Congenital Factor XIII Deficiency With the Presence of Inhibitor: A Case Study.

Coagulation factor XIII (FXIII) is a fibrin-stabilizing factor with additional roles in wound healing and interactions between the decidua and fetus. Congenital FXIII deficiency is rare bleeding disorder. Inhibitor development against FXIII in inherited FXIII deficency is also uncommon, but may cause severe, life-threatening bleeding. FXIII is the last step in the coagulation cascade with normal coagulation paramaters (PT, aPTT), the detection of inhibitor to FXIII is quite difficult. The treatment of inhibitor-positive congenital FXIII deficiency is challenging due to the lack of a role of by-pass agents such as FVII. The best known ways of treatment in these cases are the use of high-dose FXIII concentrates and immunosuppression. Herein, we report the management of postoperative bleeding diathesis in a patient with FXIII deficiency who developed inhibitors, and to follow the clinical course of the disease with FXIII concentrate and immunosuppression.

Successful Management of a Patient with Autoimmune Hemorrhaphilia due to Anti-Factor XIII/13 Antibodies Complicated by Pulmonary Thromboembolism.

Autoimmune hemophilia-like disease (hemorrhaphilia) due to anti-factor XIII (FXIII) antibodies (AH13) is a very rare, life-threatening bleeding disorder. A 77-year-old woman developed macrohematuria and a right renal pelvic hematoma. The coagulation times were not prolonged, but FXIII activity and antigen levels were severely and moderately reduced to 9 and 29% of normal values, respectively. Accordingly, the FXIII-specific activity turned out to be low. FXIII inhibitor and anti-FXIII-A subunit autoantibodies were detected by a 1:1 crossmixing test and immunoblot and immunochromatographic assays. She was therefore diagnosed with "definite AH13" and treated with plasma-derived FXIII concentrates to arrest the hemorrhage. In addition to a highly compressed inferior vena cava by a huge renal pelvic hematoma, deep vein thrombosis (DVT) and pulmonary thromboembolism (PE) were identified by systemic computed tomography. The patient was immediately started on anticoagulation therapy with low-dose heparin. Emboli disappeared quickly, probably because under-crosslinked thrombi caused by severe FXIII deficiency are vulnerable to fibrinolysis. After about 1.5 years, anti-FXIII-A subunit autoantibodies still remained despite the use of rituximab, steroid pulse therapy, oral prednisolone, and oral cyclophosphamide treatments. In conclusion, an extremely rare AH13 case complicated by DVT and PE was successfully managed by balancing anticoagulation therapy with hemostatic therapy.

Inhibitors of Factor XIII/13 in older patients.

Factor XIII/13 (FXIII or F13) is a plasma protransglutaminase, which stabilizes fibrin clots, and thus plays an important role in hemostasis. Autoimmune hemo(rrha)philia due to anti-F13 autoantibodies (AH13) has been on the rise in Japan, which has become the leading superaging society in the 21st century. The mean age of Japanese AH13 cases has risen to 70.4 years. A total of 83 AH13 cases have been diagnosed in the world as of July 2014. To raise the awareness of AH13, the author and members of the Japanese Collaborative Research Group first proposed "Criterion and Algorithm of Laboratory Tests for anti F13" in February 2012. AH13 is not just an acquired isolated defect of F13 molecule itself but a disturbance caused by autoantibodies. Accordingly, AH13 cases are diagnosed in patients with otherwise unexplained hemorrhages by a combination of a severe deficiency of F13 activity and the presence of anti-F13 autoantibodies. As patients with this disease manifest life-threatening bleeding symptoms, prompt diagnosis and proper treatment are essential. Because AH13 tends to become chronic and intractable, affected patients must be closely followed for a prolonged period.

Detection of factor XIII inhibitors in 33 patients with autoimmune factor XIII deficiency in Japan.

