Studies on factor VIII-related protein. IV. Interaction of galactose-specific lectins with human factor VIII/Von Willebrand factor.

Factor VIII of human cryoprecipitate was purified and fractionated on Sepharose CL-2B into three fractions of progressively decreasing multimer size and ristocetin cofactor activity. Following complete disulfide reduction, the resulting subunits were electrophoresed on 3% polyacrylamide gels and subsequently stained with two galactose-specific, fluorescein-labelled lectins from Ricinus communis (RCAI and RCAII). Measurements of fluorescence indicated that the reduced chains, derived from the largest factor VIII multimers, have stronger binding affinity for both lectins than those obtained after reduction of smaller factor VIII species. Ristocetin cofactor activity of purified factor VIII was competitively inhibited by both Ricinus lectins and by concanavalin A. RCAI-lectin was found to be a considerably more efficient inhibitor than RCAII or concanavalin A. Following removal of sialic acid from factor VIII, the inhibiting effect of RCAII-lectin was markedly potentiated, probably by exposing additional galactose residues, some of which must be located close to the ristocetin cofactor 'active site' of factor VIII. Ristocetin cofactor activity was also strongly inhibited with RCAII-lectin for binding sites which are located on the surface factor VIII multimers. Our results suggest that RCAI-lectin, which contains galactose-specific binding sites per molecule, and anti-factor VIII antibodies inhibit ristocetin cofactor activity by crosslinking and aggregation of factor VIII multimers.