Polymorphism in fibrinogen aggregates

Impact of Temperature on the Self-Assembly of Fibrinogen in Thrombin-Free Solutions

Self-assembly of thrombin-free solutions of fibrinogen can be triggered not only by a drop in the ionic strength but also by an appropriate decrease in temperature. Accordingly, an in situ study of self-assembly of fibrinogen in saline buffered solution is carried out by means of time-resolved light scattering providing the molar mass, geometric size, and hydrodynamic radius of the growing intermediates. The resulting data provide access to the morphology of the intermediates and to the mechanism in which these intermediates grow during the early stages of self-assembly. Modeling the results of concentration dependent experiments based on temperature gradients in terms of a chain growth mechanism leads to the corresponding molar standard enthalpy and entropy of aggregation.

D-dimer specific monoclonal antibodies react with fibrinogen aggregates

Human fibrinogen exposed to 46.5 degrees C was subjected to gel permeation chromatography. The protein eluted in two distinct peaks. The first peak appeared in the void volume containing soluble fibrinogen aggregates, while the other peak represented monomeric fibrinogen. In contrast to the monomeric peak material, the aggregate fraction reacted with a panel of monoclonal antibodies specific for fragment D-dimer using an ELISA system. Edman degradation showed that both the aggregate and the monomeric fractions were devoid of soluble fibrin, and immunoblots of SDS-PAG electrophoretic profiles disclosed no sign of stabilized high molecular weight derivatives. We have previously shown that the aggregate fraction of similarly treated fibrinogen, in contrast to the monomeric fraction, stimulates the t-PA catalyzed conversion of plasminogen to plasmin and concomitantly exposes the sequences Aalpha-(148-160) and gamma-(312-324) involved in t-PA stimulation. Our present and previous findings suggest that soluble fibrinogen aggregates possess a fibrin-like structure, and that fibrin or fibrinogen polymer formation is a prerequisite for the enhancing effect on t-PA-mediated plasminogen to plasmin conversion which is seen even with the polymers in the soluble state.

The electron microscope band pattern of human fibrin: various stains, lateral order, and carbohydrate localization

Human fibrin negatively contrasted with a variety of heavy metal compounds and examined by electron microscopy displays a distinctive, nonpolar band pattern with a repeat of 22.5 nm. These results together with a reversal of contrast observed in images of positively stained fibrin, indicate that the striations reflect the protein density along the fiber. All major features of the band pattern can be accounted for directly in terms of a model for the structure of fibrinogen. Optical and computed diffraction patterns of micrographs of fibrin show that most specimens are highly ordered along the fiber axis but have only diffuse equatorial reflections arising from the average spacing of the protofibrils, although occasional fibers have discrete reflections at about 19 nm. Finally, the resulting change in negative staining pattern upon binding of lectins to the carbohydrate moieties is distinctive and allows the carbohydrate-containing beta domain of the molecule to be localized.

A survey of surface hemorheological experiments on the inhibition of fibrinogenin formation employing surface layers of fibrinogen systems with heparins and other substances. A contribution on antithrombogenic action

In earlier studies using a modified Weissenberg Rheogoniometer, we found decreased rigidity or torque values (tau) in surface layers of heparin plasma, when compared to tau of oxalate plasma from the same blood withdrawal (Thrombosis Res. 1, 1-17, 1972). In subsequent studies of the viscoelasticity of surface layers of highly purified fibrinogen (97-100% clottability) of human and bovine origin, we found, with some heparins, marked lowering of surface viscous moduli (eta's) and of surface elastic moduli (Gs). With some heparins no changes in tau, eta's and Gs occurred. Certain low molecular weight (LMW) preparations of heparins showed decreases, but some did not. This is also the case with heparins of low and high affinity for antithrombin. Calcium heparin and Ca2+ alone always increased eta's and Gs, when added to the fibrinogen system. N-desulfated heparin both decreased or did not change eta's and Gs. Preparations of fibrinogen in dog plasma, to which sodium heparin was added, resulted in a decrease of tau values. These results appear to emphasize that plasma proteins other than fibrinogen, and other plasma constituents, may affect surface hemorheological values. These findings suggest needed interface studies of fibrinogen systems to which plasma or plasma constituents are added. We found also that other substances, i.e., dextran MW 20,000; dextran sulfate MW 17,000; sodium hyaluronate and depolymerized hyaluronate decreased tau, eta's and Gs markedly. Recent findings in the literature are discussed in relation to thrombogenesis in which fibrinogenin gelation is considered as the initial phase of blood clotting. Fibrinogenin is the new term for initial fibrinogen aggregation and subsequent fibrinogen gelation without thrombin participation. The inhibition of fibrinogenin formation extra vivum is considered to be a valid indicator of antithrombogenic activity of substances which play a significant role in investigations on the therapy and prevention of thrombotic conditions.

