[Domain structure of human von Willebrand factor, and its modulators involved in the platelet adhesion process in vitro]

Abstract

von Willebrand factor (vWF), a macromolecular glycoprotein synthesized in endothelial cells and megakaryocytes circulates in blood as heterogenous multimers and plays a critical role in the formation of platelet plugs. vWF is composed of an identical subunit with a molecular weight (MW) of 270 kDa, which is held together by disulfide bonds. Functional domains of vWF for factor VIII, heparin, platelet glycoprotein (GP) Ib, collagen, and GP IIb/IIIa locate in this order from the N-to C-terminus of subunit. The GP Ib binding domain is cryptic in normal circulation and only becomes expressed when vWF is associated with the subendothelial matrix or with fibrin. In vitro, an antibiotic ristocetin or the snake venom botrocetin isolated from Bothrops jararaca mimics the active component of subendothelial matrix and causes the binding of vWF to GP Ib. To elucidate the mechanism of in vitro process of vWF-GPIb binding, we describe here the isolation and characterization of two distinct forms of botrocetin. Since ristocetin is a heavily glycosylated proteoid, it is hard to characterize the structure on the basis of amino acid analysis. The apparent MW of the one-chain botrocetin was 28 kDa before and 32 kDa after reduction of disulfide bonds, while that of the two-chain botrocetin was 27 kDa before and 15/14.5 kDa after reduction. Amino acid composition of the two species revealed a similar high content of potentially acidic residues (greater that 60 Asx and Glx residues/molecules) but significant differences in the content of Cys and Phe residues. The N-terminal sequence of the one-chain was Ile-Ile/Val-Ser-Pro-Pro-Val-Cys-Gly-Asn-Glu-.(ABSTRACT TRUNCATED AT 250 WORDS)