A fluorimetric study of the lanthanides binding to concanavalin A

The binding of Tb3+ and other lanthanides to Con A has been studied by sensitized Tb3+ luminescence, by quenching of intrinsic fluorescence and by activity measurements. In all the experimental conditions tested, it was found that holo and apo Con A bind lanthanide ions at a site different from the binding sites of the constitutive metals, Mn2+ and Ca2+. The bound lanthanide did not affect the saccharide binding ability and the hemoagglutinating ability of Con A. The intrinsic fluorescence of Con A is quenched by the binding of Tb3+ and Gd3+. The same quenching is obtained by shifting the pH of Con A from pH 6.5 to 4.5. It is proposed that H+ and Ln3+ completely quench a tryptophan, perhaps the residue 88 or 182.