Substitution of gamma Arg-275 by Cys in an abnormal fibrinogen, "fibrinogen Osaka II". Evidence for a unique solitary cystine structure at the mutation site

In an abnormal fibrinogen with impaired fibrin monomer polymerization designed as fibrinogen Osaka II, we have identified substitution of Arg by Cys at position 275 of the gamma chain. This Cys is linked to a free cysteine molecule by a disulfide link as evidenced by fast atom bombardment mass spectrometry. This finding was supported by identification of a single cysteine released from isolated abnormal fragment D1 upon reduction. This unique cystine structure at the mutation site has not been reported heretofore in any abnormal protein including fibrinogen. The substitution may well perturb the structure required for fibrin monomer polymerization, specifically that assigned to the carboxyl-terminal D domain of fibrinogen. Indeed, isolated fragment D1 with the Cys substitution failed to inhibit thrombin-mediated clotting of normal fibrinogen and normal fibrin monomer polymerization, while normal fragment D1 inhibited them markedly. Our data seem to provide supporting evidence that the putative polymerization site(s) assigned to the D domain of fibrinogen may be structure-dependent, including the carboxyl-terminal segment of the gamma chain as well as a contiguous region that contains the gamma 275 residue.     

Expression of primary polymerization sites in the D domain of human fibrinogen depends on intact conformation

Fragments D1 and DD, plasmic degradation products of human fibrinogen and cross-linked fibrin, respectively, originate from the COOH-terminal domain of the parent molecule. Since a specific binding site for fibrin resides in the COOH-terminal region of the gamma chain, the primary structure of the two fragments was compared and their affinity for fibrin monomer measured. Fragments D1 and DD contained the same segments of the three fibrinogen chains, corresponding to the sequences alpha 105-206, beta 134-461, and gamma 63-411. Fragment DD had a double set of the same chain remnants. Fragments D1 and DD inhibited polymerization of fibrin monomer in a dose-dependent manner; 50% inhibition occurred at a molar ratio of fragment to monomer of 1:1 and 0.5:1, respectively. To prevent fibrin monomer polymerization and render it suitable for binding studies in the liquid phase, fibrinogen was decorated with Fab fragments isolated from rabbit antibodies to human fragment D1. Fibrinogen molecules decorated with 6 molecules of this Fab fragment did not clot after incubation with thrombin, and the decorated fibrin monomer could be used to measure binding of fragments D1 and DD in a homogeneous liquid phase. The data analyzed according to the Scatchard equation and a double-reciprocal plot gave a dissociation constant of 12 nM for fragment D1 and 38 nM for fragment DD. There were two binding sites/fibrin monomer molecule for each fragment. After denaturation in 5 M guanidine HCl, the inhibitory function on fibrin polymerization was irreversibly destroyed. Denatured fragments also lost binding affinity for immobilized fibrin monomer. The preservation of the native tertiary structure in both fragments was essential for the expression of polymerization sites in the structural D domain.     

Characterization of the inhibition of fibrin assembly by fibrinogen fragment D.

Fragment D (Mr 100 000) prepared from a terminal plasmin digest of fibrinogen was isolated and used to study its effect on fibrin formation. Increasing amounts of fragment D added to a solution of fibrinogen and thrombin decrease the rigidity of the resultant gel (10% of control at 2 mol of fragment D/mol of fibrinogen). Half-maximal inhibition is achieved at 1 mol of fragment D/mol of fibrinogen for non-cross-linked clots and at 1/2 mol of fragment D/mol of fibrinogen for cross-linked clots. "Clottability' decreases concomitantly with the rigidity. Only small amounts of fragment D (less than 10% for non-cross-linked gels) are incorporated into the gel. Light-scattering shows an increase in the final fibre thickness at fragment D concentrations up to 2 mol of fragment D/mol of fibrinogen, from 60 molecules/cross-section for the control to 120 molecules/cross-section. Higher fragment D concentrations lead to a decrease in the final fibre thickness. The limit fibre thickness is 8 nm, with a length of 80 nm, which is equivalent to a fibrin trimer. On the basis of results of synthetic-substrate and fibrinopeptide-release assays, it is clear that thrombin inactivation is not responsible for this effect. These data suggest that fragment D may inhibit fibrin formation by blocking the bimolecular polymerization of activated fibrin monomer molecules to form protofibrils, although additional effects on subsequent assembly steps may also be involved.     

