Immunochemical studies of the combining sites of the two isolectins, A4 and 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 [alpha]-linked 2-acetamido-2-deoxy--galactopyranose (GalNAc) but also reacts with oligosaccharides with terminal nonreducing [alpha]-linked Gal, thus accounting for its blood group A and B specificities. Of the oligosaccharides tested, A4 reacted best with GalNAc[alpha]1 --> 3Gal and a trisaccharide GalNAc[alpha]1 --> 3Gal[beta]1 --> 3GlcNAc (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 [alpha]-linked Gal with Gal[alpha]1 --> 3Gal being most active, while Fuc[alpha]l --> 2 Gal[alpha]1 --> 3Gal[beta]1 --> 4GlcNAc[beta]1 --> 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 [alpha]1 --> 3 linked Gal. The B4 site was found to be strictly B specific.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23456/1/0000407.pd