Inhibitor-neutralisation assay and electro-immuno assay of human factor IX (Christmas factor)

Binding of human blood-coagulation Factors IXa and X to phospholipid membranes

A simple centrifugation technique has been developed to study the interaction of human coagulation Factors IXa and X with phospholipid membranes. In the presence of Ca2+, equimolar phosphatidylserine/phosphatidylcholine membranes form tight complexes with Factor X (KD = 2.8 X 10(-8) M); the KD is independent of the phospholipid concentration. Binding sites are available for about 2 mmol of Factor X/mol of phospholipid. Factor IXa has a slightly higher affinity for the phospholipid membrane (KD = 1.2 X 10(-8)M), and competes with Factor X for binding. The experimentally observed competition between Factor X and Factor IXa is in agreement with a model that describes the binding of two distinct ligands to a single class of independent binding sites.

Congenital protein C deficiency and venous thromboembolism. A study of three Dutch families

Protein C is the zymogen of a vitamin K-dependent serine protease involved in blood coagulation. In the absence of protein C the inactivation of activated factors V and VIIIC is impaired, and the fibrinolytic capacity of the circulating blood is reduced. These conditions promote excessive fibrin formation and thus constitute a risk factor for thrombosis. Using an immunologic assay for protein C, we identified 18 patients (11 male and 7 female) in three unrelated Dutch families as fulfilling the criteria for an isolated protein C deficiency. In 12 patients who were not receiving oral anticoagulant treatment the mean protein C antigen concentration was 0.48 +/- 0.09 U per milliliter (+/- S.D.), and in 6 patients who were receiving adjusted doses of oral anticoagulants and had stable anticoagulation, the mean value was 0.17 +/- 0.05 U per milliliter. (The value in healthy subjects is 0.98 +/- 0.19 U per milliliter.) Fourteen of the 18 patients had a history of venous thromboembolism, with superficial thrombophlebitis as the hallmark of this condition (in 13 patients). These data are consistent with an autosomal dominant trait with variable expressivity.

New methods in coagulation

Recent advances in the elucidation of the molecular biochemistry of the coagulation proteins have provided the foundation for the development of synthetic substrates. These substrates are oligopeptide with either a chromophore or fluorophore group attached to the C-terminal end. They may be used in the laboratory to assay for a number of the serine proteases involved in either coagulation or fibrinolysis. Also, by suitably modifying the assay system, the various inhibitors can be quantitated. These substrates promise to revolutionize the coagulation laboratory allowing for more precise quantitation of trace enzymes and also improved standardization and precision of coagulation assays. In addition to these substrates, the introduction of a number of immunologic procedures into the diagnostic laboratory have been of major importance in elucidating the heterogeneity of hereditary coagulation defects. By correlating the immunologic assays, coagulation assays and clinical picture, a number of subgroups of hereditary deficiencies have been identified. Also, the immunologic assays have provided a means for identifying the carrier state of hemophilia A and have significantly contributed to the improved diagnosis of von Willebrand's disease. The use of ristocetin cofactor assays, when used in conjunction with the Factor VIII antigens, have enable the laboratory to more accurately diagnose the majority of patients with von Willebrand's disease. Ristocetin cofactor may be assayed utilizing either formalin fixed or washed platelets and recently a snake venom has been introduced to assay for this particular aspect of the Factor VIII complex. Platelet specific proteins (i.e., platelet factor 4 and beta-thromboglobulin) may be assayed utilizing either radioimmunoassays or in the case of platelet factor 4 modified coagulation assays. These proteins provide evidence of in vivo platelet activation and hopefully may, in the future, be correlated with platelet kinetics.

Coagulation factors in the human fetus of about 20 weeks of gestational age

In plasma samples of 11 fetuses of about 20 weeks of gestational age the following coagulation factors have been determined (mean values found are given in parentheses): fibrinogen (0.30 U/ml), prothrombin (+/-0.17 U/ml), factor V (0.28 U/ml), factor VII (0.21 U/ml), factor VIII coagulant activity (factor VIII:C) (0.12 U/ml), factor VIII-related antigen (factor VIIIR:Ag) (1.04 U/ml), coagulant factor VIII-related antigen (factor VII:CAg) (0.19 U/ml), factor IX coagulant activity (0.05 U/ml), factor IX antigen (less than or equal to 0.03 U/ml), factor X (0,19 U/ml) and antithrombin III (AT-III) (0.23 U/ml). Our data support the evidence that prenatal diagnosis of haemophilia A is at present possible; less optimism is warranted where haemophilia B is concerned. the number of samples is sufficient to establish normal values for the age group. Means of quality control of the sample-which is often difficult to obtain-are disscussed.