Significance of tryptophan residues in the D-domain of the fibrin molecule in fibrin polymer formation

The modification of fibrin monomer with H2O2 caused reduction of the association activity of fibrin monomer. The association activity was not reduced even by modification of approx. 16 out of the total 64 tryptophan residues in the fibrin molecule; it was then abolished by further modification of the following several residues. Fragment D obtained by proteolysis of fibrinogen with plasmin, inhibited the association activity of fibrin monomer and the modification of approx. six out of the total 21 tryptophan residues in the fragment led to the complete loss of the inhibitory effect. It was concluded from these studies that about six tryptophan residues in the D-domain of fibrin are important for the association of fibrin monomer.     

Analysis of soluble fibrin complexes by agarose gel chromatography and protamine sulfate gelation.

The molecular makeup of soluble fibrin complexes was studied by gel exclusion chromatography using radio-labelling to characterize individual components in protein mixtures. Products of limited plasmin degradation of fibrinogen and mixtures of fibrinogen and "early" fibrinogen digests formed high molecular weight soluble fibrin complexes upon incubation with thrombin. Purified, nonclottable fragment Y did not incorporate into soluble fibrin complexes, nor could we demonstrate incorporation of fragments D and E as previously described from our laboratory. Thus, under the conditions of these experiments, soluble fibrin complexes have two identifiable components, fibrin monomer and clottable fragment X monomer, although incorporation of native fibrinogen or fragment X unreacted by thrombin into soluble fibrin complexes cannot be excluded. Individual fractions of thrombin-treated early fibrinogen digests isolated by agarose gel chromatography were treated with protamine sulfate at 37 degrees C resulting in precipitation-gelation of greater than 90 per cent of high molecular weight soluble fibrin complexes; whereas, less than 10 per cent of lower molecular weight fibrinogen degradation products precipitated by protamine sulfate. These findings do not support the widely held concept that soluble fibrin complexes incorporate nonclottable degradation products of fibrinogen proteolysis, nor do they support the notion that the so-called paracoagulation reaction induced by protamine sulfate results from the splitting of complexes between fibrin monomer and nonclottable fibrinogen degradation products.     

Localization of a fibrin polymerization site

The formation of a fibrin clot is initiated after the proteolytic cleavage of fibrinogen by thrombin. The enzyme removes fibrinopeptides A and B and generates fibrin monomer which spontaneously polymerizes. Polymerization appears to occur though the interaction of complementary binding sites on the NH2-terminal and COOH-terminal (Fragment D) regions of the molecule. A peptide has been isolated from the gamma chain remnant of fibrinogen Fragment D1 which has the ability to bind to the NH2-terminal region of fibrinogen as well as to inhibit fibrin monomer polymerization. The peptide reduces the maximum rate and extent of the polymerization of thrombin or batroxobin fibrin monomer and increases the lag time. The D1 peptide does not interact with disulfide knot, fibrinogen, or Fragment D1, but it binds to thrombin-treated disulfide knot with a Kd of 1.45 X 10(-6) M at approximately two binding sites per molecule of disulfide knot. Fibrin monomer formed either by thrombin or batroxobin binds approximately two molecules of D1 peptide per molecule of fibrin monomer, indicating that the complementary site is revealed by the loss of fibrinopeptide A. The NH2-terminal sequence (Thr-Arg-Trp) and COOH-terminal sequence (Ala-Gly-Asp-Val) of the D1 peptide were determined. Therefore the gamma 373-410 region of fibrinogen contains a polymerization site which is complementary to the thrombin-activated site on the NH2-terminal region of fibrinogen.     

Self-assembly of soluble unlinked and cross-linked fibrin oligomers

Self-assembly of soluble unlinked and cross-linked fibrin oligomers formed from desA-fibrin monomer under the influence of factor XIIIa was studied in the presence of non-denaturing urea concentrations. By methods of elastic and dynamic light scattering combined with analytical ultracentrifugation, desA-fibrin oligomers formed in both the presence and absence of the factor XIIIa were shown to be ensembles consisting of soluble rod-like double-stranded protofibrils with diverse weight and size. Unlinked and cross-linked soluble double-stranded protofibrils can reach the length of 350–450 nm. The structure of soluble covalently-linked protofibrils is stabilized by isopeptide γ-dimers. Electrophoretic data indicate a complete absence of isopeptide bonds between α-chains of desA-fibrin molecules. The molecular mechanism of formation of soluble rod-like fibrin structures and specific features of its covalent stabilization under the influence of factor XIIIa are discussed.     

