Summary
Fibrin-bound thrombin is protected from inactivation by antithrombin III, while its
coagulant potential is retained. In the presence of heparin, ternary complexes between
thrombin, fibrin and heparin are formed. In these complexes the coagulant activity
of thrombin is retained, whereas the anticoagulant activity of fibrin-bound heparin
is neutralized. The limited effectiveness of heparin in the prevention of both venous
thrombosis and coronary reocclusion is probably related to the protective effect of
fibrin on the inactivation of thrombin by antithrombin III. Recently, it has been
shown that factor XI can be activated by thrombin, resulting in the generation of
additional thrombin via the intrinsic pathway. This additional thrombin is capable
of stabilizing the clot by protecting it from fibrinolysis. We studied the effect
of heparin on the activation of factor XI by fibrin-bound thrombin. First, we used
fibrin monomers coupled to Sepharose to which thrombin and unfractionated heparin
(UFH) were bound. Factor XI activation by thrombin was the same in the presence of
fibrin-Sepharose or control-Sepha-rose. The addition of heparin (0.1 U/ml) resulted
in a 91 and 15-fold enhancement in the presence of control-Sepharose and fibrin-Sepharose,
respectively. Next, we added complexes of heparin, thrombin and fibrin monomer to
factor XII and XI double-deficient plasma in the presence or absence of a reconstituting
amount of factor XI. In the presence of factor XI, additional fibrin formation was
observed indicating that factor XI activation by thrombin in complex with fibrin and
heparin can take place in plasma. We then studied the effect of other heparin-like
anticoagulants on the thrombin-mediated factor XI activation. UFH enhanced thrombin-mediated
factor XI activation 68-fold, LMWH (low molecular weight heparin, Fragmin) 12-fold,
danaparoid (Orgaran) 3-fold, while the pentasaccharide ORG 31540 did not result in
an enhancement. Binding studies of these anticoagulants to fibrin-Sepharose showed
that LMWH bound with approximately the same affinity as UFH, while danaparoid and
the pentasaccharide did not bind to fibrin.
We conclude that fibrin-bound thrombin is capable of factor XI activation. Furthermore,
heparin bound in a complex with fibrin can act as a cofactor for this activation.
This factor XI activation capacity may play a role in the limited effectiveness of
heparin. Provided that thrombin-mediated factor XI activation plays an important role
in vivo, danaparoid and especially the pentasaccharide may be better anticoagulants
than UFH and LMWH.