Pathways in the activation of human coagulation factor X

Purified human Factor X (apparent mol.wt. 72000), which consists of two polypeptide chains (mol.wt. 55000 and 19000), was activated by both Russell's-viper venom and the purified physiological activators (Factor VII/tissue factor and Factor IXa/Factor VIII). They all convert Factor X to catalytically active Factor Xa (mol.wt. 54000) by cleaving the heavy chain at a site on the N-terminal region. In the presence of Ca2+ and phospholipid, the Factor Xa formed catalyses (a) the cleavage of a small peptide (mol.wt. 4000) from the C-terminal region of the heavy chain of Factor Xa, resulting in a second active form (mol.wt. 50000), and (b) the cleavage of a peptide containing the active-site serine residue (mol.wt. 13000) from the C-terminal region of the heavy chain of Factor X, resulting in an inactivatable component (mol.wt. 59000). A nomenclature for the various products is proposed.

A genetic variant of factor IX with decreased capacity for Ca2+ binding

A genetic variant of factor IX is described that behaves identically to PIVKA/IX (the precursor factor IX molecule induced by the absence of vitamin K or presence of vitamin K antagonists, acarboxy factor IX). It shows an increased electrophoretic mobility in the presence of Ca2+, a low affinity for adsorption to A1(OH)3 and a very low specific coagulant activity. This variant of factor IX has been demonstrated in the plasma of a patient with severe haemophilia B and in the plasmas of a number of possible carriers from the probands' pedigree.

Detection of carriers of haemophilia B

Plasma levels of factor IX activity and factor IX antigen were determined in 18 definite carriers of haemophilia B-, 10 definite carriers of haemophilia B+ and 40 control subjects. Factor IX antigen was determined by the electroimmunoassay technique of Laurell using a rabbit antiserum against factor IX. In haemophilia B-, statistical tolerance regions were constructed containing 90% of the factor IX activity and factor IX antigen values for the carriers and the control subjects. Of the 18 carriers, 10 were outside the tolerance region for the control subjects, and of the 40 control subjects, 17 were outside the tolerance region for the carriers. A better separation of the probabilities of being a carrier was obtained for the B- carriers and the control subjects when factor IX antigen was determined in addition to factor IX activity. In haemophilia B+, factor IX antigen was present in excess of factor IX activity in nine of the 10 carriers. The values for factor IX activity and factor IX antigen for these nine carriers fell outside the 90% tolerance region for the control subjects. It is concluded that the quantitative determination of factor IX antigen may be of value in the detection of carriers of both haemophilia B+ and haemophilia B-.

Immunological heterogeneity of haemophilia B: a multicentre study of 98 kindreds

An electroimmunoassay with a precipitating rabbit anti-human factor IX antiserum and an inhibitor neutralization assay with a non-precipitating homologous antibody were used to measure factor IX antigen (IX:Ag) in 117 patients from 98 kindreds with haemophilia B; and to investigate in a mixed population the incidence of different immunological types of the disease. Although the two assays showed an excellent correlation, the electroimmunoassay was selected for its simplicity as a criterion for classification. 52 kindreds, referred to as haemophilia B-, were characterized by severe deficiency of factor IX coagulant activity (less than 0.01--0.03 u/ml) and unmeasurable IX:Ag (less than 0.12 u/ml): this genetic variant of the disease appears to be related to a complete or marked suppression of factor IX synthesis. In 16 kindreds, a severe or moderately severe IX:C deficiency was associated with normal or increased levels of IX:Ag (haemophilia B+): among them, a subgroup of five kindreds could be identified by the additional abnormality of a prolonged Thrombotest clotting time (haemophilia BM). These patients are likely to be the expression of normal or increased synthesis of a factor IX molecule markedly defective in the site(s) responsible for coagulant activity. Reduced levels of IX:Ag (0.12--0.65 u/ml, characterized the remaining 30 kindreds, presenting with IX:C levels ranging from less than 0.01 to 0.21 mu/ml. In 28 there was a significant excess of IX:Ag over IX:C, suggesting a reduced capacity to synthesize the factor IX molecule accompanied by a variable defect in the coagulant site; the remaining two kindreds, which showed a concomitant reduction of IX:C and IX:Ag, are likely to be examples of a true reduction of factor IX synthesis.