Interaction between complementary polymerization sites in the structural D and E domains of human fibrin.

Plasmic degradation products of human fibrin, fragments DD, D, and E, bind to fibrin. It has been inferred from this observation that the binding occurs by attraction of complementary sites located in the NH2- and COOH-terminal domains of the fibrin molecule. The interaction between fragments D1 and E1 has been investigated in this work since it represents the first step in the process of fibrin clot formation. Fragment D1, that was initially as active as fragment DD, lost most of its anticoagulant activity after purification by cation-exchange chromatography. The lability of fragment D1 function explained the previous unsuccessful attempts to form a complex between fragments D1 and E1. The loss of fragment D1 anticoagulant activity was not associated with the cleavage of the gamma 63-85 chain segment, since fragments D1A and D1 identically inhibited the fibrin monomer polymerization rate. In order to demonstrate the formation of a complex between fragments D1 and E1, three lines of experiments were advanced. First, the anticoagulant activity of fragment D1 was neutralized by fragment E1 in a dose-dependent manner, demonstrating that the association between these fragments involved polymerization sites. Second, two products, D1.E1 and D1.E1.D1, were stabilized in a reaction with bifunctional cross-linking reagents, proving the formation of D.E complexes in aqueous solution. Third, immobilized fragment D1 bound fragments E1 and E2, but not fragment E3, showing that fragments E1 and E2 attached via a polymerization site to the complementary one in fragment D1, since this association was disrupted by fibrin polymerization inhibitory peptide GPRP. These results provided direct evidence for specific binding between the structural D and E domains of fibrin mediated through complementary polymerization sites. Thus, the initial formation of fibrin clot fibers appears to be driven by specific association of these sites.     

Multidomain structure of fibrinogen and its transformations

Generalized concepts of some structural peculiarities of fibrinogen, its transformation into fibrin and assembly have been considered on the basis of author's and published data. The role of local conformational changes in different areas of fibrinogen molecule and of separate reaction centers in formation of single- and double-stranded rod-like equilibrium fibrin oligomers and flexible branched copolymers of fibrinogen with fibrin E fragment has been considered. The mechanism of compactization has been discussed.     

Structural transformations of fibrin oligomers

The morphology of equilibrium of soluble fibrin oligomers at different stages of assembly was studied. Results of Rauleigh's light scattering, analytical ultracentrifugation and viscosimetry show that fibrin-polymers throughout the entire homology range present rigid, rod-like structures dispersed by weight and dimensions. It was shown, that along with the traditional double-stranded chain protofibrills, where the monomer molecules are connected "end-to-center", there is an alternative variant, which is a result of single-stranded chain dimerization, where the monomers are formed up in an "end-to-end" fashion. Identity of physicochemical features of fibrin oligomers obtained by means of different enzyme activation of fibrinogen indicates that E1 and E2 sites interact with the complementary D1 and D2 sites only at the stage of protofibrill formation. It is suggested that the lateral aggregation is initiated by other sites that exist in fibrinogen and fibrin-monomer molecules in an accessible state. Thermodynamic reasons for the cooperative ability of protofibrill aggregation processes and gel-formation are discussed.     

Fibrin protofibril and fibrinogen binding to ADP-stimulated platelets: evidence for a common mechanism

