Clot-bound thrombin is protected from inhibition by heparin-antithrombin III but is susceptible to inactivation by antithrombin III-independent inhibitors

J Clin Invest. 1990 Aug;86(2):385-91. doi: 10.1172/JCI114723.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Chloromethyl Ketones / pharmacology
  • Antithrombin III / metabolism*
  • Blood Coagulation*
  • Fibrin / metabolism
  • Fibrinopeptide A / metabolism
  • Heparin / metabolism
  • Heparin / pharmacology
  • Hirudins / pharmacology
  • Humans
  • In Vitro Techniques
  • Kinetics
  • Peptide Fragments / pharmacology
  • Protease Inhibitors / pharmacology
  • Protein Binding
  • Solubility
  • Thrombin / antagonists & inhibitors*

Substances

  • Amino Acid Chloromethyl Ketones
  • Hirudins
  • Peptide Fragments
  • Protease Inhibitors
  • hirugen
  • Fibrinopeptide A
  • Antithrombin III
  • Fibrin
  • Heparin
  • Thrombin
  • phenylalanyl-prolyl-arginine-chloromethyl ketone