抄録
Chemical modification of histidine residues in castor bean hemagglutinin (CBH) with diethylpyrocarbonate (DEP) was studied with regard to saccharide binding. The analytical data clearly indicate that 6 out of 14 histidine residues in CBH are eventually modified with DEP at pH 6.1 in the absence of specific saccharides. Modification of histidine residues greatly decreased the cytoagglutination by CBH, and only 10 % of the residual activity was found in the derivative of CBH in which 2 histidine residues/mol were ethoxyformylated. Upon binding with galactose, the modified CBH containing 2 ethoxyformyl histidine/mol displayed an UV-difference spectrum with a maximum at 291 nm owing to the red shift of tryptophan, similar to native CBH, but its ability to bind galactose was one-fourth that of native CBH. On binding with galactopyranosides, the number of histidine residues eventually modified with DEP decreased by 2 per molecule, and the cytoagglutinating activity was retained. Hydroxylamine removed the ethoxyformyl groups from the inactive derivative of CBH which contained 6 ethoxyformyl histidine/mol, restoring the saccharide-binding ability. The results suggest that in the saccharide-binding site on each B-chain of CBH, there exists a histidine residue essential for interaction with a galactopyranosyl residue at the non-reducing end of saccharides.
