STUDIES ON THE INTERACTION BETWEEN PURIFIED BOTROCETIN AND VARIANT FORMS OF VON WILLEBRAND FACTOR (TYPE ⅡA AND TYPE ⅡB)

Recent studies demonstrate that purified botrocetin, a protein isolated from the venom of the snake Bothrops jararaca, initiates binding of von Willebrand factor (vWF) to platelet glycoprotein (GP) Ⅰb similarly to the antibiotic restocetin. More precisely, botrocetin binds to the GPⅠb binding domain of constituent vWF subunit (amino acid residue 449-728), and forms an "activated complex" with vWF, which then binds to platelet GPⅠb. The present report indicates that purified botrocetin binds to vWF from plasmas of patients with two variant forms of von Willebrand disease, ⅡA and ⅡB, and that its interaction is indistinguishable from that with vWF from normal individuals. However, an "activated complex" formed between botrocetin and ⅡB vWF expresses an enhanced biological activity for binding to GPⅠb, whereas the complex with ⅡA vWF has decreased binding activity. Among several anti-vWF monoclonal antibodies (MoAb) which inhibit ristocetin-induced platelet aggregation and/or vWF binding to GPⅠb, only two MoAbs (NMC-4 and PFF Ⅷ RAG : 1) abolished direct binding between botrocetin and vWF. This suggests that they recognize an epitope on the vWF molecule in close proximity to the botrocetin binding site (s)