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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 betaGal was the best; this compound was 1.3-fold better than its alpha-anomer. While methyl betaGal was twice as active as its alpha anomer, Gal and blood group B active disaccharides (Galalpha1-3Gal) were 2.5 times more active than GalNAc. Among the oligosaccharides tested, Galbeta1-3GalNAc (T) Gal beta1-3/4GlcNAc (I/II), Galbeta1-4Glc (L) and human blood group I Ma trisaccharide (Galbeta1-4GlcNAcbeta1-6Gal) were about equally active and the best inhibitors. They were about 2.0 and 2.4 more active than Galalpha1-4Gal (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 beta-anomer of Gal; (b) its combining site is probably of a shallow groove type and as large as a trisaccharide; (c) Galbeta--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.