Abstract
The saccharide-binding properties of abrin-a, a toxic lectin from seeds of Abrus precatorius, were studied by equilibrium dialysis and spectroscopy. Equilibrium dialysis data suggest that abrin-a has two saccharide-binding sites with different affinities for galactopyranosides, a high affinity site (HAsite) and a low affinity site (LA-site). The binding of galactopyranosides to abrin-a induced a shift of the fluorescence spectrum to a shorter wavelength by 5nm and UV-difference spectra with maxima at 283 and 292 nm. Such spectroscopic changes, however, were not induced by binding with N-acetylgalactosamine and galactosamine, suggesting a difference in the binding mode or the site to bind between these saccharides and galactopyranosides. The association constants for binding of individual saccharides to abrin-a suggested that in the HA-site there may be a subsite that can interact with the glucopyranosyl moiety of the lactose molecule in addition to the galactose-recognition site, while the LA-site may not have such a subsite structure. Based on these results, we concluded that galactopyranosides bind to abrin-a in such a manner as to induce the change in the environment of the tryptophan residue or residues located at or near the respective binding sites.
