Fibrinogen Ledyard was discovered in a 10-year-old boy with a mild
bleeding history. His father had the same defect and a
bleeding history after surgery. Both patients were heterozygous. The plasma
fibrinogen concentration was normal immunologically (335 mg/dL) and very low functionally (52 mg/dL). Purified
fibrinogen Ledyard had a prolonged polymerization, which was somewhat corrected by addition of Ca2+
ions. High performance liquid chromatography (HPLC) analyses of the fibrinopeptides released by
thrombin showed 1 mol of
fibrinopeptide A (FPA) and 2 mol of
fibrinopeptide B (FPB) released per mole of
fibrinogen Ledyard. Steady-state kinetic parameters were evaluated for release of FPA by
thrombin. When the concentration of
fibrinogen Ledyard was corrected to 50% of total
protein, because only 50% of
fibrinogen Ledyard can release FPA, the kinetic constants were similar to those of control
fibrinogen (Km = 7.5 mumol/L for A alpha chain, kcat = 54 s-1). This finding indicates that the cleavage site of the A alpha chain in these abnormal molecules may not interact with the catalytic site of
thrombin. The three chains of
fibrinogen Ledyard were isolated on reverse-phase C4-HPLC. The sequence of the amino terminus of A alpha chain showed that Arg in position 16 was replaced by Cys in the abnormal molecules. Approximately half of
fibrinogen Ledyard (52%) was clotted by
reptilase, suggesting that
fibrinogen Ledyard may consist of 50% normal homodimers (A alpha Arg16 . A alpha Arg16) and 50% abnormal homodimers (A alpha Cys16 . A alpha Cys16). Abnormal molecules could form
disulfide bond between the A alpha Cys16 residues. Thus, the abnormal molecules have a different structure that does not bind to
thrombin. Probably the abnormality of polymerization of
fibrinogen Ledyard results from the interaction of the abnormal molecules with normal
fibrin monomers, so that the growth of
fibrin protofibrils is inhibited. This abnormal
fibrinogen supports
adenosine diphosphate-induced platelet aggregation in a normal manner.