Autoimmune factor XIII (FXIII) deficiency (AiF13D) is a rare hemorrhagic disease. The anti-FXIII autoantibodies that cause this disease are classified into three types: type Aa inhibits the heterotetramer assembly and activation of FXIII, type Ab inhibits the enzymatic activity of activated FXIII, and type B enhances the elimination of FXIII from the blood. The former two are FXIII inhibitors and may be lethal if overlooked by conventional functional assays. To reliably detect both types of FXIII inhibitors, a new assay was developed by incorporating 5-(biotinamido)pentylamine (BAPA) into α2-plasmin inhibitor (PI-BAPA assay). This assay was tested on plasma samples from 128 participants, including 60 healthy controls, 35 patients with non-immune acquired FXIII deficiency, and 33 patients with AiF13D (29 with type Aa inhibitors and 4 with type Ab inhibitors). The PI-BAPA assay successfully detected type Aa and Ab inhibitors in 5-step dilution cross-mixing tests between patient and normal plasma. This assay also showed comparable or superior inhibition rates in the 1:1 mixing test compared to conventional ammonia release and amine incorporation assays. Receiver operating characteristic curve analysis confirmed the excellent specificity and sensitivity of this assay for determining inhibition rates, and the assay has already been used for AiF13D diagnosis.

Acquired FXIII inhibitors: a systematic review.

Coagulation factor XIII (FXIII) is a protein that promotes fibrin stabilization by forming multiple covalent cross-links between fibrin monomers. Beside congenital FXIII deficiency, due to FXIII gene mutations, severe acquired FXIII deficiency has been described in association with autoantibodies against coagulation FXIII. These inhibitors, which occurs very rarely but may cause life-threatening bleeding complications, may arise spontaneously or in association with autoimmune and lymphoproliferative disorders or medications. The management of patients with acquired FXIII inhibitors is very demanding and treatment regimens must be focused on eradication of the inhibitor and to increase the plasma FXIII levels. In this systematic review, we analyse all the published case-reports on anti-FXIII autoantibodies focusing on the clinical features and treatment modalities of this acquired hemorrhagic condition.

Rapid determination of blood coagulation factor XIII activity using protein arrays for serodiagnosis of human plasma.

We developed a novel on-chip assay using protein arrays for quantitative and rapid analysis of blood coagulation factor XIII (FXIII) activity in human plasma. FXIII is activated by concerted action of thrombin and Ca(2+) and plays essential roles in hemostasis, angiogenesis, and wound healing. We fabricated protein arrays by immobilizing fibrinogen onto the 3-aminopropyltrimethoxysilane layer of well-type arrays and determined FXIII activity by analyzing biotinylated fibrinogen with Cy3-conjugated streptavidin. We determined optimal concentrations of Ca(2+), thrombin, and 5-(biotinamido)pentylamine (BAPA) for the on-chip activity assay, and the detection limit was 0.01 Lowey U/mL (9.9 pM). Using the on-chip activity assay, hepatocellular carcinoma patients (n = 24), but not hepatitis (n = 24) or liver cirrhosis patients (n = 41), had significantly lower FXIII activities (p < 0.001) than normal individuals (n = 41), indicating that FXIII activity is a possible diagnostic marker for hepatocellular carcinoma. In addition, we have successfully used this activity assay to reveal individual variations (37-57%, n = 65) in the inhibition rate of FXIII activity by isoniazid, the first-line antituberculosis agent. Thus, our optimized on-chip FXIII activity assay provides a quantitative and high-throughput approach to investigating the role(s) of FXIII in human diseases. Moreover, it has a strong potential to be applied toward FXIII-related personalized medicines.

Structure-Based Design of FXIIIa-Blockers: Addressing a Transient Hydrophobic Pocket in the Active Site of FXIIIa.

Blood coagulation factor XIII (FXIII, F13) is considered to be a promising target for anticoagulants with reduced bleeding risk because of its unique position in the coagulation cascade downstream of thrombin. However, until now, no potent drug addressing FXIII has been available, indeed no compound has even entered clinical trials yet. In 2013, we published the co-crystal structure of FXIII in the active state (FXIIIa°), thereby providing a detailed map of the active site for the rational design of potent FXIIIa blockers. Here we report, for the first time, a structure-based approach to improving the affinity of FXIIIa inhibitors. FXIII was crystallized in complex with a methyl thiazole moiety to address a novel transient hydrophobic pocket close to the catalytic center. By subsequent structure-based design to rationalize the introduction of an ethyl ester, the potency of the inhibitor was improved significantly compared to that of the parent lead compound. The occupancy of the hydrophobic pocket described here might turn out to be a key step in the development of a potent reversible and orally available FXIIIa blocker.

