Comparative cross-linking activities of lactose-specific plant and animal lectins and a natural lactose-binding immunoglobulin G fraction from human serum with asialofetuin

Plant and animal lectins bind and cross-link certain multiantennary oligosaccharides, glycopeptides, and glycoproteins, This can lead to the formation of homogeneous crosslinked complexes, which may differ in their stoichiometry depending on the nature of the sugar receptor involved, As a precisely defined ligand, we have employed bovine asialofetuin (ASF), a glycoprotein that possesses three asparagine-linked triantennary complex carbohydrate chains with terminal LacNAc residues, In the present study, we have compared the carbohydrate cross-linking properties of two Lac-specific plant lectins, an animal lectin and a naturally occurring Lac-binding polyclonal immunoglobulin G subfraction from human serum with the ligand, Quantitative precipitation studies of the Lac-specific plant lectins, Viscum album agglutinin and Ricinus communis agglutinin, and the Lac-specific 16 kDa dimeric galectin from chicken liver demonstrate that these lectins form specific, stoichiometric cross-linked complexes with ASF, At low concentrations of ASF, 1:9 ASF/lectin (monomer) complexes formed with both plant lectins and the chicken lectin, With increasing concentrations of ASF, 1:3 ASF/lectin (monomer) complexes formed with the lectins irrespective of their source or size, The naturally occurring polyclonal antibodies, however, revealed a different cross-linking behavior. They show the formation of 1:3 ASF/antibody (per Fab moiety) cross-linked complexes at all concentrations of ASF, These studies demonstrate that Lac-specific plant and animal lectins as well as the Lac-binding immunoglobulin subfraction form specific stoichiometric cross-linked complexes with ASF, These results are discussed in terms of the structure-function properties of multivalent lectins and antibodies.