Inhibition of thrombin-mediated factor V activation contributes to the anticoagulant activity of fibrinogen γ'.

BACKGROUND: Besides its role in blood clotting, fibrinogen exerts a poorly understood anticoagulant function by binding thrombin and modulating its activity. In particular, the γA/γ' fibrinogen isoform binds with high affinity to thrombin exosite II through the anionic carboxyl-terminal end of the γ' chain. This interaction down-regulates thrombin-mediated factor VIII (FVIII) activation, but its effect on FV activation is unknown. OBJECTIVES: To investigate the overall anticoagulant activity of fibrinogen and particularly of fibrinogen γ' in plasma, and to verify whether the fibrinogen γ' carboxyl-terminal peptide affects thrombin-mediated FV activation. METHODS: Thrombin generation was measured by calibrated automated thrombography in whole and defibrinated plasma and in plasma supplemented with the (sulfated) fibrinogen γ' carboxyl-terminal peptide (0-500 µmol L(-1) ). The effect of the peptide on thrombin-mediated FV activation was studied in model systems and in plasma. RESULTS: Total fibrinogen prolonged the lag time of thrombin generation at low tissue factor (TF) concentrations. The fibrinogen γ' peptide dose-dependently prolonged the lag time and decreased the peak height of thrombin generation at low TF, whereas a scrambled control peptide was ineffective. These effects persisted in the presence of an anti-FVIII antibody, suggesting that the peptide may also inhibit thrombin-mediated activation of FV. This was confirmed in model systems and in plasma. CONCLUSIONS: Total fibrinogen and the fibrinogen γ' peptide have an overall anticoagulant effect on thrombin generation determined at low TF. Inhibition of thrombin-mediated FV activation by the fibrinogen γ' peptide is a novel mechanism of the anticoagulant activity of fibrinogen γ'.

'Agglutination and flocculation' of stem cells collected by apheresis due to cryofibrinogen.

Collection of peripheral stem cells by apheresis is a well-described process. Here, investigations concerning 'agglutination and flocculation' of stem cells collected from two patients are described. In both cases, cryoproteins were observed and cryofibrinogen was identified using high-resolution two-dimensional electrophoresis. In one case, peripheral stem cells were collected after a second course of mobilization, and the cells were immediately washed at 37 degrees C before being frozen, allowing their use, despite the presence of cryofibrinogen. In the other case, 'agglutination' was reversed by warming the bag, and plasma was removed before freezing.

A new type of congenital dysfibrinogen, fibrinogen Bremen, with an A alpha Gly-17 to Val substitution associated with hemorrhagic diathesis and delayed wound healing.

We have identified a new type of A alpha Gly-17 to Val substitution in a congenital dysfibrinogen, fibrinogen Bremen, derived from a 15-year-old boy having manifested easy bruising and delayed wound healing. The functional abnormality was characterized by altered fibrin monomer polymerization, which became evident by increasing the salt concentration and pH. A synthetic tetrapeptide with a sequence of the amino-terminal segment of normal fibrin alpha-chain, Gly-Pro-Arg-Val, substantially inhibited polymerization of both normal and the patient-derived fibrin monomers. A synthetic tetrapeptide with the Bremen type sequence of Val-Pro-Arg-Val inhibited polymerization of the patient's fibrin monomers partially at a peptide: fibrin monomer molar ratio of 4,000:1, and that of normal one at a much higher ratio of 10,000:1. Likewise, a synthetic peptide Ala-Pro-Arg-Val with a replacement of the Gly residue by another aliphatic amino acid Ala inhibited similarly the patient's fibrin monomer polymerization. Thus, the hypothetical two-pronged socket-like structure consisting of the alpha-amino group of the amino-terminal Gly and the guanidino group of an Arg at position 3 of the normal fibrin alpha-chain seems to be restored considerably in the mutant fibrin alpha-chain at low ionic strengths and pH's, despite the replacement of the amino-terminal Gly by another aliphatic amino acid Val.

Fibrinogen Ledyard (A alpha Arg16----Cys): biochemical and physiologic characterization.

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.