Lectinochemical Studies on the Binding Properties of a Toxic Lectin (Ricin) Isolated from the Seeds of Ricinus communis

Background: Ricin (RCA2 or RCA60) is a highly toxic heterodimeric protein found in the seeds of the castor plant Ricinus communis. It is a potential biohazard. In the present study, the fine specificity of ricin was defined. Methods: The combining site of ricin was characterized by quantitative precipitin (QPA) and precipitin inhibition assays (QPIA). Results: Of 31 glycoproteins and pneumococcus type XIV capsular polysaccharide tested, only twelve of them precipitated over 50 % of the toxin N added, reflecting poor precipitability of the lectin with the compounds tested. This can be explained by only a single chain (B chain of the molecules) participat-ing in binding. The blood group active glycoproteins after mild acid hydroly-sis or Smith degradation, as well as sialic-acid containing glycoproteins after removal of sialic acid, in general, had substantially increased activity. Of the monosaccharides tested for inhibition of precipitation of ricin, p-nitrophenyl Gal was the best; this compound was 1.3-fold better than its-anomer. While methyl Gal was twice as active as its anomer, Gal and bloo