Identification of the polypeptide chains involved in the cross-linking of fibrin

Altered fibrinogen γ-chain cross-linking in mutant fibrinogen-γΔ5 mice drives acute liver injury

BACKGROUND

Hepatic deposition of cross-linked fibrin(ogen) occurs alongside platelet accumulation as a hallmark of acetaminophen (APAP)-induced liver injury.

OBJECTIVES

We sought to define the precise role of the fibrinogen γ-chain C-terminal integrin αIIbβ3 binding domain in APAP-induced liver injury.

METHODS

Mice expressing mutant fibrinogen incapable of engaging integrin αIIbβ3 due to a C-terminal fibrinogen γ-chain truncation (mutant fibrinogen-γΔ5 [FibγΔ5] mice) and wild-type mice were challenged with APAP (300 mg/kg, intraperitoneally).

RESULTS

We observed an altered pattern of fibrin(ogen) deposition in the livers of APAP-challenged FibγΔ5 mice. This led to the unexpected discovery that fibrinogen γ-chain cross-linking was altered in the livers of APAP-challenged FibγΔ5 mice compared with that in wild-type mice, including absence of γ-γ dimer and accumulation of larger molecular weight cross-linked γ-chain complexes. This finding was not unique to the injured liver because activation of coagulation did not produce γ-γ dimer in plasma from FibγΔ5 mice or purified FibγΔ5 fibrinogen. Sanger sequencing predicted that the fibrinogen-γΔ5 γ-polypeptide would terminate at lysine residue 406, but liquid chromatography tandem mass spectrometry analysis revealed that this critical lysine residue was absent in purified fibrinogen-γΔ5 protein. Interestingly, hepatic deposition of this uniquely aberrantly cross-linked fibrin(ogen) in FibγΔ5 mice was associated with exacerbated hepatic injury, an effect not recapitulated by pharmacologic inhibition of integrin αIIbβ3.

CONCLUSION

The results indicate that fibrinogen-γΔ5 lacks critical residues essential to form γ-γ dimer in response to thrombin and suggest that hepatic accumulation of abnormally cross-linked fibrin(ogen) can exacerbate hepatic injury.

von Willebrand factor-binding protein (vWbp)-activated factor XIII and transglutaminase 2 (TG2) promote cross-linking between FnBPA from Staphylococcus aureus and fibrinogen

The secreted von Willebrand factor-binding protein (vWbp) from Staphylococcus aureus interacts with the coagulation factors prothrombin and fibrinogen (Fbg), leading to the non-proteolytic transglutaminase activation of Factor XIII (FXIII). In this study we found that vWbp-activated FXIII catalyses the incorporation of amino-donor dansylcadaverine into region A of fibronectin-binding protein A (FnBPA). Incubation of Fbg with recombinant region A of S. aureus Fbg-binding proteins FnBPA, FnBPB, ClfA or ClfB in presence of vWbp-activated FXIII resulted in the formation of high molecular heteropolymers with FnBPA only, suggesting a specificity of the cross-linking reaction between fibrin(ogen) and the staphylococcal surface. As previously observed, cross-linking sites were mapped to the α-chain and the N1 subdomain of fibrin(ogen) and region A of FnBPA, respectively. Comparable results were obtained when tissue tranglutaminase-2 (TG2) was tested for cross-linking of FnBPA and Fbg. Of note, FnBPA-mediated covalent cross-linking promoted by vWbp-activated FXIII was also observed when bacteria were allowed to attach to fibrin(ogen). Together these findings suggest a novel pathogenetic mechanism by which the transglutaminase action of FXIII and/or TG2 contributes to entrapment and persistence of S. aureus in blood and host tissues.

