Fibrinolysis

• Fibrinolysis


  The process by which fibrin clots are broken down and the flow of blood is reestablished
• Fibrinolysis is initiated as soon as the clotting process has begun
• Normally, the fibrinolytic system functions to keep the vascular system free of fibrin clots or deposited fibrin
• The fibrinolytic system and the coagulation system are in equilibrium in normal persons
• Fibrinolysis is increased whenever coagulation is increased
• Plasmin


  The active enzyme that is responsible for digesting fibrin or fibrinogen

  
• Plasmin is not normally found in the circulating blood but is present in an inactive form, plasminogen

  
• Plasminogen activators


  Proteolytic enzymes that convert plasminogen to plasmin

  
• Plasminogen activators are found in small amounts in most body tissues, in very low amounts in most body fluids, and in urine
• Fibrin degradation products (FDPs) or fibrin split products (FSPs)


  The end products of fibrin and fibrinogen formed during fibrinolysis
• 1. dimers


  Result only when fibrin that has been stabilized by FXIII crosslinking has been digested

  
• Four components of the fibrinolytic system

  • Plasminogen
  • Plasmin
  • Plasminogen Activators
  • Inhibitors of Fibrinolysis

  
• Plasminogen


  92,000 Daltons; from the liver; a single chain protein with 5 glycosylated loops called kringles

  
• Kringles


  • Enable plasminogen, along with activators TPA and UPA, to bind the lysine moieties on the fibrin molecule during the polymerization process
  • This fibrin-binding step is essential to fibrinolysis

  
• Conversion of fibrin-bound plasminogen to active plasmin


  Cleavage between arginine at position 561 and valine at position 562 by neighboring fibrin-bound TPA or UPA

  
• Plasmin


  A serine protease that systematically digests fibrin polymer by the hydrolysis of arginine-related and lysine –related peptide bonds

  
• As fibrin becomes digested, the exposed carboxy-terminal lysine residues bind additional plasminogen and TPA, which further accelerates clot digestion
• FXa, FXIIa, and kallikrein produce an alternative mechanism of plasminogen activation

  
• Bound plasmin


  • Digests clot and restores blood vessel patency (openness)
  • Its localization to fibrin through lysine binding prevents systemic activity
• Free plasmin


  Can be found in the circulation and is capable of digesting Factors 1, V, VIII, and fibronectin
• Plasma alpha2- antiplasmin


  Rapidly binds and inactivates any free plasmin
• Plasminogen Activators


  • Endogenous
  • Exogenous

  
• Tissue Plasminogen Activator (TPA)

  • Secreted by the endothelial cells, hydrolyze fibrin- bound plasminogen, converting it to plasmin
  • Has 2 glycosylated kringle regions, forms covalent lysine bonds with fibrin during polymerization and localizes at the surface of the thrombus with plasminogen to convert the latter to plasmin
  • Circulating TPA is bound to inhibitors such as PAI-1

  
• Urokinase Plasminogen Activator (UPA)


  • Secreted by urinary tract epithelial cells, monocytes, and macrophages
  • Circulates in the plasma at a concentration of 2-4ng/mL and becomes incorporated into the mix of fibrin-bound plasminogen and TPA at the time of thrombus formation
  • Converts plasminogen to plasmin
  • Has only one kringle region, so UPA does not bind firmly to fibrin and has a relatively minor physiologic effect

  
• Streptokinase


  An exogenous plasminogen activator

  
• Acyl- plasminogen streptokinase activator complex (APSAC)


  An inactive complex of human plasminogen and streptokinase that becomes activated when injected

  
• TPA, purified UPA preparations called urokinases are used therapeutically to dissolve thrombi in patients with myocardial infarction, stroke, and deep vein thrombosis
• Inhibitors of Fibrinolysis


  • Plasminogen Activator Inhibitor-1
  • Alpha-2 antiplasmin
  • Thrombin Activatable Fibrinolysis Inhibitor (TAFI)
  • Alpha2 macroglobulin
  • Thrombomodulin

  
• Plasminogen Activator Inhibitor-1 (PAI-1)

  • The principal inhibitor of plasminogen activation, inactivating both TPA and UPA, thus preventing them from converting plasminogen to plasmin
  • Produced by various cell types, including endothelial cells, megakaryocytes, smooth muscle cells, fibroblasts, monocytes, adipocytes, and hepatocytes
  • Platelets store more than 50% of the total PAI-1
  • Present in excess of the TPA concentration in plasma, and circulating TPA normally becomes bound to PAI-1
  • Binding of TPA to fibrin protects TPA from PAI-1 inhibition
  • Its deficiency is associated with chronic bleeding caused by increased fibrinolysis
  • It is an acute phase reactant and is increased in many conditions, including metabolic syndrome, obesity, atherosclerosis, sepsis, and stroke
  • Increased levels correlate with reduced fibrinolysis and increased risk of thrombosis

  
• Alpha-2 antiplasmin


  • Synthesized in the liver and is the primary inhibitor of free plasmin
  • A SERPIN
  • Free plasmin produced by activation of plasminogen can bind either to fibrin, where it is protected by AP (alpha-2 antiplasmin)
  • Therapeutic lysine analogs, tranexamic acid and e-aminocaprioc acid, are similarly antifibrinolytic through their affinity for kringles in plasminogen and TPA

  
• Thrombin Activatable Fibrinolysis Inhibitor (TAFI)

  • A plasma procarboxypeptidase produced by the liver that becomes activated by the thrombin-thrombomodulin complex
  • Functions as an antifibrinolytic enzyme by cleaving carboxy-terminal lysine residues from partially degraded fibrin, thereby preventing the binding of TPA and plasminogen to fibrin and blocking the formation of plasmin
  • In coagulation factor –deficient states, such as hemophilia, decreased thrombin production may reduce the activation of TAFI, contributing to thrombosis
  • May also play a role in regulating inflammation and wound healing

  
• Alpha2 macroglobulin


  Inactivates plasmin not inhibited by alpha2 antiplasmin

  
• Thrombomodulin


  Released by platelets, inhibits activation of fibrin-bound plasminogen

  
• Fibrin degradation products (FDPs) or fibrin split products (FSPs)


  • A series of identifiable fibrin fragments (X, Y, D, E and D-D) produced by the cleavage of fibrin (and fibrinogen) by plasmin
  • Several of these fragments inhibit hemostasis and contribute to hemorrhage by preventing platelet activation and by hindering fibrin polymerization
  • FDPs are antithrombin so that excessive amounts can cause bleeding

  
• Fragment X


  The central E domain with the 2 D fragments (D-E-D), minus some peptides cleaved by plasmin

  
• Fragment Y


  The E domain after cleavage of one D domain (D-E)
• 1. dimer


  • Composed of two D domains from separate fibrin molecules (not fibrinogen) cross-linked by the action of FXIIIa
  • A specific product of digestion of cross-linked fibrin only and is therefore a marker of thrombosis and fibrinolysis

  
• 1. dimer is present only when thrombin generation, polymerization of fibrin clot by FXIIIa, and fibrinolysis of the clot by plasmin have occurred

  
• Assessing D-dimer levels


  An important diagnostic tool to identify DIC and to rule out venous thromboembolism and pulmonary embolism
• Classification of plasminogen activators

  • Intrinsic activators (in the blood/plasma)
  • Extrinsic activators (in body tissues)
  • Exogenous activators (outside the physiologic milieu)