Further studies on the factor VIII of a patient with a variant form of von Willebrand's disease

The agglutination of human platelets by botrocetin: evidence that botrocetin and ristocetin act at different sites on the factor VIII molecule and platelet membrane

Botrocetin caused a factor VIII (FVIII) dependent platelet agglutination which was associated with a reduction in the plasma levels of all FVIII parameters as a result of specific binding of FVIII to the platelets. The site of binding of FVIII to the platelet in response to ristocetin or botrocetin involves the glycoprotein I complex. This is suggested by the inability of chymotrypsin treated platelets or platelets from patients with the Bernard-Soulier syndrome to agglutinate in response to ristocetin. These platelets responded to botrocetin , but this was greatly reduced compared to normal. Crossed immunoelectrophoretic analysis indicated that in the presence of botrocetin most multimetric forms of FVIII bound to the platelet, whereas ristocetin caused binding of high and intermediate molecular weight forms. The antibiotic vancomycin inhibited platelet agglutination by ristocetin but had no effect on that caused by botrocetin . Assays of FVIII von Willebrand factor (VIII:vWf) using botrocetin compared well with those obtained using ristocetin in plasmas from normal individuals and from patients with classical von Willebrand disease (vWd). However, a patient with variant vWd demonstrated 100% botrocetin cofactor activity and 0% ristocetin cofactor activity. This suggested that the site of interaction on the FVIII molecule for botrocetin and ristocetin are different. Therefore the diagnosis of some von Willebrand variants cannot be excluded on the basis of a normal botrocetin cofactor assay.

Properties of human asialo-factor VIII. A ristocetin-independent platelet-aggregating agent

Human Factor VIII desialylated by treatment with Vibrio cholerae neuraminidase (ASVIII) aggregated human platelets in the absence of ristocetin in platelet-rich plasma and, to a lesser extent, in washed platelet suspensions. Aggregation is accompanied by thromboxane formation and is completely inhibited by EDTA. Aspirin blocks the second phase of aggregation and abolishes thromboxane production. Subaggregating doses of ASVIII and of either ADP, epinephrine, or collagen produce prompt and complete platelet aggregation. Bernard-Soulier syndrome platelets either did not aggregate with ASVIII (Two cases) or showed markedly decreased aggregation (one cases). Factor VIII complex was prepared from the plasma of two patients with variant von Willebrand's disease (sialic acid content 142 and 75 nmol/mg, respectively); neither protein generated platelet-aggregating activity upon desialylation. [3H]ASVIII binds rapidly to platelets and 37 degrees C, while tritiated, fully sialylated factor VIII binds to a negligible extent. As little as 1--2 micrograms ASVIII bound/10(9) platelets is capable of inducing platelet aggregation. ASVIII may be a useful tool for investigating platelet-Factor VIII interactions in the absence of ristocetin. Furthermore, desialylated Factor VIII might play a physiologic role in Factor VIII-mediated platelet reactions in vivo.

Quantitation of binding of factor VIII antigen to concanavalin A

This study establishes a convenient method for screening plasma samples for abnormalities of the carbohydrate content of the factor VIII (FVIII) molecule. A radioimmuno-electrophoretic technique has been developed to quantitate the percentage binding of FVIII-related antigen (VIII-Ag) to the lectin concanavalin A (Con A). Plasma samples were electrophoresed through a strip of agarose containing Con A into agarose containing a mixture of unlabelled anti-FVIII and 125I-anti-FVIII where precipitant lines formed, the height of which was dependent upon the degree of VIII-Ag binding to Con A in the first gel. Using this system reduced binding of VIII-Ag to Con A was found in the plasma of 12 patients with moderate classical von Willebrand's disease (vWd), while the Con A binding of six haemophilia A patients fell within the normal range. The VIII-Ag in normal cryoprecipitate showed increased % binding to Con A while the VIII-Ag remaining in the cryo-supernate demonstrated reduced Con A % binding.

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.