Immunochemical studies on the combining sites of Forssman hapten reactive hemagglutinins from Dolichos biflorus, Helix pomatia, and Wistaria floribunda.

The lectin of Dolichos biflorus, a hemagglutinin previously considered to be blood group A specific, is now found to react much more strongly with the terminal disaccharide unit [alpha DGalNAc(1 leads to 3) beta DGalNAc] of the Forssman antigenic determinant. In contrast, the relative reactions of the lectins of Helix pomatia (which also agglutinates A erythrocytes) and Wistaria floribunda (which agglutinates A, B, and O erythrocytes) with the Forssman pentasaccharide were substantially weaker than that of Dolichos biflorus. The combining site of the lectin of Helix pomatia has a broader affinity for terminal 2-acetamido-2-deoxy-alpha-D-galactopyranose (alpha DGalNAc) residues than does that of Dolichos biflorus. The reactions of the lectin with terminal alpha DGalNAc units are strongly dependent on the nature of the aglycon and remain ill defined. The lectin may also react with appropriately presented terminal 2-acetamido-2-deoxy-beta-D-glucopyranose units. The broad affinity of the lectin of Wistaria floribunda which reacts both with a range of blood group specific glycoproteins (A, B, H, Lea, and Leb) and with non blood group glycoproteins [Sugii, S., & Kabat, E.A. (1980) Biochemistry 19, 1192-1199] appears best assigned to a combining site that favors pauci- or multivalent cooperative effects of clustered terminal beta-D-galactopyranose units. An attempt is made to rationalize certain of the inhibition data in terms of topographical features at the surfaces of the carbohydrate structures which are considered compatible for binding within essentially hydrophobic combining sites.