| Ricin toxin is a potent type 2 ribosome inactivating protein (RIP) from the plant, Ricinus communis. It has the potential to be used as a bioterrorism agent due to its high toxicity and the relative ease with which it can be obtained/purified. Ricin is a heterodimeric protein consisting of 2 subunits. The A-chain (RTA) is an N-glycosidase that halts protein synthesis by catalyzing the depurination of a specific adenine residue on ribosomal RNA. RTA is linked to the B-chain (RTB) via a single disulphide bridge. RTB is a lectin, containing 2 galactose binding sites ∼70A apart, that plays an essential role in mediating the binding and internalization of the toxin.;The goal of this work was to synthesize multivalent carbohydrate ligands, use them to determine whether ricin would bind to them in a multivalent fashion, and to identify general saccharide characteristics that affect binding. To account for the distance between the two galactose binding sites on RTB, bovine serum albumin (BSA) was selected as the oligosaccharide carrier. BSA-based neoglycoconjugates were synthesized and their efficacy as ligands for the toxin monitored using surface plasmon resonance (SPR). The results indicated that ricin did exhibit multivalency when it interacted with appropriately derivatized BSA neoglycoconjugates and that it appeared to bind more efficiently to specific longer chain oligosaccharide ligands than to disaccharides. These observations have important implications for the development of possible antidotes for the treatment of people inadvertently exposed to ricin, for the use of RTB in drug deliver, and for optimization of ricin biosensors.;While the binding of ricin to galactose has been studied, characterization of the binding of ricin to more complex carbohydrate ligands (i.e., glycoproteins and glycolipids) that might be present on the surface of a cell is lacking. In contrast to the low affinity of RTB for monovalent galactose (reported to be between 0.28 and 3.5 x 10-4M), its affinity for asialofetuin (ASF) is about 1,000 times greater than it is to monovalent galactose. This observation supports the hypothesis that ricin exhibits multivalency when it adheres to ASF. |
