Binding of Vibrio cholera Toxin and the Heat - labile Enterotoxin of Escherichia coli to GM 1 , Derivatives of GM 1 , and Nonlipid Oligosaccharide

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 GMl bound by cholera toxin and the heat-labile enterotoxin of E. coli were identified by determining the concentration of GMl, derivatives of GMl, 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 aldehyde of GM1, and ethanolamine-sialosylGM1 were similar (-30-50 nM) for both toxins. NDeacetylation of GMl 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 K I by 10-15-fold; and removal of the sialosyl moiety (aSialO-GM1) resulted in loss of inhibition of both toxins. Oligosaccharide isolated from GMl had a K I for both toxins that was -100-fold greater than that obtained for GMl and -1000-fold greater than that for a clustered oligosaccharide derivative having an average of 8 oligosaccharide residues (isolated from GMl) per molecule of poly-L-lysine. These results indicate that both toxins are functionally quite similar in their recognition of GMl 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 GMl blocked by a nonlipid compound, i.e. oligo-GM1-poly-L-lysine.