hemostasis & thrombosis beth a. bouchard bioc 212: biochemistry of human disease spring 2006
DESCRIPTION
Hemostasis & Thrombosis Beth A. Bouchard BIOC 212: Biochemistry of Human Disease Spring 2006. HEMOSTASIS. Hemorrhage. Thrombosis. Hemostasis. HEMOSTASIS (CONT.). 1). INITIATION Vessel wall – endothelial cells and subendothelial components 2). LOCALIZATION - PowerPoint PPT PresentationTRANSCRIPT
Hemostasis & Thrombosis
Beth A. BouchardBIOC 212: Biochemistry of Human Disease
Spring 2006
HEMOSTASIS
Hemorrhage Thrombosis
Hemostasis
HEMOSTASIS (CONT.)
1). INITIATIONVessel wall – endothelial cells and subendothelial
components
2). LOCALIZATIONPlatelets – circulating cellular elements
3). PROPAGATION/AMPLIFICATIONPlasma coagulation proteins (factors)
4). TERMINATIONPlasma coagulation protein inhibitors
5). ELIMINATIONFibrinolytic system
Vessel Wall: Endothelial cells
Antithrombotic Attributes of Vascular Endothelium
Vessel Wall (cont.)
Response to Vessel Wall Injury: Platelet adhesion
• Exposure of flowing blood and platelets to subendothelial components
• Platelets bind to the subendothelial collagen bound to von Willebrand factor (vWF), which is secreted from endothelial cells directly into the subendothelial space or adsorbed from plasma following endothelial cell secretion
• vWf also binds directly to platelets via glycoprotein Ib-IX
PLATELETS
Platelets adhered to damaged endothelium
Platelet Plug Formation = 1° hemostasis
Platelet Plug Formation: Platelet activation
• Activated via their interaction with subendothelial collagen
• Additional platelet agonists include ADP, epinephrine, thrombin, immune complexes, and high shear stress – all of the compounds interact with specific platelet membrane receptors
• Several platelet activation pathways are initiated
Platelet Plug Formation: Platelet activation events
• Platelet shape change: extend pseudopodia, which facilitates aggregation and coagulant activity
• Release of alpha and dense granule contents including a number of compounds involved in hemostasis (eg ADP, factor V and fibrinogen)
• Aggregation
Response to Vessel Wall Injury: Vasoconstriction
• Temporarily reduces local blood flow and hence, blood loss
• Mediated in part by serotonin and thromboxane A2 (TXA2) from activated plateletsSerotonin is released from platelet dense granules
TXA2 is a product of platelet prostaglandin metabolism
Activated platelets
Platelet Plug Formation: Platelet aggregation
• Platelet activation results in the functional expression of membrane receptors normally expressed in a non-functional state (glycoprotein IIb-IIIa)
• Fibrinogen from the plasma or released from activated platelet alpha-granules binds to activated glycoprotein IIb-IIIa membrane receptors effectively bridging platelets to each other
Platelets adhered to and aggregated upon collagen
Platelet Plug Formation
1). INITIATIONVessel wall – endothelial cells and subendothelial
components
2). LOCALIZATIONPlatelets – circulating cellular elements
3). PROPAGATION/AMPLIFICATIONPlasma coagulation proteins (factors)
4). TERMINATIONPlasma coagulation protein inhibitors
5). ELIMINATIONFibrinolytic system
HEMOSTASIS
BLOOD COAGULATION
BLOOD COAGULATION (CONT.)
