Carbohydrate binding specificity of the basic lectin from winged bean (Psophocarpus tetragonolobus)

The carbohydrate binding specificity of the basic lectin from winged bean (Psophocarpus tetragonolobus) was investigated by quantitative precipitin analysis using blood group A, B, H, Le and I substances and by precipitation inhibition with various mono- and oligosaccharides. The lectin precipitated best with A1 substances and moderately with B and A2 substances, but not with H or Le substances. Inhibition assays of lectin-blood group A1 precipitation demonstration that A substance-derived oligosaccharides having the common structure: D-GalNAc alpha(1----3)D-Gal-(beta 1----3/4) to a D-Glc, were the best inhibitors and about 8 and 4 times more active than D-GalNAc and D-GalNAc alpha(1----3)D-Gal, respectively. A difucosyl A-specific oligosaccharide (A-penta), a monofucosyl (A-tetra) and a non-fucosyl containing (A5II) oligosaccharide, D-GalNAc alpha(1----3)D-Gal beta(1----3)D-GlcNAc, had almost the same reactivity, suggesting that the fucose linked to the sub-terminal D-Gal or to the third sugar. D-GlcNAc, from the non-reducing end made no contribution to the carbohydrate binding. Although a terminal non-reducing D-GalNAc or D-Gal residue was indispensible for binding, the lectin bound not only to these terminal non-reducing galactopyranosyl residues, but also showed increased binding to oligosaccharides in which it was bonded to a sub-terminal D-Gal joined to a D-GlcNAc residue, as in blood group A or B substances. This defines the site, thus far, as complementary to a disaccharide plus the beta linkage to the third sugar (D-Glc or D-GlcNAc) from the non-reducing end. The role of the beta(1----3) or beta(1----4) linkage of the sub-terminal non-reducing D-Gal to the D-GlcNAc requires further study.