The sugar-combining area of the galactose-specific toxic lectin of mistletoe extends beyond the terminal sugar residue: comparison with a homologous toxic lectin, ricin.

Viscumin (the major lectin of mistletoe extract), also known as ML-1, and ricin (RCA II) belong to a group of heterodimeric toxic lectins composed of an A chain, which inhibits protein synthesis, and a B chain, which mediates entry into the cell in a galactose-specific manner. Although most of the binding force for the association of viscumin with galactose-containing ligands is generated by the nonreducing terminal galactose residue, a particular hydroxyl group on the penultimate sugar also appears to participate in the binding, suggesting that viscumin has an extended combining site. In this paper, we give further examples of affinity enhancement by the hydroxyl group situated on the penultimate sugar next to the glycosidic linkage of the terminal galactose. The structure with highest affinity for viscumin thus far discovered is beta-D-Gal-(1-->2)-beta-D-Gal. In contrast to viscumin, ricin does not have this extended binding area, as none of the disaccharides tested exhibited significant affinity enhancement.