Dimeric ristocetin flocculates proteins binds to platelets, and mediates von Willebrand factor-dependent agglutination of platelets

Ristocetin in aqueous solution dimerizes with an equilibrium dissociation constant of 5.0 × 10^-4 M, i.e. ∼1.1 mg ml^-1 (Waltho, J. P., and Williams, D. H. (1989) J. Am. Chem. Soc. 111, 2475-2480). At concentrations of about 1.0 mg ml^-1 ristocetin flocculates many proteins, lyses platelets and, in the presence of von Willebrand factor, agglutinates both fresh and formalin-fixed platelets. Because ristocetin exists as both monomeric and dimeric species, we sought to determine which of these forms flocculates proteins and agglutinates platelets. We found that: 1) the initial rate of flocculation of certain proteins, 2) the initial rate of agglutination of formalin-fixed platelets, and 3) the binding of ristocetin to formalin-fixed platelets are higher order solely with respect to the concentration of ristocetin dimers. As to the operative mechanism, it appears that bifunctional dimers cross-link proteins that possess multiple copies of a common recognition site. Preliminary evidence indicates that a recognition site is a β-turn of the form X-P-G-X′.