Identification of histidine residues in the sugar binding site of snake gourd (Trichosanthes anguina) seed lectin.

Chemical modification studies have been carried out on the galactose-specific lectin (SGSL) purified from snake gourd (Trichosanthes anguina) seeds. Modification of the imidazole side chains of histidine residues with ethoxyformic anhydride resulted in a complete loss of activity of the lectin. A total of 9.5 (+/- 0.7) histidine residues were modified per dimer of M(r) 55,000 when the reaction was carried out for 2 hours. A partial protection was observed when the modification was done in the presence of 0.1 M galactose, indicating that histidine residues are directly involved in the sugar-binding activity of the lectin. Complete recovery of the lectin activity was observed when the modification was reversed by treatment with hydroxylamine. In immunodiffusion experiments, the histidine-modified lectin reacted with rabbit antiserum raised against the native SGSL forming a precipitin line, indicating that the loss of activity upon modification was not due to changes in the overall conformation of the lectin. Modification of the side chains of lysine, cysteine and tyrosine residues did not result in any change in the activity of SGSL.