Fibrinogen Chapel Hill II: defective in reactions with thrombin, factor XIIIa and plasmin

Fibrinogen Chapel Hill II is a hereditary, abnormal fibrinogen which is characterized by poor substrate reactivity toward thrombin, factor XIIIa and plasmin. The patient has a low plasma level of clottable protein with normal antigen concentration, high amounts of fibrinogen related material in serum, and prolonged thrombin and reptilase clotting times. Fibrinopeptide release was decreased with both thrombin and ancrod, indicating that release of fibrinopeptide A from the abnormal fibrinogen was impaired. Sequence analysis indicated that the A peptide was normal. Light scattering indicated that the fibrils formed by thrombin were unusually short and thick. When clotted under crosslinking conditions gamma dimers formed normally but alpha polymer formation was defective. Under conditions which yielded complete plasmin digestion of normal fibrinogen only half of the patient fibrinogen was degraded beyond the fragment X stage. The rate of fibrinopeptide release from patient fragment X and NH2-terminal disulphide knot (N-DSK) was similar to that from the fibrinogen, indicating that the defect was contained within the N-DSK. A simple amino acid substitution could result in a conformational defect in the N-DSK sufficient to perturb the reactions involving thrombin, factor XIIIa and plasmin and also polymerization.