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) participating in binding. The blood group active glycoproteins after mild acid hydrolysis 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 βGal was the best; this compound was 1.3-fold better than its α-anomer. While methyl βGal was twice as active as its α anomer, Gal and blood group B active disaccharides (Galα1-3Gal) were 2.5 times more active than GalNAc. Among the oligosaccharides tested, Galβ1-3GalNAc (T) Galβ1-3/4GlcNAc (I/II), Galβ1-4Glc (L) and human blood group I Ma trisaccharide (Galβ1-4GlcNAcβ1-6Gal) were about equally active and the best inhibitors. They were about 2.0 and 2.4 more active than Galβ1-4Gal (E) sequence and B determinant, respectively. Conclusion: From the present results, it is concluded that: (a) this toxin has a broad range of affinity for the β-anomer of Gal; (b) its combining site is probably of a shallow groove type and as large as a trisaccharide; (c) Galβ- is the major combining site of the lectin; and (d) hydrophobic interaction gives a significant contribution for binding. This information should facilitate future usage of this lectin in glycobiological research and medical applications.