Novel fibrin-fibronectin matrix accelerates mice skin wound healing

Plasma fibrinogen (F1) and fibronectin (pFN) polymerize to form a fibrin clot that is both a hemostatic and provisional matrix for wound healing. About 90% of plasma F1 has a homodimeric pair of γ chains (γγF1), and 10% has a heterodimeric pair of γ and more acidic γ' chains (γγ'F1). We have synthesized a novel fibrin matrix exclusively from a 1:1 (molar ratio) complex of γγ'F1 and pFN in the presence of highly active thrombin and recombinant Factor XIII (rFXIIIa). In this matrix, the fibrin nanofibers were decorated with pFN nanoclusters (termed γγ'F1:pFN fibrin). In contrast, fibrin made from 1:1 mixture of γγF1 and pFN formed a sporadic distribution of "pFN droplets" (termed γγF1+pFN fibrin). The γγ'F1:pFN fibrin enhanced the adhesion of primary human umbilical vein endothelium cells (HUVECs) relative to the γγF1+FN fibrin. Three dimensional (3D) culturing showed that the γγ'F1:pFN complex fibrin matrix enhanced the proliferation of both HUVECs and primary human fibroblasts. HUVECs in the 3D γγ'F1:pFN fibrin exhibited a starkly enhanced vascular morphogenesis while an apoptotic growth profile was observed in the γγF1+pFN fibrin. Relative to γγF1+pFN fibrin, mouse dermal wounds that were sealed by γγ'F1:pFN fibrin exhibited accelerated and enhanced healing. This study suggests that a 3D pFN presentation on a fibrin matrix promotes wound healing.

Ranking reactive glutamines in the fibrinogen αC region that are targeted by blood coagulant factor XIII

Factor XIIIa (FXIIIa) introduces covalent γ-glutamyl-ε-lysyl crosslinks into the blood clot network. These crosslinks involve both the γ and α chains of fibrin. The C-terminal portion of the fibrin α chain extends into the αC region (210-610). Crosslinks within this region help generate a stiffer clot, which is more resistant to fibrinolysis. Fibrinogen αC (233-425) contains a binding site for FXIIIa and three glutamines Q237, Q328, and Q366 that each participate in physiological crosslinking reactions. Although these glutamines were previously identified, their reactivities toward FXIIIa have not been ranked. Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry and nuclear magnetic resonance (NMR) methods were thus used to directly characterize these three glutamines and probe for sources of FXIIIa substrate specificity. Glycine ethyl ester (GEE) and ammonium chloride served as replacements for lysine. Mass spectrometry and 2D heteronuclear single quantum coherence NMR revealed that Q237 is rapidly crosslinked first by FXIIIa followed by Q366 and Q328. Both (15)NH4Cl and (15)N-GEE could be crosslinked to the three glutamines in αC (233-425) with a similar order of reactivity as observed with the MALDI-TOF mass spectrometry assay. NMR studies using the single αC mutants Q237N, Q328N, and Q366N demonstrated that no glutamine is dependent on another to react first in the series. Moreover, the remaining two glutamines of each mutant were both still reactive. Further characterization of Q237, Q328, and Q366 is important because they are located in a fibrinogen region susceptible to physiological truncations and mutation. The current results suggest that these glutamines play distinct roles in fibrin crosslinking and clot architecture.

Taking the Thrombin “Fork”

The proverb that probably best exemplifies my career in research is attributable to Yogi Berra (http://www.yogiberra.com/), ie, “when you come to a fork in the road … take it.” My career is a consequence of chance interactions with great mentors and talented students and the opportunities provided by a succession of ground-breaking improvements in technology.

Functional analysis of fibrin {gamma}-chain cross-linking by activated factor XIII: determination of a cross-linking pattern that maximizes clot stiffness

Activated coagulation factor XIII (FXIIIa) cross-links the gamma-chains of fibrin early in clot formation. Cross-linking of the alpha-chains occurs more slowly, leading to high molecular weight multimer formations that can also contain gamma-chains. To study the contribution of FXIIIa-induced gamma-chain cross-linking on fibrin structure and function, we created 2 recombinant fibrinogens (gammaQ398N/Q399N/K406R and gammaK406R) that modify the gamma-chain cross-linking process. In gammaK406R, gamma-dimer cross-links were absent, but FXIIIa produced a cross-linking pattern similar to that observed in tissue transglutaminase cross-linked fibrin(ogen) with mainly alpha-gamma cross-links. In Q398N/Q399N/K406R, cross-links with any gamma-chain involvement were completely absent, and only alpha-chain cross-linking occurred. Upon cross-linking, recombinant normal fibrin yielded a 3.5-fold increase in stiffness, compared with a 2.5-fold increase by alpha-chain cross-linking alone (gammaQ398N/Q399N/K406R). gammaK406R fibrin showed a 1.5-fold increase in stiffness after cross-linking. No major differences in clot morphology, polymerization, and lysis rates were observed, although fiber diameter was slightly lower in cross-linked normal fibrin relative to the variants. Our results show that gamma-chain cross-linking contributes significantly to clot stiffness, in particular through gamma-dimer formation; alpha-gamma hybrid cross-links had the smallest impact on clot stiffness.