Coagulation factor XIII serves as protein disulfide isomerase.

Tissue transglutaminase was reported to act as protein disulfide isomerase (PDI). We studied whether plasma transglutaminase - coagulation factor XIII (FXIII) - has PDI activity as well. PDI activity was measured by determining the ability to renature reduced-denatured RNase (rdRNase). We found that FXIII can renature rdRNase, with efficiency comparable to commercial PDI. This PDI activity was inhibited by bacitracin. Like tissue transglutaminase, FXIII-mediated PDI activity is independent of its transglutaminase activity and is located on the A subunit. Surface-associated PDI has been previously shown to catalyse two distinct functions: transnitrosation with subsequent release of intracellular nitric oxide and disulfide bond rearrangement during platelet integrin ligation. Our results imply that FXIII-PDI activity may have a role in platelet function.

In vitro inhibition of factor XIII retards clot formation, reduces clot firmness, and increases fibrinolytic effects in whole blood.

BACKGROUND: Thrombelastography has received renewed interest in the perioperative setting. The main determinants of thrombelastographic results are coagulation factor concentrations (various zymogens and fibrinogen) and platelet count; thus, platelet inhibition renders these assays mainly coagulation factor dependent. Assays with and without platelet inhibition are thus increasingly used to trigger and monitor replacement therapy with blood products. In this study, we evaluated the effect of factor XIII inhibition and additional glycoprotein (GP) IIb/IIIa blockade on (platelet-inhibited) whole blood thrombelastography and whether a modified routine assay (using factor XIII antibody) can be used to detect factor XIII deficiency. METHODS: Normal whole blood was incubated with increasing amounts of a nonspecific antibody, an anti-GPIIb/IIIa antibody, or a neutralizing anti-factor XIII antibody; samples were analyzed with a tissue factor-activated and platelet-inhibited whole blood thrombelastographic assay. Clotting time, clot formation time, maximum clot firmness, and clot lysis at 60 min were evaluated in triplicate. Also, 25 whole blood routine samples were evaluated for factor XIII deficiency using a new thrombelastographic assay incorporating a factor XIII antibody and using a standard factor XIII assay for comparison. RESULTS: Although GPIIb/IIIa inhibition did not alter the results of the platelet-inhibited whole blood thrombelastography, factor XIII inhibition significantly reduced maximum clot firmness (P = 0.020) and increased clot formation time (P = 0.025) and clot lysis (P = 0.007), leaving clotting time unchanged; a ceiling effect seemed to be present with increasing antibody concentrations in whole blood (but not plasma). The thrombelastographic assay for factor XIII deficiency (<70% activity) had a 90% sensitivity and negative predictive value (area under receiver operating characteristic curve 0.803, P = 0.0015); for a deficiency <60%, sensitivity and negative predictive value were 100% (area under receiver operating characteristic curve 0.84, P = 0.0037). CONCLUSION: Factor XIII has significant impact on platelet-inhibited activated whole blood thrombelastography. This phenomenon should be considered when interpreting thrombelastographic results in the bleeding patient, especially when the results trigger procoagulant therapy. Antibody-mediated factor XIII inhibition can be used to establish thrombelastography-based assays to detect factor XIII deficiency.

Flow-dependent remodeling of small arteries in mice deficient for tissue-type transglutaminase: possible compensation by macrophage-derived factor XIII.

