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

Abstract

The lectin of D. 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 .fwdarw. 3).beta.DGalNAc] of the Forssman antigenic determinant. In contrast, the relative reactions of the lectins of H. pomatia (which also agglutinates A erythrocytes) and W. floribunda (which agglutinates A, B and O erythrocytes) with Forssman pentasaccharide were substantially weaker than that of D. biflorus. The combining site of the lectin of H. pomatia has a broader affinity for terminal 2-acetamido-2-deoxy-.alpha.-D-galactopyranose (.alpha.DGalNAc) residues than does that of D. 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 W. floribunda which reacts both with a range of blood group specific glycoproteins (A, B, H, Lea and Leb) and with non-blood group glycoproteins 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.