The molecular basis of platelet-fibrin binding has been elucidated by studying interactions between platelets and protofibrils, soluble two-stranded polymers of fibrin which are intermediates on the fibrin assembly pathway. The fibrinogen degradation product, fragment D, has been used to block fibrin assembly, thus enabling the preparation of stable solutions of short protofibrils, composed of fewer than twenty fibrin monomer molecules per polymer. Fibrin protofibrils bound to ADP-activated platelets in a time- and concentration-dependent process which was effectively blocked by excess unlabelled fibrinogen, i.e., the binding was specific and appeared to involve a common receptor. ADP-stimulated cells bound approx. 3 micrograms of fibrin protofibrils/10(8) platelets, compared to 4 micrograms of fibrinogen/10(8) cells, following a 30-min incubation period at room temperature. Binding of both ligands was inhibited by high concentrations of fragment D, further indicating a similar mechanism. The kinetic data obtained were well described by an apparent first-order mechanism in which the rate constant for fibrin protofibril binding was found to be 5-fold slower than that measured for fibrinogen. Two monoclonal antibodies, each directed against the platelet glycoprotein IIb-IIIa complex, inhibited the binding of fibrin protofibrils and fibrinogen in a similar, concentration-dependent manner, providing strong evidence for a common receptor. Binding of GPRP-fibrin (soluble fibrin oligomers formed in the presence of 1 mM Gly-Pro-Arg-Pro) to ADP-stimulated platelets was also inhibited by a monoclonal antibody directed against the GPIIb-IIIa complex. Neither fibrin protofibrils nor fibrinogen bound to Glanzmann's thrombasthenic platelets, which lack normal quantities of functional glycoprotein IIb-IIIa complex, further supporting the hypothesis that fibrinogen and fibrin bind to a common platelet receptor present on the glycoprotein IIb-IIIa complex.     

Structural Effects of Methionine Oxidation on Isolated Subdomains of Human Fibrin D and αC Regions

Oxidation of key methionine residues on fibrin leads to altered fibrin polymerization producing severely altered fibrin gel structure and function. This is important because fibrinogen and its modification by oxidative stress have been implicated as key contributors to both pathological thrombotic and hemorrhagic diseases ranging from cardiovascular thrombosis to the acute coagulopathy of trauma. However, how oxidation leads to altered fibrin polymerization remains poorly understood at the molecular level. We have applied a powerful and novel well-tempered ensemble parallel tempering (PT-WTE) technique along with conventional molecular dynamics (MD) simulation to investigate the molecular-level consequences of selective methionine oxidation of fibrinogen. We offer new insights into molecular mechanisms of oxidation-induced changes in fibrin polymerization, while focusing on the D region knob ‘B’ and hole ‘b’ interaction and αC-domain interactions, both of which are hypothesized to contribute to the lateral aggregation mechanism of fibrin fibrils. Methionine oxidation did not alter the native state or the stability of a bound knob ‘B’ surrogate when interacting with hole ‘b’ in the D region. However, applying PT-WTE simulation to a human homology model of the bovine N-terminal subdomain fragment from the αC-domain revealed that methionine oxidation altered the conformation of the hairpin-linking region to favor open rather than closed hairpin structures. We attribute this alteration to the disruption of the hairpin-linking region''s conformation, with oxidation increasing the radius of gyration for this segment. This result is in agreement with experimental data demonstrating decreased fibrin protofibril lateral aggregation when methionine oxidation is present in the same αC-domain fragment. Therefore, single methionine oxidation within the αC-domain is a likely molecular mechanism.     

Localization of segments essential for polymerization and for calcium binding in the gamma-chain of human fibrinogen

We have isolated an intermediate plasmic degradation product, D2, of fibrinogen that does not inhibit the polymerization of fibrin monomer but does bind Ca2+. Fibrinogen was digested to a limited extent with plasmin in the presence of Ca2+, and a "large" fragment D (fragment D1A) was isolated with a gamma-chain remnant consisting of residues 63-411. Fragment D1A was digested further in the presence of Ca2+, yielding fragment D1 (with its gamma-chain containing residues 86-411). The digestion of fragment D1 [in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) to complex Ca2+] led to a gradual shortening of the carboxyl-terminal portion of the gamma-chain. Fragment D2 (with its gamma-chain containing residues 86-335/356) was isolated from an intermediate digest in the presence of EGTA. The Lys-338-Cys-339 peptide bond of the gamma-chain is intact in this preparation of D2, even though it is split in the isolated peptide gamma303-355 (with an intact disulfide bond at Cys-326-Cys-339). Fragment D2 does not interfere with the polymerization of fibrin monomer, whereas fragment D1 is a potent inhibitor of this polymerization. We conclude that the gamma-chain segment 356/357-411, present in fragment D1 but absent from fragment D2, is essential for maintenance of a polymerization site located in the outer (D) nodule of fibrinogen. This segment (356/357-411) is longer than two shorter ones reported earlier [Olexa, S.A., & Budzynski, A. Z. (1981) J. Biol. Chem. 256, 3544-3549; Horwitz, B.H., Váradi, A., & Scheraga, H.A. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 5980-5984]; the data for the earlier reports are reinterpreted here. Finally, fragment D2 possesses a single Ca2+ binding site, as revealed by equilibrium dialysis binding studies. Since fragment D3 (with its gamma-chain containing residues 86-302) fails to bind Ca2+, we conclude that segment gamma 303-355/356 plays a crucial role in Ca2+ binding.     

