IMMUNOCHEMICAL STUDIES OF THE COMBINING SITES OF THE 2 ISOLECTIN-A4 AND ISOLECTIN-B4, ISOLATED FROM BANDEIRAEA-SIMPLICIFOLIA

The specificity of two isolectins, A4 and B4, of Bandeiraea simplicifolia lectin I (BS-I) was studied by quantitative precipitin, precipitin inhibition, as well as by competitive binding assays using various blood group substances and tritium-labeled human B substance. A4 precipitated well with A1, A2, B, and precursor substances, with A2 precipitating less strongly than did A1 substance; H, Lea and Leb substances did not react. Precipitin inhibition and competitive binding assays confirmed the precipitin data that A4 is most specific for terminal nonreducing α-linked 2-acetamido-2-deoxy-d-galactopyranose (dGalNAc) but also reacts with oligosaccharides with terminal nonreducing α-linked dGal, thus accounting for its blood group A and B specificities. Of the oligosaccharides tested, A4 reacted best with dGalNAcα1 → 3dGal and a trisaccharide dGalNAcα1 → 3dGalβ1 → 3dGlcNAc (A5II) was equally active, suggesting that the A4 site is no larger than a disaccharide. B4 precipitated well with B substances and with a precursor substance to a lesser extent, while A1 A2, H, Lea, and Leb substances were inactive. Precipitin and competitive binding assays showed that it reacted well with oligosaccharides with terminal α-linked dGal with dGalα1 → 3dGal being most active, while lFucαl → 2 dGalα1 → 3dGalβ1 → 4dGlcNAcβ1 → 6-R (BRL 0.44) was much less active, indicating a substitution at the subterminal residue affects the binding substantially and indicating that the B4 site involves at least the subterminal α1 → 3 linked dGal. The B4 site was found to be strictly B specific. © 1979.