The incipient stage in thrombin-induced fibrin polymerization detected by FCS at the single molecule level

We used fluorescence correlation spectroscopy (FCS) to study the activation of fibrinogen by thrombin and the subsequent aggregation of fibrin monomer s into fibrin polymers at a very low and at physiological fibrinogen concen trations. In the labeling procedure used the fibrinogen was randomly labele d and the label was bound to the fibrinopeptide A and/or to the part of fib rinogen which after activation takes part in fibrin formation. We measured a diffusion coefficient for fibrinogen of 2.48 x 10(-7) +/- 0.10 x 10(-7) c m(2)/s. After activation with thrombin both fibrinopeptide A and fibrin pol ymerization products could be demonstrated. From our findings we suggest a model for the formation of a three-dimensional network as two parallel proc esses, elongation and branching and that fibrin oligomers are not only inte rmediates in the polymerization process but also are substrates for branchi ng, (C) 1999 Academic Press.