Further characterization of the combining sites of Bandeiraea (Griffonia) simplicifolia lectin-I, isolectin A₄

Bandeiraea (Griffonia) simplicifolia lectin-I, isolectin A₄ (GSI-A₄), which is cytotoxic to the human colon cancer cell lines, is one of two lectin families derived from its seed extract. It contains only a homo-oligomer of subunit A, and is most specific for GalNAcα1→. In order to elucidate the GS I-A₄-glycoconjugate interactions in greater detail, the combining site of this lectin was further characterized by enzyme linked lectino-sorbent assay (ELLSA) and by inhibition of lectin-glycoprotein interactions. This study has demonstrated that the Tn-containing glycoproteins tested, consisting of mammalian salivary glycoproteins (armadillo, asialo-hamster sublingual, asialo-ovine, -bovine, and -porcine submandibular), are bound strongly by GS I-A₄. Among monovalent inhibitors so far tested, p-NO2-phenylα GalNAc is the most potent, suggesting that hydrophobic forces are important in the interaction of this lectin. GS I-A₄ is able to accommodate the monosaccharide GalNAc at the nonreducing end of oligosaccharides. This suggests that the combining site of the lectin is a shallow cavity. Among oligosaccharides and monosaccharides tested as inhibitors of the binding of GS I-A₄, the hierarchy of potencies are: GalNAcα1→3GalNAcβ1→3Galα1→4Galβ1→ 4Galβ1→4Glc (Forssman pentasaccharide) > GalNAcα1→3(LFucα1→2)Gal (blood group A) > GalNAc > Galα1→4Gal > Galα1→3Gal (blood group B-like) > Gal.