Summary
Clot-bound thrombin proteolyses fibrinogen and amplifies the coagulation cascade at
its close vicinity, thereby ensuring the growth of fibrin-rich thrombus. The present
study compares the ability of various glycosaminoglycans (GAGs) to inhibit these 2
properties. Unfractionated heparin (UH), 3 low molecular weight heparins (LMWHs) with
increasing antifactor Xa/antifactor Ha ratio, the synthetic pentasaccharide (PS),
devoid of antifactor Ha activity, and dermatan sulfate (DS), a catalyst of thrombin
inhibition by heparin cofactor II, were selected on the basis of their different properties.
Proteolysis of fibrinogen by clot-bound thrombin was evaluated by measuring fibrinopeptide
A (FPA) generation after an incubation of standardized washed clots in plasma for
120 min in absence or in presence of increasing concentrations of heparins or of DS.
The results were compared to those obtained when free a-thrombin (0.4 nM) was added
to plasma in the same experimental conditions. On the basis of equivalent antithrombin
units, UH and LMWHs gave identical results. To inhibit by 70% fibrinogen proteolysis
induced by clot-bound thrombin (IC 70), 5- to 9-fold higher concentrations of UH or
of LMWHs were required in comparison with those required to inhibit free thrombin.
For DS, only a 1.3 times higher concentration was required. PS (final concentration
1 anti Xa U • ml-1) was devoid of any inhibitory effect. The amplification of the coagulation cascade
induced by dot-bound thrombin was evaluated by measuring the shortening of whole blood
clotting time (WBCT) resulting from the incubation of washed clots in native blood.
In absence of GAG, clot-bound thrombin reduced WBCT from 18 ± 2 min to 9 ± 1 min.
Each GAG prolonged WBCT in a dose-dependent manner but these prolongations were smaller
in presence of washed clots. The most potent agent to suppress the shortening of WBCT
was DS. LMWH and UH were less effective and PS (final concentration 1 anti Xa U/ml)
was almost ineffective. Therefore, in these in vitro experiments, DS is a more potent
inhibitor of clot-bound thrombin than heparin. Whether or not these observations are
relevant for the treatment of established deep-vein thrombosis requires comparative
clinical studies.
