Val34Leu polymorphism of the A subunit of coagulation factor XIII (FXIII-A) is located in the activation peptide (AP) just 3 amino acids away from the thrombin cleavage site. This mutation has been associated with a protective effect against occlusive arterial diseases and venous thrombosis; however, its biochemical consequences have not been explored. In the current study it was demonstrated that the intracellular stability and the plasma concentration of FXIII of different Val34Leu genotypes are identical, which suggests that there is no difference in the rate of synthesis and externalization of wild-type and mutant FXIII-A. In contrast, the release of AP by thrombin from the Leu34 allele proceeded significantly faster than from its wild-type Val34 counterpart. By molecular modeling larger interaction energy was calculated between the Leu34 variant and the respective domains of thrombin than between the Val34 variant and thrombin. In agreement with these findings, the activation of mutant plasma FXIII by thrombin was faster and required less thrombin than that of the wild-type variant. Full thrombin activation of purified plasma FXIII of different genotypes, however, resulted in identical specific transglutaminase activities. Similarly, the mean specific FXIII activity in the plasma was the same in the groups with wild-type, heterozygous, and homozygous variants. Faster activation of the Leu34 allele hardly could be associated with its presumed protective effect against venous thrombosis. No such protective effect was observed in a large group of patients with familial thrombophilia.

Acquired factor XIII (FXIII) deficiency due to autoantibody against FXIII is a very rare severe hemorrhagic diathesis. Antibodies directed against the A subunit of FXIII, which interfere with different functions of FXIII, have been described. Here, for the first time, we report an autoantibody against the B subunit of FXIII (FXIII-B) that caused lifethreatening bleeding in a patient with systemic lupus erythematosus. FXIII activity, FXIII-A 2 B 2 complex, and individual FXIII subunits were undetectable in the plasma, whereas platelet FXIII activity and antigen were normal. Neither FXIII activation nor its activity was inhibited by the antibody, which bound to structural epitope(s) on both free and complexed FXIII-B. The autoantibody highly accelerated the elimination of FXIII from the circulation. FXIII supplementation combined with immunosuppressive therapy, plasmapheresis, immunoglobulin, and anti-CD20 treatment resulted in the patient's recovery. FXIII levels returned to around 20% at discharge and after gradual increase the levels stabilized above 50%. IntroductionBlood coagulation factor XIII (FXIII) is a protransglutaminase of tetrameric structure (FXIII-A 2 B 2 ). 1,2 Its potentially active A subunit (FXIII-A) is synthesized in cells of bone marrow origin; it is also present in platelets and monocytes/macrophages in dimeric form (FXIII-A 2 ). The noncatalytic B subunit (FXIII-B) is in excess and it is essential for the stabilization of FXIII-A 2 in plasmatic conditions. FXIII is converted into an active transglutaminase (FXIIIa) by limited proteolysis of FXIII-A and by Ca 2ϩ -induced dissociation of FXIII-B. Cross-linking of fibrin ␣-and ␥-chains and ␣ 2 -plasmin inhibitor to fibrin by FXIIIa stabilizes fibrin and protects it from prompt elimination by plasmin. 3 Inherited FXIII-A deficiency is a severe bleeding diathesis with the high risk of intracranial bleeding in nonsupplemented patients. 4 Only 5 cases of inherited FXIII-B deficiency with mild-to-moderate bleeding tendency have been reported. [5][6][7][8] In the absence of FXIII-B, plasma FXIII activity and FXIII-A concentration were considerably decreased, whereas in platelets a normal amount of FXIII-A was measured. 9 Thirty-six cases of severe FXIII deficiency due to an autoantibody against FXIII-A have been reported. In a few cases, the autoantibody was characterized and classified into subgroups according to its inhibition of FXIII activation, FXIIIa activity, or binding to fibrin. [10][11][12][13][14][15] In about one third of the cases the autoantibody was associated with systemic lupus erythematosus (SLE). No report on an autoantibody directed against FXIII-B has been published so far. MethodsA 28-year-old woman suffering from SLE with end-stage kidney disease was on hemodialysis. For preparation of cubital fistula she was admitted to a county hospital. In the proximity of the surgical wound, hematomas developed that were explored. The wounds showed no tendency of healing and remained open. A few days later extensive intramuscular hematoma ...

SummaryA new highly sensitive sandwich ELISA assay was developed for the determination of plasma factor XIII (FXIII). Plasma FXIII is a tetrameric complex of two types of subunits (A2B2). A biotinylated monoclonal capture-antibody against the B-subunit and a peroxidase-labelled monoclonal tag-antibody against the A-subunit were added to the plasma dilution and the amount of the complex attached to streptavidincoated microplate was quantitated by measuring peroxidase activity. Only the tetrameric plasma FXIII reacted in the assay, non-complexed A or B subunits showed no reaction. The assay is linear up-to 40 µg/L of FXIII in the assay mixture. It is a quick one-step assay which can be performed within two hours. At normal and low FXIII concentration within batch reproducibility was 2.0% and 3.3%, day to day variation was 5.5% and 8.7%, respectively. Its high sensitivity allows reliable measurement at FXIII concentrations below 1% of normal average. Plasma samples can be stored for the assay at –20° C for at least one month. Plasma levels of healthy individuals were normally distributed and no gender difference was observed. A reference interval of 14-28 mg/L (67-133%) was established.

SummaryThe cellular form of blood coagulation factor XIII (FXIII) is present in platelets, monocytes and macrophages. During long-term stimulation of platelets by thrombin cellular FXIII becomes activated and crosslinks proteins, however, the mechanism of its activation has not been elucidated. It was shown that, contrary to plasma FXIII, the intracellular activation of platelet FXIII does not involve proteolysis. FXIII remained intact in thrombin-activated platelets, i.e., the activation peptide was not removed from the molecule. Part of the zymogen FXIII molecules, however, assumed an active configuration as was demonstrated both by the measurement of transglutaminase activity and by active-site-SH titration. These findings clearly indicate that during platelet activation, when intracellular Ca2+ concentration is raised, a slow non-proteolytic transformation of FXIII zymogen into an active transglutaminase occurs.