Thrombin-mediated Feedback Activation of Factor XI on the Activated Platelet Surface Is Preferred over Contact Activation by Factor XIIa or Factor XIa

Abstract

To study the pathways for initiation of intrinsic blood coagulation, activated human platelets were compared with dextran sulfate as surfaces for factor XI activation by factor XIIa, factor XIa, or thrombin. Activated gelfiltered platelets promoted the activation of factor XI (60 nM) by thrombin (0.02-10 nM, EC 50 ϳ100 pM, threshold concentration ϳ10 pM) at initial rates 2-to 3-fold greater than those obtained with dextran sulfate in the presence of either high molecular weight kininogen (45 nM) and ZnCl 2 (25 M) or prothrombin (1.2 M) and CaCl 2 (2 mM). The maximum rates of factor XI activation achieved in the presence of activated gel-filtered platelets were 30 nM⅐min ؊1 with thrombin, 6 nM⅐min ؊1 with factor XIIa and 2 nM⅐min ؊1 with factor XIa. Values of turnover number calculated at various enzyme concentrations (0.05-1 nM) were 24 -167 (mean ‫؍‬ 86) min ؊1 for thrombin, 4.6 -50 (mean ‫؍‬ 21) min ؊1 for factor XIIa, and 1.3-14 (mean ‫؍‬ 8) min ؊1 for factor XIa. A physiological concentration of fibrinogen (9.0 M) inhibited factor XI activation by thrombin (but not by factor XIIa) in the presence of dextran sulfate but not in the presence of gel-filtered platelets. Compared with factors XIIa and XIa, thrombin is the preferred factor XI activator, and activated platelets are a relevant physiological surface for thrombinmediated initiation of intrinsic coagulation in vivo.Human coagulation factor XI is a disulfide-linked homodimer consisting of two identical polypeptide chains each containing 607 amino acids. Factor XI is present in human plasma as a zymogen that requires proteolytic activation to develop serine protease activity (1-5). It circulates in human plasma in a noncovalent complex with high molecular weight kininogen (HK) 1 (6) and can be activated by three biologically relevant proteases: factor XIIa, factor XIa, and thrombin (1,7,8). The primary structure of factor XI has been determined (9, 10), including the identification of four tandem repeat sequences, designated Apple (A1, A2, A3, and A4) domains in the heavy chain region of factor XI. Binding sites (11-15) for thrombin, the Kringle 2 domain of prothrombin, and HK are present in the A1 domain, whereas both heparin-and plateletbinding sites exist within the A3 domain (16 -18) and a binding site for factor XIIa is located in the A4 domain (19).Factor XI can participate in the contact phase of blood coagulation in a reaction that requires the presence of anionic surfaces for optimal activation in vitro by factor XIIa (6,[20][21][22]. However, deficiencies in factor XII, prekallikrein, and HK are not associated with hemostatic abnormalities, whereas a deficiency in factor XI produces abnormal bleeding complications (23-25). Therefore, it has been suggested that the physiologically relevant pathway for factor XI activation might constitute feedback activation either by thrombin or by factor XIa (7,8,15). All three proteases cleave each monomer of factor XI at the Arg 369 -Ile 370 bond generating the new amino-terminal sequence (IVGG) of the cataly...

Thrombin Activation of Factor XI on Activated Platelets Requires the Interaction of Factor XI and Platelet Glycoprotein Ibα with Thrombin Anion-binding Exosites I and II, Respectively

“…Thus, FXI activation is required for normal hemostasis only in cases of significant hemostatic challenge, which is consistent with clinical observations that patients with congenital FXI deficiency generally have much milder bleeding than those with FVIII deficiency (hemophilia A) (15,16). Consistent with these observations is the demonstration that thrombin is more efficient than FXIIa or FXIa in activating FXI in vitro (13).…”

“…For 10 mutants demonstrating diminished (Ͻ50% compared with WT thrombin) FXI activation, dose-response curves were constructed over a range of thrombin concentrations using the above protocol. Linear initial rates of FXIa generation with respect to time and enzyme (thrombin) concentration were observed in the presence of dextran sulfate as previously reported (8,9,13). EC 50 values were calculated using SigmaPlot software.…”

“…FXIIa, a component of the contact phase of blood coagulation, is unlikely to be a physiologically relevant activator because FXII deficiency is not associated with clinical bleeding (10,11). Recent data suggest that thrombin is the physiological activator of FXI on the activated platelet (12)(13)(14). Normal hemostasis is initiated by the exposure of tissue factor at sites of vascular injury, followed by the formation of the FVII-tissue factor complex and the subsequent activation of FX.…”

Characterization of the H-kininogen-binding Site on Factor XI

“…Several recent studies have shed light on this conundrum. It has been shown that HK facilitates FXI binding to the surface of activated platelets, where it is rapidly activated to FXIa (18,20,43). Prothrombin may serve as a substitute for HK in this role, providing a plausible explanation as to why HK-deficient humans do not bleed (19).…”

“…Along similar lines, it has been shown that HK facilitates the binding of FXI to activated platelets (17,18). Once bound to the platelet surface, FXI is efficiently activated to FXIa by coagulation proteases such as thrombin and factor XIIa (FXIIa) (4,19,20).…”