Structure, Biology, and Genetics of Factor VIII. Factor VIII Function, страница 8

Regulation of Factor VIII Activation

The present understanding of the mechanism by which factor VIII functions is presented in Figure 105-3. The two chains of factor VIII are associated by a divalent metal ion bridge. vWF interacts with the NH2 terminus and the COOH terminus of the light chain. The mechanism by which initial activation of factor VIII occurs is unknown, but recent evidence has accumulated showing that the extrinsic pathway may be the most significant physiologic initiator of factor VIIIa generation. MLID92351144  151 Patients deficient in factor VII have low levels of factor Xa compared with patients who have deficiencies in factor VIII. MLID90028725  152 In addition, infusion of VIIa into a chimpanzee increases the level of circulating factor Xa. MLID87101524  155 These results indicate that the primary mechanism of factor Xa generation in vivo is via the extrinsic pathway. Since patients deficient in factor VIII or factor IX do have a bleeding diathesis, there is a need for an intrinsic pathway for hemostasis in vivo. Two possible mechanisms could explain the need for factors VIII and IX in vivo. First, since factor Xa generation via the tissue factor pathway can bind tissue factor pathway inhibitor (TFPI) and subsequently inhibit the factor VIIa/tissue factor complex, only small amounts of factor Xa can be generated before further extrinsic factor X activation is inhibited. MLID88108141 MLID87101524  154,155 Since TFPI does not inhibit the intrinsic formation of factor IXa or its activity, the intrinsic route is required to amplify the response. Alternatively, since the extrinsic pathway can activate factor IX in vitro, this pathway may primarily activate factor IX as opposed to factor X in vivo. MLID78094386  156 Either mechanism could explain the need for factors VIII and IX for effective hemostasis.

Factor VIII activation by thrombin requires cleavage at both residues 372 and 1,689. Cleavage at 1,689 releases activated factor VIII from vWF. Activated factor VIII is a heterotrimer composed of the 50,000 MW, 43,000 MW, and 73,000 MW polypeptides. Activated factor VIII is transferred from vWF to a factor VIII receptor present on activated platelets to participate in assembly of the active factor IXa complex. MLID92011498  57 Activated factor VIII is stabilized by assembly into the factor Xa complex. MLID84204051  134 After activation of factor VIII, there is a first-order decay of activity that, in vitro, likely results from dissociation of the A2 domain. In vivo, inactivation may also occur through proteolytic inactivation by activated protein C, factor Xa, or thrombin at residue 336 or 562, or both.

Activated factor VIII enhances the catalytic efficiency of factor IXa by 10,000-fold. Enhanced efficiency occurs through several mechanisms. Binding of factor VIII induces a conformational change in the factor IXa active site and greatly increases the kcat. MLID92381007  99 The phospholipid membrane also binds factor IXa and factor X so that the enzyme complex and substrate are concentrated within a two-dimensional surface to reduce the Km. It is also likely that factor VIIIa interacts with factor X to enhance the extended substrate recognition site of factor IXa, analogous to the interaction of factor Va with meizothrombin. MLID90202893  157 Finally, membrane fluidity may also influence the kinetics of factor X activation, similar to thrombin activation, possibly by influencing the mechanism of catalysis. MLID93003146  158

Hemophilia A

The gene for factor VIII is on the human X chromosome, and therefore hemophilia A is a classic example of X-linked recessive inheritance. It occurs almost exclusively in males; females with one abnormal copy of the factor VIII gene are carriers because the other X chromosome contains a normal copy of the gene. The frequency of the disorder is 1 in 5,000–10,000 male births, and no particular ethnic group has an unusually lower or higher incidence of the disease. The severity and frequency of bleeding in patients correlate with the factor VIII activity in plasma. MLID87107570  159,160 Of particular interest for the understanding of factor VIII function is a category of patients who have a considerable amount of factor VIII protein in their plasma (E30% of normal) but whose protein is nonfunctional (i.e., the factor VIII activity is much less than the factor VIII plasma level [usually <2% of normal]). Approximately 5% of patients belong to this category, termed cross-reacting material (CRM) positive. MLID79048618  161 Another category is called CRM reduced: the factor VIII antigen and activity are reduced to approximately the same level.