Tryptophan fluorescence properties of cholera toxin upon interacting with ganglioside GD1b

A blue shift of the tryptophan fluorescence emission spectra of cholera toxin or the B protomer is induced by disialoganglioside GD1b with a capacity similar to that of monosialoganglioside GM1. Both gangliosides were also capable of decreasing or reversing the fluorescence quenching by iodide ion of the toxin. The quenching constants (Ksv) for the toxin fluorescence in absence of gangliosides was 2.8 M-1; in presence of GM1 or GD1b, Ksv was 0.8 M-1 and 0.7 M-1, respectively. Gangliosides GD1a and GT1b were unable to decrease the quenching effect. The results suggest that GD1b induces a perturbation in the tryptophan environment of the toxin molecule similar to that induced by GM1.