Inactivation of factor XIa in human plasma assessed by measuring factor XIa-protease inhibitor complexes: major role for C1-inhibitor

Abstract

From experiments with purified proteins, it has been concluded that factor XIa (FXIa) is inhibited in plasma mainly by alpha 1-antitrypsin (a1AT), followed by antithrombin III (ATIII), C1-inhibitor (C1Inh), and alpha 2-antiplasmin (a2AP). However, the validity of this concept has never been studied in plasma. We established the relative contribution of different inhibitors to the inactivation of FXIa in human plasma, using enzyme-linked immunosorbent assays (ELISAs) for the quantification of complexes of FXIa with a1AT, C1Inh, a2AP, and ATIII. We found that 47% of FXIa added to plasma formed complexes with C1Inh, 24.5% with a2AP, 23.5% with a1AT, and 5% with ATIII. The distribution of FXIa between these inhibitors in plasma was independent of whether FXIa was added to plasma, or was activated endogenously by kaolin, celite, or glass. However, in the presence of heparin (1 or 50 U/mL), C1Inh appeared to be the major inhibitor of FXIa, followed by ATIII. Furthermore, at lower temperatures, less FXIa-C1Inh and FXIa-a1AT complexes but more FXIa-a2AP complexes were formed. These data demonstrate that the contribution of the different inhibitors to inactivation of FXIa in plasma may vary, but C1Inh is the principal inhibitor under most conditions.

Initiation of contact system activation in plasma is dependent on factor XII autoactivation and not on enhanced susceptibility of factor XII for kallikrein cleavage

“…by assessment of the generation of complexes of C1inhibitor and activated FXIIa, factor XIa and kallikrein. Since C1-inhibitor is the major inhibitor in plasma of all three proteases of the contact system (Schapira et al, 1982;Pixley et al, 1985;Wuillemin et al, 1995), the generation of protease-C1-inhibitor complexes reflected the contact system activation in plasma (Nuijens et al, 1987;Lewin et al, 1983;Kaplan et al, 1985;Wuillemin et al, 1995).…”

“…Sample dilutions were tested in duplicate and the concentrations of FXIIa-C1-inhibitor and kallikrein-C1-inhibitor complexes generated were calculated by comparison with normal pooled human plasma fully activated by DS500 (50 mg/ml) (Nuijens et al, 1988). Generation of factor XIa-C1-inhibitor complexes was determined by an enzyme-linked immunosorbent assay (ELISA) using kaolin-activated normal plasma as an in-house standard (Wuillemin et al, 1995).…”

Hereditary and acquired angioedema: Problems and progress: Proceedings of the third C1 esterase inhibitor deficiency workshop and beyond

“…As discussed in greater detail in the Pathogenesis and Pathobiology of HAE and AAE section, C1-INH controls several proteases, including C1r and C1s, the mannosebinding protein associated serine protease (MASP) system, kallikrein, coagulation factors XIIa and XIa, plasmin, and tissue plasminogen activator. [139][140][141][142][143][144] Therefore, C1-INH plays a key role in regulating the early steps of complement and the contact system of kinin formation. 89 This broad inhibitory ability ensues from a property unique to the serpin class: highly efficient complex formation with target proteases.…”

“…Few studies have attempted to measure bradykinin according to such protocols in HAE, but these studies have shown increased bradykinin generation during attacks and not during symptom-free periods, particularly when samples are drawn from the site of the edematous swelling. 13,14,27 C1-INH is a major inhibitor of factor XIa of the intrinsic pathway of the intrinsic pathway of coagulation 143,224 and also inhibits thrombin, 199 although inhibition of this latter enzyme is likely physiologically inconsequential. The role of factor XI in coagulation is not yet resolved.…”

Activation of coagulation factor XI, without detectable contact activation in dengue haemorrhagic fever

“…Upon activation in plasma, factor XI forms complexes with several protease inhibitors, the dominant inhibitor being C1-inhibitor (Wuillemin et al, 1995). Hence, to assess activation of factor XI in the patients, levels of factor XIa± C1-inhibitor complexes were measured according to a method described previously (Wuillemin et al, 1995). In the plasma of normal volunteers, levels of these complexes are below the detection limit (10 pmol/l) of this assay.…”