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Blood coagulation is essential for hemostasis, however excessive coagulation can lead to thrombosis. Factor XII starts the intrinsic coagulation pathway and contact-induced factor XII activation provides the mechanistic basis for the diagnostic aPTT clotting assay. Despite its function for fibrin formation in test tubes, patients and animals lacking factor XII have a completely normal hemostasis. The lack of a bleeding tendency observed in factor XII deficiency states is in sharp contrast to deficiencies of other components of the coagulation cascade and factor XII has been considered to have no function for coagulation in vivo. Recently, experimental animal models showed that factor XII is activated by an inorganic polymer, polyphosphate, which is released from procoagulant platelets and that polyphosphate-driven factor XII activation has an essential role in pathologic thrombus formation. Cumulatively, the data suggest to target polyphosphate, factor XII, or its activated form factor XIIa for anticoagulation. As the factor XII pathway specifically contributes to thrombosis but not to hemostasis, interference with this pathway provides a unique opportunity for safe anticoagulation that is not associated with excess bleeding. The review summarizes current knowledge on factor XII functions, activators and inhibitors.

Background: Thrombin activity increases in patients treated with coronary thrombolysis for acute myocardial infarction, but the mechanisms are not well defined. We have shown that thrombin activity increases in plasma and whole blood incubated with plasminogen activators and appears to be plasmin mediated and dependent on activity of the factor VIIIa/IXa complex., Methods and Results: In the present study, increases in thrombin activity induced by incubation of recalcified citrated plasma with 0.16 to 0.5 mumol/L plasmin at 37 degrees C were markedly attenuated in recalcified citrated plasma deficient in factors XI or XII, prekallikrein, or high molecular weight kininogen, as well as in plasma incubated with plasmin in the presence of 3.5 mumol/L corn trypsin inhibitor, a specific factor XIIa inhibitor. Increases in thrombin activity also occurred in nonanticoagulated whole blood incubated with pharmacological concentrations of plasminogen activators and were markedly attenuated in the presence of corn trypsin inhibitor. Plasmin-mediated (0.25 mumol/L) activation of purified factor XII occurred in 0.05 mol/L Tris-HCl and 0.012 mol/L NaCl (pH 7.8) at 37 degrees C, resulting in equimolar quantities of two fragments that corresponded to cleavage of factor XII at Arg353-Val354, the site involved in kallikrein-mediated activation of factor XII, and cleavage at Lys346-Ser347, an apparently novel site of plasmin-mediated hydrolysis of factor XII. Contact activation was also demonstrated in plasma samples from patients after treatment with fibrinolytic agents for myocardial infarction, by demonstrating cleavage of high molecular weight kininogen from its one-chain to its two-chain form by ligand blotting with 125I-prekallikrein., Conclusions: Plasmin-mediated activation of the contact system of coagulation appears to account, at least in part, for increases in procoagulant activity in patients treated with fibrinolytic agents. It may also explain hypotension, by release of bradykinin from high molecular weight kininogen, and complement activation, by activated factor XII, that has been demonstrated in these patients.

Stearic acid causes hypercoagulability of the blood by activation of factor XII and by aggregation of blood platelets. Injection of unbound stearic acid (sodium salt) into the systemic circulation of dogs was followed by massive generalized thrombosis and sudden death. Similar infusions into birds, which are deficient in factor XII, did not cause hypercoagulability or thrombosis. The effects of the long-chain saturated fatty acids could be prevented by using albumin to bind the stearic acid at a molar ratio of free fatty acid (FFA) to albumin of < 2. The major issue is whether eating foods rich in stearic acid can cause a thrombogenic effect. We have no experimental evidence to support this concept. If a thrombogenic effect of long-chain saturated fatty acids exists in humans, it is most likely to occur as an aberration of fatty acid transport in which the FFA-albumin molar ratio exceeds 2 either as a result of very high plasma FFA concentrations from lipid mobilization or a low concentration of albumin in the blood as found in disease states such as the nephrotic syndrome.