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

Chapter 105

Structure, Biology, and Genetics of Factor VIII

Randall J. Kaufman and Stylianos E. Antonarakis

Introduction

Hemophilia A, a bleeding disorder resulting from a deficiency in factor VIII, was documented >1,700 years ago in the Talmud. MLID69190620  1 The genetics of hemophilia A were described during the early 1800s, 2 and transfusion of whole blood was shown to treat a hemophilia-associated bleeding episode successfully by 1840. 3 The presence of factor VIII in plasma was demonstrated in 1911, 4 and in 1937 Patek and Taylor 5 described its role in hemostasis. However, a detailed structural characterization of the factor VIII gene and protein product was only recently achieved.

Since Hemophilia A and von Willebrand disease are both associated with factor VIII deficiency, their relationship was confused for many years. Early preparations of antihemophilic factor were demonstrated not only to correct the clotting time of hemophilic plasma, but also to restore platelet adhesion and agglutination defects in the plasma from patients with von Willebrand disease. Over the last decade it has been demonstrated that factor VIII and von Willebrand factor (vWF) are two separate proteins that exist as a complex in plasma and that are under separate genetic control. These proteins have distinct biochemical and immunologic properties, as well as unique and essential physiologic functions. Factor VIII is an X-linked gene product that accelerates the activation of factor X by factor IXa in the presence of calcium and phospholipid. vWF is an autosomal gene product that is essential for platelet adhesion to subendothelium and for ristocetin-induced platelet agglutination. Since vWF and factor VIII are found in the plasma as a complex and vWF serves to stabilize factor VIII and regulate its activity, the activities of these two proteins are intimately intertwined. Based on a greater understanding of factor VIII and vWF, in 1985 the Subcommittee on factor VIII and vWF of the International Committee on Thrombosis and Haemostasis formulated nomenclature guidelines MLID86123641  6: factor VIII protein is designated VIII; factor VIII antigen is designated VIII:Ag; factor VIII procoagulant activity is designated VIII:C; von Willebrand factor is designated vWF; and von Willebrand factor antigen is designated vWF:Ag.

Factor VIII Function

The physiologic response to blood vessel injury is the sequential activation of plasma proteases of the blood coagulation cascade, leading to the localized generation of thrombin and the conversion of fibrinogen to fibrin (see Ch. 100). Thrombin generation requires the interaction of proteases, protein cofactors, and substrate zymogens that assemble on a phospholipid surface or the cell surface. Factor VIII is proteolytically activated by factor Xa or thrombin, or both, to yield factor VIIIa, which serves as a cofactor for factor IXa-mediated activation of factor X.

The mechanism by which factor VIIIa functions in the factor Xa-generating complex is poorly understood. Factor VIII has no enzymatic activity of its own but acts as a cofactor to increase the Vmax of factor X activation by factor IXa by 10,000-fold in the presence of negatively charged phospholipids and calcium. MLID81142359  7 The mechanism by which factor VIIIa accelerates the proteolysis of factor X by factor IXa likely involves the interaction of factor VIIIa and factor IXa on a phospholipid surface to facilitate a conformational change in the enzyme that favors catalysis. MLID92381008  8

The reported specific activity of pure factor VIII ranges from 2,300 U/mg MLID82174657  9 to 8,000 U/mg. MLID87000647  10 The definition of factor VIII activity is complicated, since thrombin converts the cofactor into a much more active form. However, for standardization, 1 U of factor VIII is defined as that amount of activity in 1 ml of normal pooled human plasma measured in a factor VIII assay using factor VIII-deficient plasma. 11 For greater convenience and precision, factor VIII activity can be measured by its ability to promote activation of factor X in the presence of factor IXa, phospholipid, and Ca2+. Factor Xa is measured directly by monitoring cleavage of a synthetic chromogenic substrate. MLID77151595  12 Factor VIII antigen can be measured using factor VIII-specific antibodies in specific immunoassays. MLID88183964  13

Characterization of factor VIII has been hampered by its low concentration in plasma, its heterogeneity in size, and its exquisite sensitivity to degradation. Recent advances in our understanding of factor VIII resulted from the use of immunoaffinity chromatography for the successful purification of factor VIII from porcine MLID82135730  14 and human plasma, MLID82174657  9,15 and from the cloning of the human factor VIII gene and elucidation of the primary structure of factor VIII. MLID85061549 MLID85061548 MLID85061550  16–18

Factor VIII Structure