• Deficiencies in all of the factors, except factor XII, lead to a bleeding tendency in the affected individual
• Described as a ‘waterfall’ or ‘cascade’ sequence of zymogen (pro-enzyme) to enzyme conversions, with each enzyme activating the next zymogen in the seqeunce
• Activated factor enzymes are designated with an “a”, e.g. factor Xa
Common constituents of coagulation complexes
Vitamin K-dependent (VKD) zymogen
Ca2+
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
Protein cofactor
Common constituents of coagulation complexes
Vitamin K-dependent (VKD) zymogen
Ca2+
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
Protein cofactor
Functional Domains of the Vitamin K-dependent Zymogens
Gamma ()-carboxyglutamic acid
Formation of Gla residues subsequent to protein synthesis (post-translational)
• Group of related, fat soluble compounds, which differ in the number of side-chain isoprenoid units
• Plant derived (vitamin K1) and synthesized by intestinal bacteria (vitamin K2)
H2
Common constituents of coagulation complexes
Vitamin K-dependent (VKD) zymogen
Protein cofactor
Ca2+
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
Ca2+
FXaCa2+ 30
Ca2+
FXa
FVa HC
FVa LC
Ca2+
300,000
-Thrombin
ProthrombinaseComponents
Relative Rateof Prothrombin
Activation
FXa 1Prothrombin
Ca2+
FXa
Prothrombinase
FVa HC
FVa LC
Ca2+
FXa 300FVa HC
FVa LC
Ca2+
Relevance of complex formation and its constituents
Common constituents of coagulation complexes
Vitamin K-dependent (VKD) zymogen
Protein cofactor
Ca2+
Appropriate membrane surface
- activated platelets (VIIIa/IXa complex, Va/Xa complex)
- subendothelial cells, typically fibroblasts (TF/VIIa complex)
** Express anionic phospholipids and membrane receptors for coagulation proteins.
In platelets, the expression of this membrane surface is activation-dependent.
Activate platelets
HEMOSTASIS (CONT.)
1). INITIATIONVessel wall – endothelial cells and subendothelial
components
2). LOCALIZATIONPlatelets – circulating cellular elements
3). PROPAGATION/AMPLIFICATIONPlasma coagulation proteins (factors)
4). TERMINATIONPlasma coagulation protein inhibitors
5). ELIMINATIONFibrinolytic system
INHIBITORS
INHIBITORS (cont.)
FIBRINOLYSIS
FIBRINOLYSIS (CONT.)
Platelet Plug Formation
• Measured clinically as the bleeding time
• Normal bleeding time is from 2 – 10 min
• Usually the bleeding time is sufficient to detect defects of platelet adhesion and aggregation, in which it is prolonged
Intrinsic Pathway of Blood Coagulation
• No factors extrinsic to the blood are involved
• Clinical test to assess the functionality of this pathway is the activated partial thromboplastin time (aPTT)– Kaolin and cephalin are added to the test plasma sample– The normal range is ~30 – 50 seconds (varies slightly
depending on the laboratory)– Prolongations in the aPTT are observed in deficiencies of
factors XI, IX, VIII, X, and V, prothrombin, or fibrinogen.– Used to test for common congenital hemophilias
(deficiencies in IX, VIII, or XI) and to monitor heparin treatment
Extrinsic Pathway of Blood Coagulation
• Extrinsic refers to tissue factor, which is expressed on subendothelial cells
• Clinical test to assess the functionality of this pathway is the prothrombin time (PT)– Lipidated tissue factor is added to test plasma sample– The normal range is ~10-15 seconds (varies slightly
depending on the laboratory)– Prolongations in the PT are observed in deficiencies of
factors VII, X, V, prothrombin, or fibrinogen.– Used to test for the rare congenital deficiencies in these
factors: More often it is used to diagnose acquired bleeding disorders resulting from vitamin K deficiency, oral anticoagulants (e.g. warfarin), and liver disease
Thrombin Time (TT)
In this test, thrombin is added to plasma
– The normal range is ~10-15 seconds (varies slightly depending on the laboratory)
– Prolongations in the TT are observed in congenital fibrinogen deficiency or acquired fibrinogen deficiency resulting from consumption of fibrinogen in DIC (disseminated intravascular coagulation), or may occur following treatment with fibrinolytic drugs
