A study of the specificity of Bandeiraea simplicifolia lectin I by competitive-binding assay with blood-group substances and with blood-group A and B active and other oligosaccharides.

The specificity of Bandeiraea simplicifolia lectin I (BS I) has been studied by competitive-binding assays (CBA) using tritium-labeled human B and hog A substances. Blood-group B substances isolated from horse gastric mucosae and from human ovarian-cyst fluids were much better inhibitors of binding of tritiated blood-group B substance to insoluble BS I-Sepharose 2B than were human blood-group A substances from saliva and ovarian-cyst fluid. A and B active blood-group substances showed the same range of potency in inhibiting binding of tritium-labeled hog A substance to BS I-Sepharose 2B. CBA with BS I-Sepharose 2B, labeled human blood-group B substance, and human blood-group A and B active aligosaccharides separated the haptens into two groups differing in slope. Group 1, containing methyl alpha-D-GalNAcp, D-GalNAcp, and an A active pentasaccharide ARL 0.52, with 3, 19, and 25 nmol respectively needed for 50% inhibition of binding, has a lower slope than group 2, which contains alpha-D-GalNAcp-(1 leads to 3)-2-acetamido-2-deoxy-D-galactitol and p-nitrophenyl alpha-D-GalNAcp, with 3 nmol of each required for 50% inhibition of binding, as well as ten glycosides with terminal, nonreducing, alpha-linked D-Galp. The most potent inhibitors of this group were p-nitrophenyl alpha-D-Galp, alpha-D-Galp-(1 leads to 3)-D-Galp, alpha-D-Galp-(1 leads to 6)-D-Glcp, and methyl alpha-D-Galp, with 5, 7.4, 9.6, and 11 nmol respectively needed to inhibit binding by 50%. The difference in slopes was explainable in terms of a recent finding that BS I exists as a mixture of five isolectins composed of two subunits having different specificities; subunit A is most specific for alpha-linked, terminal, nonreducing D-GalNAcp, but it also reacts with alpha-linked, terminal, nonreducing D-Galp, whereas subunit B tends to be more specific for terminal, nonreducing, alpha-linked D-Galp.