Chronic changes in blood flow induce an adaptation of vascular caliber. Thus, arteries show inward remodeling after a reduction in blood flow. We hypothesized that this remodeling depends on the crosslinking enzyme tissue-type transglutaminase (tTG). Flow-dependent remodeling was studied in wild-type (WT) and tTG-null mice using a surgically imposed change in blood flow in small mesenteric arteries. WT mice showed inward remodeling after 2 days of low blood flow, which was absent in arteries from tTG-null mice. Yet, after continued low blood flow for 7 days, inward remodeling was similar in arteries from WT and tTG-null mice. Studying the alternative pathways of remodeling, we identified a relatively high expression of the plasma transglutaminase factor XIII in arteries of WT and tTG-null mice. In addition, vessels from both WT and tTG-null mice showed the presence of transglutaminase-specific crosslinks. An accumulation of adventitial monocytes/macrophages was found in vessels exposed to low blood flow in tTG-null mice. Because monocytes/macrophages may represent a source of factor XIII, tTG-null mice were treated with liposome-encapsulated clodronate. Elimination of monocytes/macrophages with liposome-encapsulated clodronate reduced both the expression of factor XIII and inward remodeling in tTG-null mice. In conclusion, tTG plays an important role in the inward remodeling of small arteries associated with decreased blood flow. Adventitial monocytes/macrophages are a source of factor XIII in tTG-null mice and contribute to an alternative, delayed mechanism of inward remodeling when tTG is absent.

Difference between type I autoimmune inhibitors of fibrin stabilization in two patients with severe hemorrhagic disorder.

Inhibitors of fibrin stabilization of apparently autoimmune origin, found in two severely bleeding unrelated patients (W. G. and G. A.), were compared with regard to their biological target specificities, potencies and immunological characteristics. Both interfered only with the activation of fibrin stabilizing factor (coagulation Factor XIII) and, while totally preventing the conversion of this zymogen to the functional transamidating enzyme, fibrinoligase (Factor XIII(a)), they showed very little inhibition toward the enzyme itself. Thus, according to the classification of Lorand concerning biological specificities, both can be characterized as Type I inhibitors of fibrin stabilization. Potencies of the two inhibitors were quite similar when measured in conjunction with the plasma zymogen, but they differed remarkably in tests with platelet Factor 13. The inhibitor of patient W. G. prevented the activation of the zymogen from platelets, but that of G. A. had no effect on the platelet factor. It may therefore be concluded that the inhibitor of W. G. is directed exclusively against the a subunit which is a common constituent of plasma as well as platelet factors. The inhibitor of G. A., however, must be targeted against determinants uniquely characteristic for the ab ensemble of the plasma zymogen including the b subunit. On the basis of this difference in target specificity, the inhibitor of W. G. is designated as Type I-1 and that of G. A. as Type I-2. The inhibitors of both patients were isolated as immunoglobulins, and neutralization tests revealed that the antibody of W. G. comprised mainly heavy chains of the IgG1 and light chains of the kappa class. The antibody of G. A. proved to be considerably more heterogeneous and contained IgG1 and IgG3 heavy chains as well as kappa- and lambda-light chains. The finding that the antibody of W. G. inhibited conversion of platelet Factor 13 and also its thrombinmodified form, but had no effect on the thrombin and Ca(2+)-activated factor, is an indication that antigenic determinants existing both on the native zymogen and on its hydrolytically modified form become buried in the Ca(2+)-dependent step of activation. This is clear evidence for the occurrence of a significant conformational change in the protein structure attendant to the process of unmasking of its enzymic activity.

Autoimmune acquired factor XIII deficiency due to anti-factor XIII/13 antibodies: A summary of 93 patients.

Autoimmune acquired factor XIII (F13) deficiency or autoimmune hemophilia-like disease (hemorrhaphilia) resulted from the generation of anti-F13 antibodies (AH13) is a severe bleeding disorder that occurs mainly in the elderly. Although rare, the number of patients diagnosed with AH13 has recently increased. To improve understanding of this disease, the author summarized 93 ever reported/diagnosed AH13 cases. About 50% of cases were idiopathic. In the remaining half of the patients, autoimmune diseases and malignancies were the most common underlying diseases. Intramuscular and subcutaneous bleeding were the most frequently reported symptoms. Hemorrhage was the cause of death in 13 patients. In 4 patients, the diagnosis was established after hemorrhagic death. Therefore, physicians/hematologists must raise the awareness of AH13 as a life-threatening disease. Most patients were treated with F13 concentrates to arrest bleeding and with prednisolone and cyclophosphamide to eradicate anti-F13 autoantibodies. AH13 cases tend to become chronic and intractable and require close follow-up over an extended period.

