The fibrin polymers formed in solution during the earliest phase of the fibrinogen–fibrin conversion are shown to be stable soluble molecules at pH7.4 and 0.15m- or 0.3m-NaCl. The various sequential soluble fibrin polymers produced from the fibrinogen–thrombin reaction can be observed by gel chromatography and can be isolated for characterization. The mechanism of fibrin polymerization proposed from the present studies suggests that the initial event is the thrombin activation at only one of the A α-chains in fibrinogen. The resulting highly reactive intermediate is the true fibrin monomer and it rapidly, and irreversibly, self-associates to form the stable fibrin dimer (s20.w=12S). Fibrin dimer possesses the N-terminal pattern alanine/glycine/tyrosine (1:1:2) per 340000 molecular weight, and possesses the chain structure [(α)A α)(B β)2(γ)2]2. The fibrin dimer is a soluble inert molecule, but additional thrombin activation of its remaining intact A α-chains leads to new associations into larger inert soluble fibrin polymers. In this manner progressively larger fibrin oligomers are constructed with thrombin continually in control of the process because of the necessity to repeatedly re-activate the various fibrin polymers in solution. The inert character of the soluble fibrin polymers can be explained by the reciprocal alignment of the associating molecules, which mutually consumes their active surfaces and leaves an intact A α-chain at either end of each fibrin oligomer. The soluble fibrin polymers will proceed to further association only if thrombin activates these remaining A α-chains, otherwise the fibrin molecules are stable indefinitely. The intermolecular associations within the soluble fibrin polymers are essentially irreversible under these nearly physiological conditions. However, the bonding is not covalent. This mechanism accounts for the clinical observations of stable fibrinogen-derived polymers in the plasma from patients undergoing thrombotic processes. Since it is shown that the intermediate fibrin polymers, themselves, are stable soluble molecules, it is no longer necessary, nor warranted, to invoke hypothetical ‘fibrinogen–fibrin complexes’ to explain observations of fibrin solubility.

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