Chemical modification of tryptophan residues in castor bean hemagglutinin.

Modificationof tryptophan residues in castor bean hemagglutinin (CBH) with Nbromosuccinimide (NBS) was investigated in detail.Tryptophan residues accessible to NBS increased with lowering pH and six tryptophan residues/mol were oxidized at pH 3.0, while two tryptophan residues/mol were oxidized at pH 5.0.From the pH-dependence curve for tryptophan oxidation, we suggest that the extent of modification of tryptophan in CBHis influenced by an ionizable group with pKa= 3.6.The saccharide-binding activity was decreased greatly by modification of tryptophan concomitantly with a loss of fluorescence.A loss of the saccharide-binding activity was found to be principally due to the modification of two tryptophan residues/mol located on the surface of the protein molecule.In the presence of raffinose, two tryptophan residues/mol remained unmodified with retention of fairly high saccharide-binding activity.The results suggest that one tryptophan residue is involved in each saccharide-binding site on each B-chain of CBH.Lectins are divalent or multivalent carbohydrate-binding proteins that bind to the carbohydrate moiety of the cell membraneand mediate a veriety of biological effects.1~3)Castor bean hemagglutinin (CBH), from Ricinus communis seeds, has a molecular weight of 130,000 and consists of two sets of heterodimers of two polypeptide chains (Aand B-chains).B-chains are responsible for the f This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.* To whomcorrespondence should be addressed.