The structure of fibrinogen and fibrin: II. Architecture of the fibrin clot

Our present low resolution model for fibrinogen based on electron microscopy and x-ray diffraction data has been described by Cohen et al. A unique aspect of the structural analysis of fibrous proteins is that the molecular packing in ordered arrays reflects biologically significant intermolecular interactions. We have shown that the orthogonal sheet microcrystals, which are closely related to fibrin, are made up of a highly regular arrangement of two-stranded protofibrils, and we have visualized aspects of both the substructure of the protofibrils as well as their packing to form the fibrin clot. By correlation of structural data with biochemical studies we have begun to identify certain functional regions of the fibrinogen model related to fibrin. Many aspects of fibrinogen's physiological activity remain to be related to its structure. As our present model is improved by higher resolution studies, we will see with increasing clarity molecular features critical for clot formation and fibrinolysis.

Band patterns seen by electron microscopy in ordered arrays of bovine and human fibrinogen and fibrin after negative staining

When fibers of fibrin clots or fragments of fibrinogen pellets are negatively stained they exhibit in the electron microscope characteristic patterns of cross-striations or bands. Those found in pellet material are indistinguishable from those seen in thrombin-induced fibrin fibers. The pattern seen in fibrin from bovine sources contains three equally spaced faint bands between every two of the broad prominent ones, spaced 23 nm apart. Human material shows a different pattern, one wherein no central faint band is seen, whereas the two remaining ones are broader. Its character is unaffected by crosslinking following fiber formation and preceding negative staining. The bovine pattern, however, is converted by such crosslinking to one that closely resembles the human. It is suggested that the striation pattern in human fibrin is due to juxtapositions of E domains of the parallel-aligned fibrin monomers with tightly coiled COOH-terminal regions of beta and gamma polypeptide chains, with no discernible contribution to the pattern from the alpha chain. In negatively stained untreated fibers of bovine fibrin, however, it is proposed that the COOH-terminal region of the alpha chain becomes tightly coiled, thereby contributing the faint central striation to the band pattern. Crosslinking prevents this conformational change in the alpha chain.

Quantitation of the inhibitory effect of fibrinogen and its degradation products on fibrin polymerization

Anticlotting activities of fibrinogen and its plasmin degradation products--fragments X,Y and D--have been measured. On the molar basis fragments Y and D are found equally active whereas fragment X acts about 2 times stronger. We suggest that the inhibitory effect depends on a set of specific binding sites characteristic of domain D. As fragment X possesses two such sets its activity is twice as high as that of the single-set fragments Y and D. In spite of its two domains D fibrinogen inhibits clotting much weaker than fragment X. This is probably due to the presence in fibrinogen of large COOH-terminal sections of the two A infinity-chains interfering with the inhibitory effect. The possible role of these A infinity-chain sections in fibrinogen-fibrin system is discussed.

A model from electron microscopy for the molecular structure of fibrinogen and fibrin

The overall architecture of the fibrinogen molecule and aspects of its packing to form fibrin have been derived from a study of electron microscope images of ordered arrays. The molecule, of length 450 A, is seen in more detail than the Hall-Slayter model and is made up of seven globular domains connected by rod-like segments.

Fibrinogen from human plasma: preparation by precipitation with heavy-metal coordination complex

Potassium tetrathiocyanato-(S)mercurate II [K(2)Hg(SCN)(4)] is used in a mild and rapid procedure for the isolation of human fibrinogen from fresh plasma. The final product, 94 to 99 percent of which is coagulable by thrombin, represents an average yield of 80 percent and is stable in solution. It is free of plasmin, streptokinase-activatable plasminogen, and coagulation factors II, V, VIII, X and XIII. Sedimentation analysis reveals a single peak with a sedimentation coefficient equal to 7.0S at infinite dilution. Immunodiffusion on cellulose acetate results in two precipitin lines with rabbit antiserum to whole human serum. The fibrinogen precipitates are unusual in that they are flocculent and readily redissolve.