BINDING OF SACCHARIDES TO ABRIN-B ISOLATED FROM ABRUS-PRECATORIUS SEEDS

The nature of the binding of saccharides to arbin-b, a toxic lectin is olated from Abrus precatorius seeds, was studied by equilibrium dialys is and fluorescence spectroscopy. Equilibrium dialysis data indicate t hat abrin-b has two saccharide-binding sites, a high affinity site (HA -site) and a low affinity site (LA-site), to which both galactopyranos ides and N-acetylgalactosamine can bind. With excitation at 290 nm, ab rin-b displayed a fluorescence spectrum with an emission maximum at 34 5 nm. Upon binding with specific saccharides, this spectrum shifted to a wavelength shorter by 5 nm, suggesting that saccharides bind to abr in-b in such a manner as to induce a change in the environment of the tryptophan residue or residues at or near the respective binding sites . From the variation of fluorescence at 320 nm with saccharide concent rations, the association constants for binding of saccharides to the r espective sites were measured. The results suggest that the HA-site ha s a subsite favorable for saccharides having beta-1,4 linked galactopy ranoside at the non-reducing end like lactose in addition to the galac tose-recognition site, while the LA-site may not have such a subsite.