Interaction of linear manno-oligosaccharides with three mannose-specific bulb lectins. Comparison with mannose/glucose-binding lectins

Abstract Three new mannose-binding lectins, isolated from daffodil (NPA), amaryllis (HHA), and snowdrop (GNA) bulbs, are capable of precipitating with a linear mannopentaose (Manα1-3Manα1-3Manα1-3Manα1-2Man). NPA and HHA reacted strongly with the mannopentaose whereas GNA gave a precipitate only at concentrations > 500 μM. A phosphate group at C-6 of the nonreducing terminal mannosyl group prevented precipitation in all three cases. The reduced (NaBH 4 ) mannopentaose, Man 4 Man-ol, did not precipitate with GNA or NPA, but was active with HHA. This activity was lost when Man 4 Man-ol was converted (NaIO 4 then NaBH 4 ; mild acid hydrolysis of the reduced product) into trisaccharide derivatives. With α- d -Man p -OMe the three lectins gave UV difference spectra having large positive peaks at 292–293 and 283–284 nm, and a small positive peak at 275 nm, characteristic of tryptophanyl and tyrosyl residues. The association constants for the interaction with α- d -Man p -OMe were very low (NPA, 86; HHA, 66; and GNA, 41 M −1 ), but the lectins bound methyl (1 → 3)-α-mannobioside with increased affinity ( K for NPA 540, for HHA 2400, and for GNA 200 M −1 ). The bulb lectins lack binding sites for hydrophobic ligands, as judged by their failure to interact with the fluorescent probes 8-anilino-1-naphthalensulfonic acid (ANS) and 6- p -toluidino-2-naphthalenesulfonic acid (TNS).