Characterization of soluble polymerized fibrin formed in the presence of excess fibrinogen fragment D

Polymerization of fibrin is inhibited in the presence of excess fibrinogen fragment D. This study was performed in order to test the proposal that these inhibited solutions contain short linear polymers of fibrin (protofibrils) whose further polymerization is prevented as a result of attachment of a molecule of fragment D at each end. Negative-stain electron micrographs, intrinsic viscosities, angular dependence of light scattering intensity, and kinetics of the increase of the scattered intensity with polymerization all were found to support the above model of the inhibited polymer and to reflect the presence of a broad distribution of the lengths of the inhibited fibrin polymers. Furthermore, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of polymers stabilized with gamma-dimer cross-links introduced by factor XIIIa demonstrates cross-linking of fragment D to fibrin oligomers. Cross-linked polymers have been separated from excess fragment D by gel exclusion chromatography in 1 M urea. (In the absence of urea, the purified polymers very slowly associate to fibers.) The observation of the relative stability of short isolated inhibited protofibrils and the decrease or absence of inhibition of fibrin gelation when fragment D was added to solutions in which fibrin had been given time to polymerize to long protofibrils demonstrate that the inhibitory effect of fragment D occurs as a result of inhibition of the first fibrin polymerization step.     

Interaction of phosphatidyl serine with fibrin monomer]

Phosphatidyl serine induces a concentration-dependent inhibition of polymerization of fibrin monomer and forms a complex with it, which is stable to gel-filtration and chloroform treatment. During plasmin proteolysis phosphatidyl serine remains tightly bound to the fragments of the fibrin monomer molecule formed. A correlation between the amount of amino acids responsible for phospholipid binding and that of phosphatidyl serine bound to the fragment of the fibrin monomer molecule was observed. The introduction of phosphatidyl serine into the blood flow causes a decrease of the thrombin-precipitated fibrinogen and fibrin monomer obtained from animal plasma. At the same time phosphatidyl serine is present in fibrinogen and in high amounts in the fibrin monomer. It is assumed that phosphatidyl serine which controls thrombinogenesis and enzymatic and non-enzymatic steps of fibrin production can thus be regarded as a natural stabilizer of the blood.     

The antifibrinolytic action of the D-dimer fragment

The effect of D-dimer on the process of plasmin hydrolysis of unstabilized and crosslinked fibrin has been studied. Less degraded early, intermediate, and late products of fibrin cleavage have been revealed by electrophoresis of reduced and nonreduced samples. The molecular mechanism of antifibrinolytic effect of the D-dimer is supposed to be determined by shielding of peptide regions of monomer fibrin, localized both in N-terminal area of beta chain and in alpha, beta, and gamma chains between D and E domains. A notion has been proposed of autoinhibition of fibrinolytic reaction as a phenomenon related to the physical-chemical regulation of fibrinogen transformation into fibrin.     