A proposed stoichiometrical calibration procedure to achieve transferability of D-dimer measurements and to characterize the performance of different methods

BACKGROUND

There is little transferability between D-dimer levels obtained by different reagents today. This makes it difficult to compare results from different clinical studies.

OBJECTIVES

We give a comprehensive proposal for calibration of D-dimer assays. All crucial steps and underlying assumptions are made explicit.

METHODS

The new approach is based on using a set of fibrinolysates of patient samples clotted and treated with tPA to obtain maximal conversion to D-dimers. Their expected maximal D-dimer concentrations are calculated stoichiometrically from their different fibrinogen values and the published molecular masses of fibrinogen and average D-dimer. The characteristics of five latex enhanced D-dimer immunoassays were also tested against early and late fibrin fragments using this procedure. These were produced by prolonged fibrinolysis of a set of patient samples of varying fibrinogen concentrations.

RESULTS

These varied typically between methods and lysis times. One of the methods showing the highest yield irrespective of lysis time was used for calibration. A linear standard curve with zero intercept and R2 = 0.95 was obtained.

CONCLUSION

Following this procedure will allow better transferability of D-dimer in future clinical trails.

Fibrin but not adsorbed fibrinogen supports fibronectin assembly by spread platelets. Effects of the interaction of alphaIIb beta3 with the C terminus of the fibrinogen gamma-chain

We investigated the assembly of soluble fibronectin by lysophosphatidic acid-activated platelets adherent to fibrinogen or fibrin. More fibronectin was assembled by activated platelets spread on fibrin matrices than by platelets spread on adsorbed fibrinogen. The difference between platelets adherent to fibrinogen and fibrin occurred under both static and flow conditions. Similar differences were seen in binding of the 70-kDa N-terminal fragment of fibronectin that recognizes fibronectin assembly sites on adherent cells. Antibody and peptide blocking studies demonstrated that alphaIIb beta3 integrin mediates platelet adhesion to fibrinogen, whereas both alphav beta3 and alphaIIb beta3 mediate platelet adhesion to fibrin. The hypothesis that engagement of the C-terminal QAGDV sequence of the fibrinogen gamma-chain by alphaIIb beta3 inhibits the ability of the platelet to assemble fibronectin was tested by several experiments. Activated platelets adherent to adsorbed mutant fibrinogen lacking the QAGDV sequence (gammadelta5FG) were assembly-competent, as were platelets adherent to adsorbed normal fibrinogen that had been pretreated with the 7E9 antibody to the C terminus of the gamma-chain. Moreover, adsorbed normal fibrinogen but not gammadelta5FG suppressed the ability of co-adsorbed fibronectin to direct assembly of soluble fibronectin by spread platelets. The suppressive effect was lost when a surface of co-adsorbed fibronectin and fibrinogen was pretreated with 7E9. These results support a model in which the engagement of alphaIIb beta3 by the C-terminal sequence of the fibrinogen gamma-chain initiates signals that suppress subsequent fibronectin assembly by spread platelets. This interaction is less dominant when platelets adhere to fibrin, resulting in enhanced fibronectin assembly.

The impaired polymerization of fibrinogen Longmont (Bbeta166Arg-->Cys) is not improved by removal of disulfide-linked dimers from a mixture of dimers and cysteine-linked monomers

This study identified a new substitution in the Bbeta chain of an abnormal fibrinogen, denoted Longmont, where the residue Arg166 was changed to Cys. The variant was discovered in a young woman with an episode of severe hemorrhage at childbirth and a subsequent mild bleeding disorder. The neo-Cys residues were always found to be disulfide-bridged to either an isolated Cys amino acid or to the corresponding Cys residue of another abnormal fibrinogen molecule, forming dimers. Removing the dimeric molecules using gel filtration did not correct the fibrin polymerization defect. Fibrinogen Longmont had normal fibrinopeptide A and B release and a functional polymerization site "a." Thus, the sites "A" and "a" can interact to form protofibrils, as evidenced by dynamic light-scattering measurements. These protofibrils, however, were unable to associate in the normal manner of lateral aggregation, leading to abnormal clot formation, as shown by an impaired increase in turbidity. Therefore, it is concluded that the substitution of Arg166-->Cys-Cys alters fibrinogen Longmont polymerization by disrupting interactions that are critical for normal lateral association of protofibrils. (Blood. 2001;98:661-666)

