Clot-bound thrombin is protected from inhibition by heparin-antithrombin III but is susceptible to inactivation by antithrombin III-independent inhibitors.
Propagation of venous thrombi or rethrombosis after coronary thrombolytic therapy can occur despite heparin administration. To explore potential mechanisms, we set out to determine whether clot-bound thrombin is relatively protected from inhibition by heparin-antithrombin III but susceptible to inactivation by antithrombin III-independent inhibitors. Using plasma fibrinopeptide A (FPA) levels as an index of thrombin activity, we compared the ability of thrombin inhibitors to block FPA release mediated by fluid-phase thrombin with their activity against the clot-bound enzyme. Incubation of thrombin with citrated plasma results in concentration-dependent FPA generation, which reaches a plateau within minutes. In contrast, there is progressive FPA generation when fibrin clots are incubated with citrated plasma. Heparin, hirudin, hirudin dodecapeptide (hirugen), and D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone (PPACK) produce concentration-dependent inhibition of FPA release mediated by fluid-phase thrombin. However, heparin is much less effective at inhibiting thrombin bound to fibrin because a 20-fold higher concentration is necessary to block 70% of the activity of the clot-bound enzyme than is required for equivalent inhibition of fluid-phase thrombin (2.0 and 0.1 U/ml, respectively). In contrast, hirugen and PPACK are equally effective inhibitors of fluid- and solid-phase thrombin, while hirudin is only 50% as effective against the clot-bound enzyme. None of the inhibitors displace bound 125I-labeled thrombin from the clot. These studies indicate that (a) clot-bound thrombin is relatively protected from inhibition by heparin, possibly because the heparin binding site on thrombin is inaccessible when the enzyme is bound to fibrin, and (b) clot-bound thrombin is susceptible to inactivation by antithrombin III-independent inhibitors because the sites of their interaction are not masked by thrombin binding to fibrin. For these reasons, antithrombin III-independent inhibitors may be more effective than heparin in certain clinical settings.
Weitz
J I
JI
Department of Medicine, McMaster University, Hamilton, Ontario, Canada.
Hudoba
M
M
Massel
D
D
Maraganore
J
J
Hirsh
J
J
D000590:Amino Acid Chloromethyl Ketones / Q000494:pharmacology; D000990:Antithrombin III / Q000378:metabolism*; D001777:Blood Coagulation; D005337:Fibrin / Q000378:metabolism; D005344:Fibrinopeptide A / Q000378:metabolism; D006493:Heparin / Q000378:metabolism / Q000494:pharmacology; D006629:Hirudins / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D010446:Peptide Fragments / Q000494:pharmacology; D011480:Protease Inhibitors / Q000494:pharmacology; D011485:Protein Binding; D012995:Solubility; D013917:Thrombin / Q000037:antagonists & inhibitors*
D000590:Amino Acid Chloromethyl Ketones; D006629:Hirudins; D010446:Peptide Fragments; D011480:Protease Inhibitors; C062510:hirugen; D005344:Fibrinopeptide A; D000990:Antithrombin III; D005337:Fibrin; D006493:Heparin; D013917:Thrombin; C040308:phenylalanyl-prolyl-arginine-chloromethyl ketone