Assessment of the protein interaction between coagulation factor XII and corn trypsin inhibitor by molecular docking and biochemical validation.

Essentials Corn Trypsin Inhibitor (CTI) is a selective inhibitor of coagulation Factor XII (FXII). Molecular modelling of the CTI-FXIIa complex suggested a canonical inhibitor binding mode. Mutagenesis revealed the CTI inhibitory loop and helices α1 and α2 mediate the interaction. This confirms that CTI inhibits FXII in canonical fashion and validates the molecular model. SUMMARY: Background Corn trypsin inhibitor (CTI) has selectivity for the serine proteases coagulation factor XII and trypsin. CTI is in widespread use as a reagent that specifically inhibits the intrinsic pathway of blood coagulation but not the extrinsic pathway. Objectives To investigate the molecular basis of FXII inhibition by CTI. Methods We performed molecular docking of CTI, using its known crystal structure, with a model of the activated FXII (FXIIa) protease domain. The interaction model was verified by use of a panel of recombinant CTI variants tested for their ability to inhibit FXIIa enzymatic activity in a substrate cleavage assay. Results The docking predicted that: (i) the CTI central inhibitory loop P1 Arg34 side chain forms a salt bridge with the FXIIa S1 pocket Asp189 side chain; (ii) Trp22 from CTI helix α1 interacts with the FXIIa S3 pocket; and (iii) Arg43 from CTI helix α2 forms a salt bridge with FXIIa H1 pocket Asp60A. CTI amino acid substitution R34A negated all inhibitory activity, whereas the G32W, L35A, W22A and R42A/R43A substitutions reduced activity by large degrees of 108-fold, 41-fold, 158-fold, and 100-fold, respectively; the R27A, W37A, W39A and R42A substitutions had no effect. Synthetic peptides spanning CTI residues 20-44 had inhibitory activity that was three-fold to 4000-fold less than that of full-length CTI. Conclusions The data confirm the validity of a canonical model of the FXIIa-CTI interaction, with helix α1 (Trp22), central inhibitory loop (Arg34) and helix α2 (Arg43) of CTI being required for effective binding by contacting the S1, S3 and H1 pockets of FXIIa, respectively.

A novel assay for factor XIII based on cross-linking of synthetic peptides: analysis of different substrates.

A novel assay for factor XIII is described that utilizes exclusively small synthetic peptides as substrates for the cross-linking reaction catalyzed by activated factor XIII (FXIIIa). The acyl donor substrate (selection peptide) is immobilized on a microplate via biotin while the acyl acceptor substrate (detection peptide) is labeled with the fluorochrome Oregon green to allow sensitive detection without the need for secondary enzyme systems for signal amplification. Starting with an amino acid sequence from the fibrin gamma-chain (GQQHHLGGAKQAGDV) as a prototype peptide, the influence of amino acid exchanges were investigated with respect to their impact on the FXIIIa-catalyzed reaction. It was found that FXIIIa readily accepts a broad range of substrate peptides, with a proline neighboring the essential lysine having the most detrimental effect. The assay appears to be valuable for the molecular characterization of factor XIII and may be used for a deeper investigation into the substrate requirements of this final enzyme of wound repair, and eventually also for the characterization of other transglutaminases.

Factor XIII deficiency in BALB/c mice with plasmacytoma.

These studies examined the fate of Factor XIII (fibrin-stabilizing factor) in mice with plasmacytoma (MOPC-300, MOPC-384, MOPC-467, and J-558). Plasma Factor XIII levels in these mice decreased progressively with tumor expansion. No plasma inhibitors of Factor XIII activity could be detected. Factor XIII was found on plasmacytoma cell membranes and in the cytoplasm of the malignant cells by immunofluorescence. The inverse relationship between tumor load and plasma Factor XIII levels suggested that the fibrin-stabilizing factor was absorbed by the malignant cells.

b-chains prevent the proteolytic inactivation of the a-chains of plasma factor XIII.

