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

Abstract

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-Ga1NAcα(1 → 3)D-Gal-(β1 → ¾) to a D-Glc, were the best inhibitors and about 8 and 4 times more active than D-Ga1NAc and D-Ga1NAcα(1 → 3)D-Ga1, respectively. A difucosyl A-specific oligosaccharide (A-penta), a monofucosyl (A-tetra) and a non-fucosyl containing (A₅ II) oligosaccharide, D-Ga1NAcα(1 → 3)D-Ga1β(1 → 3)D-G1cNAc, had almost the same reactivity, suggesting that the fucose linked to the sub-terminal D-Ga1 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-Ga1NAc or D-Ga1 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-Ga1 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 β linkage to the third sugar (D-Glc or D-GlcNAc) from the non-reducing end. The role of the β(1 → 3) or β(1 → 4) linkage of the sub-terminal non-reducing D-Gal to the D-GlcNAc requires further study.