Role of D and E domains in the migration of vascular smooth muscle cells into fibrin gels

The structure of fibrin plays an important role in the organization of thrombi, the development of atherosclerosis, and restenosis after PTCA. In this study, we examined the mechanisms of the migration of vascular smooth muscle cells (SMCs) into fibrin gels, using an in vitro assay system. Cultured SMCs from bovine fetal aortic media migrated into fibrin gels prepared with thrombin, which cleaves both fibrinopeptides A and B from fibrinogen, without other chemotactic stimuli. Both desA fibrin gels prepared with batroxobin, which cleaves only fibrinopeptide A, and desB fibrin gels prepared with Agkistrodon contortrix thrombin-like enzyme (ACTE), which cleaves only fibrinopeptide B, similarly induced the migration of SMCs compared to fibrin gels prepared with thrombin. These results suggest that the cleavage of fibrinopeptides is not necessary, but rather that the three-dimensional structure of the gel may be important for the migration of SMCs. Furthermore, gels prepared with protamine sulfate, which forms fibrin-like gels non-enzymatically, similarly induced the migration of SMCs compared to the gels prepared with thrombin. Both anti-fibrin(ogen) fragment D and anti-fibrin(ogen) E antibodies inhibited the migration of SMCs into fibrin gels, suggesting that both the D and E domains of fibrin(ogen) are involved in the migration of SMCs into fibrin gels. The addition of GRGDS, a synthetic RGD-containing peptide, but not that of GRGES, a control peptide, partially inhibited the migration of SMCs into fibrin gels, suggesting that the migration of SMCs into fibrin gels is at least in part dependent on the RGD-containing region of the alpha chain. The migration of SMCs into fibrin gels was also inhibited by a monoclonal antibody for integrin alpha v beta 3 and alpha 5 beta 1, indicating that migration is dependent on these integrins. Furthermore, both fibrin(ogen) fragments D and E inhibited the migration of SMCs into fibrin gels, suggesting that these fragments, generated during fibrino(geno)lysis, may be relevant in the regulation of SMC migration into fibrin gels.     

Characterization of an apparently lower molecular weight gamma-chain variant in fibrinogen Kyoto I. The replacement of gamma-asparagine 308 by lysine which causes accelerated cleavage of fragment D1 by plasmin and the generation of a new plasmin cleavage site

Congenitally abnormal fibrinogen Kyoto I with impaired fibrin monomer polymerization contains a normal gamma-chain and a gamma-chain variant (gamma Kyoto I) that has an apparently lower Mr on sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the Laemmli system (Laemmli, U. K. (1970) Nature 227, 680-685) but migrates with apparently normal Mr in the Weber and Osborn system (Weber, K., and Osborn, M. (1969) J. Biol. Chem. 244, 4406-4412). Reverse-phase high performance liquid chromatographic analyses of the cyanogen bromide or lysyl endopeptidase cleavage fragments of the purified gamma-chains of fibrinogen Kyoto I showed the presence of peptides not seen from normal fibrinogen. Amino acid sequence analysis of these peptides indicated that gamma Asn308 of the gamma-chain variant is replaced by lysine. Purified fragment D1 of fibrinogen Kyoto I also contains two types of D1 gamma-remnants: normal and apparently lower Mr types. Abnormal fragment D1 is cleaved faster to fragments D2 and D3 by plasmin in the presence of [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) than normal fragment D1, as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by immunoblotting using anti-gamma-chain monoclonal antibody. Analysis of peptides released from fragment D1 by plasmin in the presence of EGTA demonstrated the cleavage of the gamma Lys308-Gly309 bond. Fragment D1 of fibrinogen Kyoto I has normal calcium binding properties. The data suggest that a region or conformation containing gamma Asn308 affects the polymerization of fibrin monomers and that the gamma Asn308----Lys replacement causes a conformational change in the gamma-chain which results in the accelerated cleavage of gamma Lys356-Ala357 and gamma Lys302-Phe303 bonds by plasmin and also results in the generation of a new plasmin cleavage site between Lys308 and Gly309 in the presence of EGTA. During these studies, we found that part of the gamma Lys212-Glu213 bond in fragment D1 is cleaved by plasmin in the presence of EGTA.     

Fibrinogen fragment D - inhibitor of fibrinolytic processes

The effect of fragment D, the end product of fibrinogen degradation, on the course of fibrinolytic reactions and fibrinogenolysis induced by plasmin was studied. It was shown that fragment D beside a high antipolymerizing activity also exerts antifibrinolytic and antifibrinogenolytic action. It was demonstrated electrophoretically that exogenous fragment D can inhibit plasmin degradation of fibrin and fibrinogen at all stages of proteolysis without having direct influence on plasmin. It is assumed that the nature of the antipolymerizing and antifibrinolytic activities of fragment D is determined by dissociating fibrin monomer-fragment D complexes.