Structure, Biology, and Genetics of Factor VIII. Factor VIII Function, страница 7

Factor VIII is inactivated by proteolytic cleavage. Cleavage after residue 336 is mediated by factor Xa, 109 factor IXa, MLID92156105  74,76 activated protein C, MLID84105265  109,135 and also thrombin in the presence of phospholipid. MLID92207952  25 Additionally, factor Xa and factor IXa cleave the light chain after residue 1719, 74,109 and activated protein C cleaves the heavy chain after residue 562. MLID92041838  88 Because of the multiple cleavages that occur with these enzymes, it is difficult to attribute the significance of any single cleavage to its role in factor VIII inactivation. The activation of protein C by thrombin is subject to regulation by thrombomodulin on the endothelial cell surface and may represent a significant mechanism to prevent factor VIIIa from escaping the localized area of vessel wall damage. MLID94054288  136 Protein C deficiency is associated with severe thrombotic disease, suggesting that this feedback mechanism may be physiologically significant. MLID82053492 MLID83042075  137,138 The relative contributions of proteolytic inactivation and chain dissociation in regulating factor VIII activity in vivo are not known. The isolation of thrombin-activated factor VIIIa in a stable form should provide a means of characterizing the inactivation process directly.

Regulation of Factor VIII Activity by Biologic Membranes

Procoagulant activity is profoundly affected by the presence of cellular surfaces. On platelet activation by thrombin, the platelet surface exhibits procoagulant activity. This activity results from the exposure of negatively charged phospholipids and possibly specific receptors for the coagulation factors. Negatively charged phospholipids are usually confined to the inner leaflet of cellular membranes and are exposed on cellular lysis at sites of injury. MLID79000433 MLID82138870  139,140 Unactivated platelets exhibit binding sites for factor Va and Xa. MLID79005680 MLID80094513 MLID81094038  141–143 Platelet activation is associated with exposure of factor VIII and factor IXa binding sites. MLID89034121 MLID87206779 MLID90241883 MLID91373342  144–147 There are approximately 400 factor VIII sites per activated platelet, and factor V cannot compete with factor VIII binding. MLID89034121  144 The specificity in binding of factor VIII suggests that a specific receptor for factor VIII is involved. In addition, the kinetics of thrombin generation mediated by the prothrombinase complex on pure phospholipid surfaces compared with activated platelets suggest that a specific saturable receptor exists for prothrombinase, and probably also for the factor Xa-generating enzyme complex. MLID94054298  148 Platelet binding is mediated by the factor VIII light chain and is inhibited by the presence of vWF. A factor VIII mutant protein that cannot bind vWF with high affinity retains its ability to bind platelets. MLID92011498  57 Thus the vWF-binding site and the platelet-binding site appear to be distinct within the factor VIII molecule. After activation, factor VIIIa is released from vWF and is available to bind to activated platelets. Thus, one result of thrombin-mediated cleavage within the light chain on factor VIII activation is release of factor VIII from vWF to allow binding to platelets.

The anticoagulant properties of the endothelial cell surface is maintained by several independent mechanisms: (1) heparin sulfate on the surface of endothelial cells accelerates the inactivation of thrombin by antithrombin III; (2) thrombomodulin expressed on endothelial cells can alter the proteolytic specificity of thrombin to activate protein C; and (3) endothelial cells secrete prostacyclin, an inhibitor of platelet aggregation, and tissue plasminogen activator, an initiator of fibrinolysis. Endothelium also exhibits dramatic procoagulant activities. Activated endothelial cells induce expression of tissue factor and can mediate the activation of factor X. Endothelial cells contain high-affinity receptors for factor IX or IXa, as well as factor X. MLID84144814 MLID83247428  149,150 At present it is not known whether specific binding sites for factor VIII exist on endothelial cells. Thus, the endothelial cell surface may positively or negatively influence factor VIII activity through the generation of activated protein C or by the binding of factor IXa and X to initiate assembly of the factor X-activating complex.