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-s pecific lectin (SGSL) purified from snake gourd (Trichosanthes anguina ) seeds. Modification of the imidazole side chains of histidine residu es with ethoxyformic anhydride resulted in a complete loss of activity of the lectin. A total of 9.5 (+/-0.7) histidine residues were modifi ed per dimer of M-r 55,000 when the reaction was carried out for 2 hou rs. A partial protection was observed when the modification was done i n the presence of 0.1 M galactose, indicating that histidine residues are directly involved in the sugar-binding activity of the lectin. Com plete recovery of the lectin activity was observed when the modificati on was reversed by treatment with hydroxylamine. Zn immunodiffusion ex periments, 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 chain s of lysine, cysteine and tyrosine residues did not result in any chan ge in the activity of SGSL.