Chemical modification of ricin D with N-acetylimidazole.

Tyrosine residues in ricin D were modified with TV-acetylimidazole and the saccharide binding properties of the resulting derivatives were examined.The cytoagglutinating activity of ricin D was not altered by acetylation of one tyrosine residue/mol, but decreased greatly upon modification of two tyrosine residues/mol.In the cytoagglutination test, only 6% residual activity was found in the derivative, 2-Ac-ricin D, in which 2.4 tyrosine residues/mol were acetylated.In the presence of lactose, however, one tyrosine residue/mol was protected from acetylation by TV-acetylimidazole with a retention of the saccharide binding ability, suggesting the involvement of one tyrosine residue in the saccharide binding.Fluorescence and I V -difference spectroscopic data indicate that the binding ability of the low affinity saccharide-binding site (LA-site) of ricin D remained unchanged after modification of 2 tyrosine residues/mol.The affinity chromatography of the acetylated derivatives of ricin D on the lactamyl-and galactosamine-cellulofine columns suggests that 2-Ac-ricin D binds galactopyranosides only at the LA-site, but lacks the binding ability at the high affinity saccharide-binding site (HA-site).It is postulated that introduction of the bulky acetyl group into the hydroxyl group of the tyrosine residue at the HA-site of ricin D may sterically hinder the binding of saccharides.