Confirmation of mendelian properties of heterodimeric fibrinogen molecules in a heterozygotic dysfibrinogenemia, "fibrinogen Amarillo," using gprphoresis to differentiate semifibrin molecules from fibrinogen and fibrin

The fibrinogen molecule consists of two sets of Aalpha, Bbeta, and gamma chains assembled into a bilateral disulfide linked (Aalpha, Bbeta, gamma)2 structure. Cleavage of the two A-fibrinopeptides (FPA, Aalpha1-16) from normal Aalpha chains with arginine at position 16 (RFPA) by thrombin or the venom enzyme atroxin transforms fibrinogen into self-aggregating fibrin monomers (alpha, Bbeta, gamma)(2). Mutant Aalpha16R-->H fibrinopeptide (HFPA) cannot be cleaved from fibrinogen by atroxin. Many studies on heterozygous dysfibrinogenemias with this mutation suggested that incorporation of the mutant chains into the molecules was ordered in a manner yielding only (1) homodimeric normal (RFPARFPA) atroxin-coagulable molecules and (2) homodimeric abnormal (H(FPA)HFPA) atroxin-incoagulable molecules in equal quantities. Although heterodimeric molecules (RFPAHFPA) could not be found in studies on the intact protein, Meh et al. demonstrated their existence by showing that CNBr digests of fibrinogens from atroxin-treated Aalpha16R-->H heterozygotic dysfibrinogenemias consistently yielded N-terminal fragments (NDSKs) with partially resolved electrophoretic bands predominantly in between the NDSKs of fibrinogen and alpha-fibrin. An opportunity to confirm and better quantify the heterodimers arose with the recent development of a method (GPRphoresis) for identifying molecules lacking only one FPA, which is applied here in study of a newly presenting case of an Aalpha16R-->H dysfibrinogenemia, "fibrinogen Amarillo." GPRphoresis uses electrophoretic shifts, staged with GPRP-NH(2) to separate the self-aggregating fibrin monomers lacking both FPAs from weakly aggregating "semifibrin" molecules lacking one FPA An antifibrin alpha17-23 antibody is used to measure and differentiate the semifibrin from fibrinogen with FPA fully intact. Applying GPRphoresis to atroxin digests of fibrinogen Amarillo clearly demonstrated RFPARFPA, RFPAHFPA, and HFPAHFPA molecules in nearly perfect Mendelian 1:2:1 proportions. In turn, the high levels of the semifibrin in the terminal atroxin digests provide genetic phenotypic evidence supporting fidelity of the GPRphoresis method.

The role of gamma A/gamma ' fibrinogen in plasma factor XIII activation

Factor XIII zymogen activation is a complex series of events that involve fibrinogen acting in several different roles. This report focuses on the role of fibrinogen as a cofactor in factor XIII activation by thrombin. We demonstrate that fibrinogen has two distinct activities that lead to an increased rate of factor XIII activation. First, the thrombin proteolytic activity is increased by fibrin. The cleavage rates of both a small chromogenic substrate and the factor XIII activation peptide are increased in the presence of either the major fibrin isoform, gammaA/gammaA fibrin, or a minor variant form, gammaA/gamma' fibrin. This enhancement of thrombin activity by fibrin is independent of fibrin polymerization and requires only cleavage of the fibrinopeptides. Subsequently, gammaA/gamma' fibrinogen accelerates plasma factor XIII activation by a non-proteolytic mechanism. This increased rate of activation results in a slightly more rapid cross-linking of fibrin gammaA and gamma' chains and a significantly more rapid cross-linking of fibrin alpha chain multimers. Together, these results show that although both forms of fibrin increase the rate of activation peptide cleavage by thrombin, gammaA/gamma' fibrinogen also increases the rate of factor XIII activation in a non-proteolytic manner. A revised model of factor XIII activation is presented below.

