Reduction of ricin toxicity without impairing the saccharide-binding properties by chemical modification of the carboxyl groups.

The usefulness of ricin as a research tool is handicapped by its extremely biohazardous nature. In this work, ricin toxicity has been reduced by chemical modification of carboxyl groups using 1-ethyl-3(3-dimethylaminopropyl) carbodiimide and [14C]glycine methyl ester. The reaction was carried out in 8 M urea and in the presence of 0.1 M lactose to protect the groups involved in saccharide binding. Together with carboxyl groups, tyrosine residues were also modified. The maximum modification achieved was 13 carboxyl groups and 7 tyrosines per molecule (about 30% of total carboxyls and tyrosines). The modification did not alter substantially the strength and specificity of the carbohydrate-binding ability of the lectin, as observed by hemagglutination tests and by inhibition assays with different carbohydrate structures. However, the LD50 decreased 90-fold when the highest modification was achieved. Therefore, the modified lectin can be used more safely in the study of galactose-containing carbohydrates.