alpha 2PI). The ability of Marburg (and control) alpha chains to serve as a substrate for factor XIIIa and undergo cross- linking was examined in an in vitro plasma clotting system. The capacity for alpha-chain cross-linking was evaluated both as the covalent incorporation of the small synthetic peptide, NQEQVSPLTLLK (which represents the first 12 amino acids of alpha 2PI and includes the factor XIIIa-sensitive glutamine residue responsible for the cross- linking of alpha 2PI to fibrin), and as the appearance of native (ie, natural), high-molecular-weight, cross-linked alpha-chain species. Antibodies specific for the (A)alpha and gamma/gamma-gamma chains of fibrin(ogen) and for the peptide and its parent protein, alpha 2PI (68 kD), were used as immunoblotting probes to visualize the various cross- linked products formed during in vitro clotting. Recalcification of Marburg plasma in the presence of increasing concentrations of peptide resulted in the formation of peptide-decorated Marburg alpha-chain monomers. Their size at the highest peptide concentration examined indicated the incorporation of a maximum of 3 to 4 mol of peptide per mole of alpha-chain. In the absence of alpha 2PI 1–12 peptide, the alpha chains of Marburg fibrin cross-linked to form oligomers and polymers, as well as heterodimers that included alpha 2PI. Both the peptide-decorated monomers and the native cross-linked alpha-chain species of Marburg fibrin were smaller than their control plasma counterparts, consistent with the truncated structure of the parent Marburg A alpha chain. Collectively, the findings indicate that, although deletion of the A alpha chain region no. 461–610 in fibrinogen Marburg prevents formation of an extensive alpha polymer network (presumably due to the absence of critical COOH-terminal lysine residues), it does not interfere with initial events in the fibrin stabilization process, namely, factor XIII binding and the ability of alpha chains to undergo limited cross-linking to one another and to alpha 2PI.