Studies on the carbohydrate binding sites of the hemolytic lectin CEL-III isolated from the marine invertebrate Cucumaria echinata

The binding of carbohydrates to the hemolytic lectin CEL-III isolated from the marine invertebrate Cucumaria echinata was studied. Equilibrium dialysis data suggest that CEL-III has two carbohydrate-binding sites with equal affinity. The binding of specific carbohydrates to CEL-III induces a decrease in the fluorescence intensity at 339 nm and the shift of the fluorescence emission maximum to a wavelength shorter by 3 nm, owing to the change in the environment of tryptophan. By analyzing the change in the fluorescence intensity at 339 nm as a function of the concentration of carbohydrates, the association constants for binding of individual carbohydrates to CEL-III were calculated. The results indicate that GalNAc, lactulose, and lactose are bound by CEL-III with fairly high affinity among the carbohydrates tested. The pH-dependence profile of the association constant of lactose suggests that CEL-III binds carbohydrates with highest affinity around pH 5.0. Modification of CEL-III with N-bromosuccinimide produces an oxidized derivative, in which four tryptophan residues/mol were oxidized and had no hemolytic activity. However, two out of these four tryptophans escaped from the modification in the presence of specific saccharides and the resulting derivative retained fairly high hemolytic activity.