While the transglutaminase activity is associated exclusively with the thrombin-cleaved a chains of plasma Factor XIII, there is little information regarding the role of the b-chains. The present investigations were undertaken to clarify the role of the b-chains during proteolytic activation of plasma factor XIII a-chains. The a-chains of platelet Factor XIII (a2) were extremely sensitive to alpha-thrombin proteolysis, especially in the presence of 5 mM EDTA, resulting in two major fragments with molecular masses 51 +/- 3 kDa and 19 +/- 4 kDa. Furthermore, fibrin enhanced the alpha-thrombin proteolysis of thrombin-cleaved platelet Factor XIII a-chains in presence of CaCl2 or EDTA, resulting in several peptide fragments with molecular masses from 51 +/- 3 kDa to 14 +/- 4 kDa. By contrast, thrombin-cleaved a-chains of plasma Factor XIII (a2b2) were not further degraded by alpha-thrombin in presence of 5 mM EDTA. Even in the combined presence of 5 mM EDTA and 0.1 mg/ml fibrin, alpha-thrombin proteolysis of plasma Factor XIIIa was limited to the formation of a 76 kDa fragment (= Factor XIIIa), a 51 +/- 3 kDa fragment and trace amounts of a 14 +/- 4 kDa species. Platelet Factor XIII proteolyzed by 500 nM alpha-thrombin in presence of 5 mM EDTA expressed less than 20% of enzymatic activity obtained when platelet Factor XIII was activated in presence of 5 mM CaCl2. In contrast, plasma Factor XIII activated by 500 nM apha-thrombin in presence of 5 mM EDTA expressed nearly 65% of original transglutaminase activity. Likewise, when plasma Factor XIII was proteolyzed by 100-1000 nM gamma-thrombin in presence of 5 mM CaCl2 or 5 mM EDTA, maximal transglutaminase activity was observed. However, when platelet Factor XIII was similarly treated with gamma-thrombin in presence of 5 mM EDTA, only one-half the original transglutaminase activity was obtained. The b-chains thus appear to mimic the function of Ca2+ in preserving transglutaminase activity of thrombin-cleaved a-chains. The b-chains of plasma Factor XIII were not degraded by either alpha- or gamma-thrombin treatment, in presence of 5 mM EDTA or 5 mM CaCl2. Both platelet and plasma Factor XIII a-chains were degraded by trypsin to fragments with molecular masses of 51 +/- 3 kDa and 19 +/- 4 kDa in presence of 5 mM CaCl2 and to fragments with molecular masses of 19 +/- 4 kDa and lower, in presence of 5 mM EDTA.(ABSTRACT TRUNCATED AT 400 WORDS)

Gly-Pro-Arg-Pro modifies the glutamine residues in the alpha- and gamma-chains of fibrinogen: inhibition of transglutaminase cross-linking.

