Identification of the tryptophan residue located at the saccharide binding site of castor bean hemagglutinin.

The tryptophan residue present at the saccharide-binding site of castor bean hemagglutinin (CBH) was identified.A peptide containing a modified tryptophan residue was isolated from the tryptic digest of S-carboxymethylated B-chain obtained from an inactive derivative of CBH(2-Oxa-CBH), in which two tryptophan residues/mol were oxidized with Af-bromosuccinimide, by gel filtration on a Sephadex G-50 followed by high performance liquid chromatography.Analytical data for the isolated peptide indicated that the tryptophan residue at position 131 on the B-chain was modified in 2-Oxa-CBH.From these and earlier results, it is suggested that the tryptophan residue at 131 on each Bchain is closely associated with the saccharide-binding activity of CBH.The specific role of tryptophan residue at 131 in the saccharide-binding site of CBHis also discussed.