Further studies on the binding characteristics of rabbit liver galactose/N-acetylgalactosamine-specific lectin

The affinity of various carbohydrates for the galactose/N-acetylgalactosamine-specific lectin of the rabbit liver was assessed by determining the effect of these carbohydrates on the binding of [125I]asialoorosomucoid (125I-ASOR) by the lectin. To obtain the concentration of the inhibitor that causes 50% reduction in the 125I-ASOR binding (I50), we carried out inhibition assays with fixed concentrations of 125I-ASOR and the purified, detergent-solubilized lectin, while the concentrations of the inhibitors were varied. The concentrations of the 125I-ASOR and the lectin were chosen such that the I50 value obtained closely approximates the dissociation constant of the inhibitor. Previously, we had shown that equatorial 2-hydroxyl (or acetamido), equatorial 3-hydroxyl, and axial 4-hydroxyl groups of a D-galactopyranosyl (or 2-acetamido-2-deoxy-D-galactopyranosyl) residue in the neoglycoprotein ligand participate in the binding to the lectin [Stowell, C. P., Lee, R. T., & Lee, Y. C. (1980) Biochemistry 19, 4904-4908; Lee, R. T. (1982) Biochemistry 21, 1045-1050]. In this study, we demonstrate that the methylene group (C-6) and certain aglycons also contribute to the binding. The presence of an unsaturated group such as C = NH at the gamma position to the anomeric carbon enhances the binding of an equatorially oriented aglycon. In addition, there seems to be a nonspecific hydrophobic interaction between some aglycons and the lectin binding site. Thus altogether five groups (aglycon, 2-OH or 2-NHAc, 3-OH, 4-OH, and 6-CH2-) in a galactopyranoside (or N-acetylgalactosaminide) have been shown to participate in lectin-ligand interactions. However, not all five groups are absolutely necessary for binding, since significant binding to the liver lectin occurs when only four of these groups are present.