Binding of Vibrio cholera toxin and the heat-labile enterotoxin of Escherichia coli to G(M1), derivatives of G(M1), 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 G(M1) bound by cholera toxin and the heat-labile enterotoxin of E. coli were identified by determining the concentration of G(M1), derivatives of G(M1), oligosaccharide isolated from G(M1), or clustered oligosaccharide needed to inhibit toxin binding to G(M1)-coated plastic wells. The K(I)S for G(M1), the C(7) sialosyl aldehyde of G(M1), and ethanolamine-sialosyl-G(M1) were similar (~ 30-50 nM) for both toxins. N-Deacetylation of G(M1) resulted in a small increase in K(I); formation of the sialosyl methyl ester increased the K(I) 2-5 fold; loss of the terminal galactosyl residue (G(M2)) increased the K(I) by 10-15-fold; and removal of the sialosyl moiety (asialo-G(M1)) resulted in loss of inhibition of both toxins. Oligosaccharide isolated from G(M1) had a K(I) for both toxins that was ~ 100-fold greater than that obtained for G(M1) and ~ 1000-fold greater than that for a clustered oligosaccharide derivative having an average of 8 oligosaccharide residues (isolated from G(M1)) per molecule of poly-L-lysine. These results indicate that both toxins are functionally quite similar in their recognition of G(M1) 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 G(M1) blocked by a nonlipid compound, i.e. oligo-G(M1)-poly-L-lysine.