Localization of binding sites within human von Willebrand factor for monomeric type III collagen

Purified human plasma von Willebrand factor (vWf) binds to pepsin-digested monomeric type III collagen in a saturable (KD = 1 X 10(-8) M), specific, and rapid manner with a stoichiometry of approximately 1:15 [vWf subunit (Mr 270,000):collagen trimer (Mr 300,000)]. Two reduced and alkylated CNBr peptides of vWf, termed M11 residues 542-622 and M20 residues 948-998 [Titani, K., Kumar, S., Takio, K., Ericsson, L. H., Wade, R. D., Ashida, K., Walsh, K. A., Chopek, M. W., Sadler, J. E., & Fujikawa, K. (1986) Biochemistry 25, 3171-3184], inhibited vWf binding to collagen. With 125I-vWf (2 X 10(-9) M) as ligand, M11, M20, fragment III (a dimeric, V8 protease, NH2-terminal fragment, Mr 320,000 referenced above), and unlabeled vWf inhibited binding to collagen with EC50 values of 4.8 X 10(-7), 9.4 X 10(-7), 1.1 X 10(-7), and 0.2 X 10(-7) M, respectively. M11 and M20 bind to collagen directly when 125I-labeled peptides are used as ligands. Other CNBr fragments of vWf were less effective as inhibitors (5-fold or less) and bound less avidly to collagen (5-fold or less) compared to M11 and M20. A murine anti-human vWf monoclonal antibody (MR5), which blocks the binding of vWf to collagen, bound selectively to both M11 and M20 when tested in an enzyme-linked immunoadsorbent assay.(ABSTRACT TRUNCATED AT 250 WORDS)