Structural origins of fibrin clot rheology

The origins of clot rheological behavior associated with network morphology and factor XIIIa-induced cross-linking were studied in fibrin clots. Network morphology was manipulated by varying the concentrations of fibrinogen, thrombin, and calcium ion, and cross-linking was controlled by a synthetic, active-center inhibitor of FXIIIa. Quantitative measurements of network features (fiber lengths, fiber diameters, and fiber and branching densities) were made by analyzing computerized three-dimensional models constructed from stereo pairs of scanning electron micrographs. Large fiber diameters and lengths were established only when branching was minimal, and increases in fiber length were generally associated with increases in fiber diameter. Junctions at which three fibers joined were the dominant branchpoint type. Viscoelastic properties of the clots were measured with a rheometer and were correlated with structural features of the networks. At constant fibrinogen but varying thrombin and calcium concentrations, maximal rigidities were established in samples (both cross-linked and noncross-linked) which displayed a balance between large fiber sizes and great branching. Clot rigidity was also enhanced by increasing fiber and branchpoint densities at greater fibrinogen concentrations. Network morphology is only minimally altered by the FXIIIa-catalyzed cross-linking reaction, which seems to augment clot rigidity most likely by the stiffening of existing fibers.

Influence of a natural and a synthetic inhibitor of factor XIIIa on fibrin clot rheology

We investigated the origins of greater clot rigidity associated with FXIIIa-dependent cross-linking. Fibrin clots were examined in which cross-linking was controlled through the use of two inhibitors: a highly specific active-center-directed synthetic inhibitor of FXIIIa, 1,3-dimethyl-4,5-diphenyl-2[2(oxopropyl)thio]imidazolium trifluoromethylsulfonate, and a patient-derived immunoglobulin directed mainly against the thrombin-activated catalytic A subunits of thrombin-activated FXIII. Cross-linked fibrin chains were identified and quantified by one- and two-dimensional gel electrophoresis and immunostaining with antibodies specific for the alpha- and gamma-chains of fibrin. Gamma-dimers, gamma-multimers, alpha(n)-polymers, and alpha(p)gamma(q)-hybrids were detected. The synthetic inhibitor was highly effective in preventing the production of all cross-linked species. In contrast, the autoimmune antibody of the patient caused primarily an inhibition of alpha-chain cross-linking. Clot rigidities (storage moduli, G') were measured with a cone and plate rheometer and correlated with the distributions of the various cross-linked species found in the clots. Our findings indicate that the FXIIIa-induced dimeric cross-linking of gamma-chains by itself is not sufficient to stiffen the fibrin networks. Instead, the augmentation of clot rigidity was more strongly correlated with the formation of gamma-multimers, alpha(n)-polymers, and alpha(p)gamma(q)-hybrid cross-links. A mechanism is proposed to explain how these cross-linked species may enhance clot rigidity.

Resistance of gammaA/gamma' fibrin clots to fibrinolysis

Elevated plasma fibrinogen levels are a major risk factor for thrombosis. This report shows two mechanisms by which fibrinogen can affect the fibrinolysis rate in vitro and thus may lead to thrombosis. First, the lysis rate of fibrin decreases as the initial concentration of fibrinogen increases. Second, a minor variant form of fibrinogen decreases the rate of fibrinolysis. This variant, gammaA/gamma' fibrinogen, has one altered gamma chain and is known to bind to factor XIII zymogen. In a fibrinolysis assay containing purified thrombin, fibrinogen, tissue-type plasminogen activator, and plasminogen, clots from gammaA/gammaA and gammaA/gamma' fibrinogen lysed at similar rates. However, when factor XIII was added, slower lysis was seen in gammaA/gamma' fibrin clots when compared with gammaA/gammaA fibrin clots. A D-dimer agglutination assay showed that the gammaA/gamma' clots were more highly cross-linked than the gammaA/gammaA clots. The lysis rates of gammaA/gamma' clots were similar to gammaA/gammaA clots in the presence of N-ethylmaleimide, a specific inhibitor of factor XIIIa. The gammaA/gamma' fibrin clots made in the presence of factor XIII showed increased proteolytic resistance to both plasmin and trypsin. Clots made from afibrinogenemic plasma reconstituted with gammaA/gamma' fibrinogen also showed significant resistance to lysis compared with gammaA/gammaA fibrinogen. These data demonstrate gammaA/gamma' fibrin is resistant to fibrinolysis, possibly as a result of concentrating factor XIII on the clot. The total fibrinogen concentration and the amount of gammaA/gamma' fibrinogen increase clot stability in vitro and thus may contribute independently to the risk of thrombosis in humans.

