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Ricinus communis agglutinin II is an extremely toxic glycoprotein consisting of two disulfide-linked chains of about 28,000 daltons and 32,000 daltons, termed A and B chain, respectively. The B chain can bind to cell surfaces via galactose or N-acetylgalactosamine residues of membrane glycoconjugates and facilitates the transport of the lectin into the cell. The A chain has an enzyme activity which can catalytically block protein synthesis and is so toxic that only a single molecule of A chain is required to kill a cell. Alone, the A chain is incapable of entering the cell and is thus not toxic. The A chain can be separated from the B chain and then linked by a cleavable disulfide bond to other proteins, such as antibodies. These antibody-toxin hybrid molecules can be used to kill specific cells with the appropriate antigenic determinant exposed, such as tumor markers on surfaces of malignant cells.
RCA60 appears to be a family of related lectins with different toxicities. Our RCA60 is the most toxic lectin of this family, probably ricin D, having an I.P. L.D50 in 20 g mice of less than 50 ng.
Agarose bound* Ricinus Communis Agglutinin II is prepared from affinity-purified lectin. Heat stable, cross-linked 4% agarose beads with a molecular weight exclusion limit of about 2x107 are used as the solid-phase matrix to which the lectin is covalently bound. The attachment of the lectin to the solid phase is carefully controlled in order to preserve the activity of the lectin as well as to minimize conformational changes of the bound lectin which might result in nonspecific ionic or hydrophobic interactions. |