Binding of Vibrio cholera toxin and the heat-labile enterotoxin of Escherichia coli to GM1, derivatives of GM1, and nonlipid oligosaccharide polyvalent ligands.

Vibrio cholera toxin and the heat-labile enterotoxin of Escherichia coli have been shown to differ somewhat in their ligand specificity and in the antigenicity of their binding sites. Therefore, the components of the oligosaccharide portion of GM1 bound by cholera toxin and the heat-labile enterotoxin of E. coli were identified by determining the concentration of GM1, derivatives of GM1, oligosaccharide isolated from GM1, or clustered oligosaccharide needed to inhibit toxin binding to GM1-coated plastic wells. The KIs for GM1, the C(7) sialosyl alcohol [corrected] of GM1, and ethanolamine-sialosyl-GM1 were similar (approximately 30-50 nM) for both toxins. N-Deacetylation of GM1 resulted in a small increase in KI; formation of the sialosyl methyl ester increased the KI 2-5 fold; loss of the terminal galactosyl residue (GM2) increased the KI by 10-15-fold; and removal of the sialosyl moiety (asialo-GM1) resulted in loss of inhibition of both toxins. Oligosaccharide isolated from GM1 had a KI for both toxins that was approximately 100-fold greater than that obtained for GM1 and approximately 1000-fold greater than that for a clustered oligosaccharide derivative having an average of 8 oligosaccharide residues (isolated from GM1) per molecule of poly-L-lysine. These results indicate that both toxins are functionally quite similar in their recognition of GM1 as a ligand in that each requires the free carboxyl group of sialic acid for optimum binding, does not need carbons 8 and 9 of the sialosyl moiety nor the acetyl groups associated with the sialic acid and galactosamine residues, and can have its binding to GM1 blocked by a nonlipid compound, i.e. oligo-GM1-poly-L-lysine.