Crystal structure of a 30 kDa C-terminal fragment from the gamma chain of human fibrinogen

BACKGROUND

Blood coagulation occurs by a cascade of zymogen activation resulting from minor proteolysis. The final stage of coagulation involves thrombin generation and limited proteolysis of fibrinogen to give spontaneously polymerizing fibrin. The resulting fibrin network is covalently crosslinked by factor XIIIa to yield a stable blood clot. Fibrinogen is a 340 kDa glycoprotein composed of six polypeptide chains, (alphabetagamma)2, held together by 29 disulfide bonds. The globular C terminus of the gamma chain contains a fibrin-polymerization surface, the principal factor XIIIa crosslinking site, the platelet receptor recognition site, and a calcium-binding site. Structural information on this domain should thus prove helpful in understanding clot formation.

RESULTS

The X-ray crystallographic structure of the 30 kDa globular C terminus of the gamma chain of human fibrinogen has been determined in one crystal form using multiple isomorphous replacement methods. The refined coordinates were used to solve the structure in two more crystal forms by molecular replacement; the crystal structures have been refined against diffraction data to either 2.5 A or 2.1 A resolution. Three domains were identified in the structure, including a C-terminal fibrin-polymerization domain (P), which contains a single calcium-binding site and a deep binding pocket that provides the polymerization surface. The overall structure has a pronounced dipole moment, and the C-terminal residues appear highly flexible.

CONCLUSIONS

The polymerization domain in the gamma chain is the most variable among a family of fibrinogen-related proteins and contains many acidic residues. These residues contribute to the molecular dipole moment in the structure, which may allow electrostatic steering to guide the alignment of fibrin monomers during the polymerization process. The flexibility of the C-terminal residues, which contain one of the factor XIIIa crosslinking sites and the platelet receptor recognition site, may be important in the function of this domain.

Identification of covalently linked trimeric and tetrameric D domains in crosslinked fibrin

Following proteolytic conversion of fibrinogen to fibrin, clot assembly commences with formation of double-stranded fibrils that subsequently branch extensively in forming a three-dimensional network. Plasmin digests of fibrin clots that had first been covalently crosslinked by plasma transglutaminase (factor XIIIa) contained multimeric proteolytic fragments composed of crosslinked outer (D) domains of neighboring fibrin molecules. Two of these were larger than the well-known "D dimer" fragment and corresponded to D trimers and D tetramers, respectively. Whereas D dimers originate from crosslinked D domains at bimolecular junctions within two-stranded fibrils, D trimers and D tetramers evidently arise through crosslinking of contiguous D domains at trimolecular and tetramolecular junctions or at fibril branch points, respectively. Measurement of the widths of fibrils comprising trifunctional branches in thin fiber networks revealed tetramolecular branch points, which are formed by bifurcation of two double-stranded fibrils. In addition, another type of trifunctional structure, which we term the trimolecular branch point, was composed of three double-stranded fibrils. Crosslinking of D domains to form trimers may occur at this type of junction. These findings add to our understanding of the crosslinking arrangements that stabilize fibrin clot structure and the ways that fibrin molecules polymerize to form branches in the clot matrix.

Coagulation and fibrinolysis

The two final phases in the haemostatic process, plasma coagulation with the formation of a fibrin clot, and fibrinolysis leading to the dissolution of fibrin clots, are reviewed. Coagulation may be initiated either by reactions occurring between components of the blood alone, the intrinsic pathway, or by reactions which also involve tissue components, termed the extrinsic pathway. In the diagnosis of coagulation disorders, it is convenient to divide the intrinsic pathway into three phases. In phase 1, resulting in the activation of factor (f) X, are involved f XII, XI, VIII and IX, platelet phospholipids, and calcium. In phase 2, prothrombin is converted to thrombin by f Xa in conjunction with f V, phospholipids, calcium. In phase 3, thrombin converts fibrinogen to fibrin, which is then stabilized by f XIII. Antithrombin III is the most important inhibitor. The key component in fibrinolysis is plasminogen, which under the influence of various activators is converted to plasmin. Plasmin is a serine protease and its main in vivo target is fibrin. Alpha 2-antiplasmin and a fast-acting inhibitor of tissue plasminogen activator are the most important inhibitors.

