Five alpha-D-galactopyranosyl-binding isolectins from Bandeiraea simplicifolia seeds

The alpha-D-galactopyranosyl-binding lectin previously purified from Bandeiraea simplicifolia seeds (Hayes, C.H., and Goldstein, I.J. (1974) J. Biol. Chem. 249, 1904) is shown to consist of five isolectins separable on polyacrylamide gel electrophoresis at pH 9.5. The isolectins are tetrameric structures composed of various combinations of two different glycoprotein subunits designated A and B. The A and B subunits appear to be immunochemically indistinguishable against rabbit antisera prepared from the isolectin mixture. The A subunit contains no methionine, whereas the B subunit contains 1 residue. The subunits migrate differently on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and, although each subunit contains 1 residue of cysteine, they react differently toward 5,5'-dithiobis(2-nitrobenzoic acid). The carbohydrate binding specificity of the two subunits differs significantly: the A subunit exhibits a primary specificity for alpha-D-GalNAcp but also reacts with alpha-D-Galp units, whereas the B subunit shows a sharp specificity toward alpha-D-Galp residues. The differences in carbohydrate binding specificity were exploited in separating the isolectins. B. simplicifolia I isolectins (A4) and (A3B) were purified on a Bio-Gel melibionate column, and (A2B2), (AB3), and (B4) were separated on a column of insolubilized blood group A substance.