During blood clotting Factor XIIIa, a transglutaminase, catalyzes the formation of covalent bonds between the epsilon-amino group of lysine and the gamma-carboxamide group of peptide-bound glutamine residues between fibrin molecules. We report that glycyl-L-prolyl-L-arginyl-L-proline (GPRP), a tetrapeptide that binds to the fibrin polymerization sites (D-domain) in fibrin(ogen), inhibits transglutaminase cross-linking by modifying the glutamine residues in the alpha- and gamma-chains of fibrinogen. Purified platelet Factor XIIIa, and tissue transglutaminase from adult bovine aortic endothelial cells were used for the cross-linking studies. Gly-Pro (GP) and Gly-Pro-Gly-Gly (GPGG), peptides which do not bind to fibrinogen, had no effect on transglutaminase cross-linking. GPRP inhibited platelet Factor XIIIa-catalyzed cross-linking between the gamma-chains of the following fibrin(ogen) derivatives: fibrin monomers, fibrinogen and polymerized fibrin fibers. GPRP functioned as a reversible, noncompetitive inhibitor of Factor XIIIa-catalyzed incorporation of [3H]putrescine and [14C]methylamine into fibrinogen and Fragment D1. GPRP did not inhibit 125I-Factor XIIIa binding to polymerized fibrin, demonstrating that the Factor XIIIa binding sites on fibrin were not modified. GPRP also had no effect on Factor XIIIa cross-linking of [3H]putrescine to casein. This demonstrates that GPRP specifically modified the glutamine cross-linking sites in fibrinogen, and had no effect on either Factor XIIIa or the lysine residues in fibrinogen. GPRP also inhibited [14C]putrescine incorporation into the alpha- and gamma-chains of fibrinogen without inhibiting beta-chain incorporation, suggesting that the intermolecular cross-linking sites were selectively affected. Furthermore, GPRP inhibited tissue transglutaminase-catalyzed incorporation of [3H]putrescine into both fibrinogen and Fragment D1, without modifying [3H]putrescine incorporation into casein. GPRP also inhibited intermolecular alpha-alpha-chain cross-linking catalyzed by tissue transglutaminase. This demonstrates that the glutamine residues in the alpha-chains involved in intermolecular cross-linking are modified by GPRP. This is the first demonstration that a molecule binding to the fibrin polymerization sites on the D-domain of fibrinogen modifies the glutamine cross-linking sites on the alpha- and gamma-chains of fibrinogen.

Inhibition of platelet factor XIIIa-catalyzed reactions by calmodulin.

Calmodulin was found to exhibit an inhibitory effect on platelet factor XIIIa-catalyzed incorporation of pseudodonor amines into dimethylcasein, platelet actin and myosin. The inhibitory action of calmodulin on the calcium-dependent enzyme reactions was analogous to the effects of EGTA and parvalbumin on these reactions. The extent of inhibition of factor XIIIa activity was a function of calmodulin concentration when factor XIII and Ca2+ concentrations were held constant. These results indicate that calmodulin inhibits platelet factor XIIIa-catalyzed reactions by sequestering calcium.

Aspirin alters the cardioprotective effects of the factor XIII Val34Leu polymorphism.

BACKGROUND: The mechanism underlying decreased risk for myocardial infarction in carriers of the Leu34 polymorphism of the factor (F) XIII A-subunit is unclear. Given that acetylation of fibrinogen by aspirin can alter its clotting properties and the presence of fibrin stimulates thrombin-mediated activation of FXIII, we have tested the hypothesis that treatment with aspirin differentially modulates the influence of the FXIII Val34Leu polymorphism on its activation in vivo. METHODS AND RESULTS: The rates of the disappearance of FXIIIA chain and the appearance of its activated form (FXIIIAa) in sequential 30-second blood samples collected at the site of microvascular injury were compared in 14 healthy carriers of the Leu34 allele and 23 Val34 homozygotes both before and after a 7-day aspirin ingestion (75 mg/d), with the use of quantitative Western blotting. The presence of the Leu34 allele was associated with a significant increase in the maximum rate of FXIII activation by thrombin. Although the Leu34-positive and -negative subjects were similar with respect to aspirin-related impairment of thrombin generation, aspirin led to a more pronounced inhibition of the activation of FXIII in the Leu34 carriers as compared with the Val34 homozygotes. CONCLUSIONS: Inhibition of FXIII activation by aspirin is enhanced in the Leu34 carriers in vivo, suggesting that these subjects might benefit more than the Leu34-negative subjects from the reduction in risk for myocardial infarction with low-dose aspirin.

3-Substituted imidazo[1,2-d][1,2,4]-thiadiazoles: a novel class of factor XIIIa inhibitors.

A new class of selective FXIIIa inhibitors with a bicyclic [1,2,4]-thiadiazole pharmacophore is described. At 160 muM, compound 8 caused 50% reduction in fibrin gamma-chain cross-linking and suppressed the polymerization of alpha chains in platelet-depleted human plasma clots. Fibrinolysis rates in response to tissue plasminogen activator were directly proportional to the concentration of 8 in plasma at the time of clotting.