Determination of the relative positions of amino acids by partial specific cleavages of end-labeled proteins

We have developed a new method for obtaining information about protein sequences that uses an approach analogous to that used to determine DNA sequences. In essence, three steps are involved. First, a detectable label is attached exclusively to the amino terminus of a polypeptide. Next, the labeled chain is subjected to partial specific cleavage in a way that produces roughly equimolar amounts of fragments of different sizes. Cleavages for methionine, tryptophan, arginine, aspartyl-proline bonds, and asparaginyl-glycine bonds have been employed. Lastly, the labeled fragments are separated according to size by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The distribution of target amino acids along the polypeptide chain can be deduced from the specific pattern of labeled bands by reading the "ladder" in the same way that DNA sequencing gels are read. Although the method can be conducted with a radioactive label, we have chosen to use a fluorescent label. We have applied the method successfully to the three subunit chains of two different fibrinogens.

The amino acid sequence of the alpha-chain of human fibrinogen

The amino acid sequence of the human fibrinogen alpha-chain reveals a structure that can be divided into three zones of unique amino acid composition. The middle of these contains the two primary alpha-chain cross-linking acceptor sites and consists of a remarkable series of internal duplications.

Studies on the structural abnormality of fibrinogen Paris I

The structural properties of an inherited fibrinogen abnormality designated fibrinogen Paris I were investigated. Dodecyl sulfate gel electrophoresis of unreduced samples revealed no discernible differences in molecular weight from normal; this implied that in fibrinogen Paris I, the normal fibrinogen architecture of six covalently linked chains per molecule is preserved. Examination of dithiothreitol reduced samples before and after treatment with Reptilase or thrombin revealed that the Aalpha- and Bbeta-chains could release the A and B peptides, respectively. A mutant chain (mol wt 52,500, termed gammaParis I) which replaces a large proportion of gamma-chains (mol wt 49,400) was shown, like normal gamma-chains, to lack thrombin- and Reptilase-sensitive sites. The gamma-chains and alpha-chains of Paris I fibrin underwent Factor XIIIa-catalyzed cross-linking slowly; this behavior was not attributable to an intrinsic abnormality of these chains themselves but rather to the inhibitory effect of the mutant gammaParis I chains on this process. Results of DEAE-cellulose gradient elution chromatography of Paris I fibrinogen preparations revealed the presence of small amounts of normal fibrinogen molecules and also indicated that the gammaParis I chains possessed structural overlap with gamma-chains. Unlike gamma-chains however, the gammaParis I chains did not incorporate dansylcadaverine in the prescence of Factor XIIIa, nor, as previously reported, did they undergo cross-linking. The observations indicate that the amine acceptor site found in the COOH-terminal region of the gamma-chain is either not present on the gammaParis I chain or is unavailable for cross-linking. Further support for localization of the abnormality in the COOH-terminal region of the molecule was obtained from the observation that during plasmic hydrolysis of Paris I fibrinogen, at least one unique form of core Fragment D (DParis I) was evolved, whereas Fragment E did not differ from normal.

Amino acid sequence studies on the alpha chain of human fibrinogen. Location of four plasmin attack points and a covalent cross-linking site

The amino acid sequence of a 38-residue midsection piece of the alpha chain of human fibrinogen has been determined using a combination of plasmin-derived peptides and cyanogen bromide fragments. The segment contains several important features, including four early plasmin attack points, one of the two alpha-chain cross-linking acceptor sites, and a peptide homologous to one isolated from plasmin digests of bovine fibrinogen, and reported to have anticoagulant activity. The segment is sequentially adjacent to and overlapping with a large molecular weight (20000-25000) fragment released during plasminolysis. This latter material is very rich in glycine and serine and deficient in nonpolar amino acids. It also contains the other alpha-chain cross-linking acceptor site.

Isolation, characterization, and location of a donor-acceptor unit from cross-linked fibrin

The cross-linking systems of bovine and human fibrins were studied by the introduction of a radioactive substitute donor as an inhibitor of fibrin cross-linking, separation of the constituent polypeptide chains after sulfitolysis, and tryptic digestion of the labeled gamma-chains. The information gathered from this approach enabled us to isolate and characterize the complete donor-acceptor unit in tryptic digests of fibrin gamma-gamma cross-linked systems. In both bovine and human fibrin, this kind of cross-linking is accomplished by reciprocal bridging between overlapping carboxy-terminal segments of neighboring gamma-chains. The amino acid sequence of the carboxy-terminal heptadecapeptide of the bovine gamma-chain was determined and an alignment of the corresponding region of the human gamma-chain established.