An immunochemical study of the combining sites of the second lectin isolated from Bandeiraea simplicifolia (BS II)

The binding specificity of a second lectin purified from seeds of Bandeiraea simplicifolia (BS II) was studied by quantitative precipitin and inhibition assays. The lectin is not blood group specific and did not precipitate with A₁, A₂, Le(a) and a precursor blood group substance with I and i activity. Individual human B and H substances reacted to different extents due to their heterogeneity, those with terminal non-reducing αDGlcNAc reacting well; those lacking such residues did not precipitate. Glycoproteins with terminal βDGlcNAc such as agalactoorosomucoid also precipitated the lectin. Inhibition of precipitation showed phenyl αDGlcNAc and pNO₂ phenyl αDGlcNAc to be the best inhibitors, while their β anomers were relatively inactive. Of the free sugars tested only DGlcNAc showed considerable activity; methyl αDGlcNAc was twice as good as DGlcNAc but only 1/3 as active as phenyl αDGlcNAc, while methyl βDGlcNAc was relatively inactive. DGlcNAcβ1→3 or β1→6 linked to DGal and DGlcNACβ1→3[DGlcNAcβ1→6]DGal were not active. DGlcNAcα1→4DGal was 80% as active as methyl αDGlcNAc; reduction of the DGal to galactitol reduced its activity greatly. The presence of a third sugar giving DGlcNAcα1→4DGalβ1→4DGlcNAc made it as active as methyl αDGlcNAc. N,N'-diacetylchitobiose, N,N',N''-triacetylchitotriose and N,N',N'',N'''-tetraacetylchitotetraose which were all equal and as active as methyl αDGlcNAc and DGlcNAcα1→4DGalβ1→4DGlcNAc. However, DGlcNAcα1→4DGalβ1→3DGalNAc had much lower activity. DGlcNAcα1→5DGlcf had the same activity as phenyl αDGlcNAc. To explain the unusual finding that α and β linked oligosaccharides of DGlcNAc were of comparable activity, molecular models were constructed. The best inhibitors showed a basic similarlity in three dimensional structure, the overall planarity of the molecule and hydrophobic interactions are of importance.