Glycoconjugates containing the disaccharide unit GlcNAc beta 1 leads to 3Gal beta were suggested as receptors for pneumococci adhering to human pharyngeal epithelial cells. The receptor activity was detected both by inhibition of adhesion by an excess of free oligosaccharide and by induction or increase of adhesion after coating of target cells with glycolipid. Studies with free natural and synthetic oligosaccharides identified the disaccharide GlcNAc beta 1 leads to 3Gal beta as one critical binding site. The specificity of recognition was shown inter alia by the lack of inhibitory activity of GlcNAc beta 1 leads to 4Gal beta, which differs only in the linkage of the two sugars. Specific interference with pneumococcal adhesion by administration of soluble receptor sugar may improve our understanding of the role of adhesion in vivo.

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Galectin-1, a β-galactoside binding lectin involved in immunoregulation and cancer, binds natural and many synthetic multivalent glycoconjugates with an apparent glycoside cluster effect, that is, affinity above and beyond what would be expected from the concentration of the determinant sugar. Here we have analyzed the mechanism of such cluster effects in solution at physiological concentration using a fluorescence anisotropy assay with a novel fluorescent high-affinity galectin-1 binding probe. The interaction of native dimeric and monomeric mutants of rat and human galectin-1 with mono- and divalent small molecules, fetuin, asialofetuin, and human serum glycoproteins was analyzed. Surprisingly, high-affinity binding did not depend much on the dimeric state of galectin-1 and thus is due mainly to monomeric interactions of a single carbohydrate recognition domain. The mechanism for this is unknown, but one possibility includes additional interactions that high-affinity ligands make with an extended binding site on the carbohydrate recognition domain. It follows that such weak additional interactions must be important for the biological function of galectin-1 and also for the design of galectin-1 inhibitors.

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