Hemorrhage
Bleeding disorders can span the spectrum from weeping blood vessels to full-fledged internal and external hemorrhage
Genetic defects:platelet abnormalitiesblood vessel wall abnormalitiesclotting factor deficiencies (hemophilias)excess clot breakdown (fibrinolysis)
Acquired defects:liver disease (site of clotting factor synthesis)vitamin K deficiencyautoimmune disease (platelet destruction)trauma
Disorders of Platelet Adhesion or Aggregation
• Affecting constituents of the vessel wall
• Affecting the ability of the platelet to interact with the subendothelium at sites of blood vessel injury
• Affecting the ability of the platelet to interact with other platelets
Vessel Wall Defects
• von Willebrand’s disease: a group of autosomal dominant disorders that result in reduced or abnormal synthesis of vWF
• Defects in collagen synthesisEhlers-Danlos Syndrome: congenital defect in collagen synthesis
Scurvy: results from vitamin C (ascorbic acid) deficiency, which is involved in collagen synthesis
Excess exogenous or endogenous corticosteroids: also leads to acquired deficiency in collagen synthesis
Platelet Defects
• Bernard-Soulier Syndrome: expression of low levels of or defective glycoprotein Ib-IX on the platelet surface
• Glanzmann’s thrombasthenia: expression of low levels of or defective glycoprotein IIb-IIIa on the platelet surface
Other Defects
• Fibrinogen: deficiency of or production of abnormal protein
• Acquired disorders include low platelet count (thrombocytopenia) as a result of defective formation of platelets by the bone marrow or excessive destruction of platelets
Hemorrhage
Bleeding disorders can span the spectrum from weeping blood vessels to full-fledged internal and external hemorrhage
Genetic defects:platelet abnormalitiesblood vessel wall abnormalitiesclotting factor deficiencies (hemophilias)excess clot breakdown (fibrinolysis)
Acquired defects:liver disease (site of clotting factor synthesis)vitamin K deficiencyautoimmune disease (platelet destruction)trauma
Vitamin K deficiency
• Deficiency of vitamin K is rare because of its wide distribution in nature, and its production by intestinal bacteria
• Found in individuals with liver disease and fat malabsorption - it is associated with bleeding disorders
• Newborn infants (especially preemies) are also at risk- Placenta is insufficient in the transfer of maternal vitamin K- Concentration of circulating vitamin K drops immediately after birth, and it recovers upon absorption of food- Gut of the newborn is sterile
Thus, newborns are given an injection of vitamin K following birth.
Hemophilias A and B
• Hemophilias A and B are cause by deficiencies in factors VIII or IX, respectively
• Affect ~1 in 10,000 males
• Inherited as a recessive X-linked trait (Mom would be an unaffected carrier)
• Treated by administration of factor VIII or factor IX concentrates
• Recombinant factor VIII or XI• Gene therapy trials
Hemorrhage
Bleeding disorders can span the spectrum from weeping blood vessels to full-fledged internal and external hemorrhage
Treated by factor replacement
Thrombosis
Thrombosis can be manifested as a transient, short-term or episodic event in individuals with chronic or recurring clotting. It is the major cause of both stroke and heart attacks.
Genetic defects:clotting factor INHIBITOR deficienciesdecreased fibrinolysis
Acquired defects:atherosclerosis
Pharmacologic Approaches to Prevent Thrombosis
Antiplatelet agents - block activation, aggregation or intraplatelet agonist synthesis
Effective anticoagulant therapy includes both antiplatelet and antithrombin agents
Antiplatelet Drugs
Blood “thinners” - coumadin (warfarin): inhibition of “gla” formation in the liver
Coumadin blocks reformation of reduced vitamin K, which essentially stops the post-translational modification of the glutamic acid residues at the amino-termini of the VKDP’s, since vitamin K is oxidized during the reaction.
Blood “thinners” - heparin: potent cofactor for ATIII-catalyzedinhibition of procoagulant serine proteases
Snakes
Leeches
Blood-sucking Insects