Ribosome-inactivating toxins

Due to the extraordinary toxicity of intact ribosome-inactivating toxins like ricin, abrin, and modeccin, purification and handling of these proteins must be done with extreme care. Even dust from crude seed powders or lyophilized proteins should be considered dangerous. During... 

BOX 29-A THE DIPHTHERIA TOXIN AND OTHER RIBOSOME-INACTIVATING PROTEINS... 

Examination of genotoxicity of pharmaceuticals is required to assess the interaction of the drug with DNA. These studies are generally not applicable to immunotoxins. Unlike chemotherapeutics that cause cell death through DNA interaction, immunotoxins mediate cell death by preventing protein synthesis. However, immunotoxins use a linker to connect the toxin to the antibody that may need to be examined if it is an organic linker and has the ability to bind DNA (per ICH S6). The majority of immunotoxins use either a nonreducible thioether linker for intact toxins or a disulfide bond for A chains and ribosome-inactivating proteins and do not interact with DNA. 

Toxin entry retrograde transport through the secretory pathway (Lord and Roberts, 1998) Ribosome-inactivating proteins a plant perspective (Nielsen and Boston, 2001)... 

Like ricin, abrin is a type 2 ribosome inactivating protein with A and B chains linked by a disulphide bond. The abrin A chain comprises 251 amino acid residues compared to 267 in the ricin A chain, both having three folding structural domains and a molecular weight of approximately 30 kDa. The abrin B chain has a molecular weight of 35 kDa with 60% of its amino acid residues identical to those of ricin s B chain. Both B chains have two saccharide binding sites which are highly conserved between the two toxins. 

Various plant toxins, mostly ribosome-inactivating proteins (RIPs), have been identified that bind to any mammalian cell surface expressing galactose units and are subsequently internalized by RME (67). Toxins such as nigrin b (68), a-sarcin (69), ricin and saporin (70), viscumin (71), and modeccin (72) are highly toxic upon oral administration (i.e., are rapidly internalized). The possibility exists, therefore, that modified and, most important, less toxic subunits of these compound can be used to facilitate the uptake of macro-molecular compounds or microparticulates. 

Likewise, the A chains of other plant [42-44] and bacterial [45-47] hetero-dimeric toxins are responsible for toxicity. These toxins contain a single A chain moiety which, in each case, has catalytic activity and efficiently inactivates its intracellular target [44]. The A chain is only toxic to intact cells when combined with B chain. The function of the B chain is to bind the toxin to cell-surface receptors, in the case of ricin to appropriate surface glycoproteins or glycolipids. This is the essential first step in the transfer of ricin A chain into the cytosol, where ribosome inactivation occurs [48]. Additionally, the B chain is believed to have a second function during the intoxication process in which it facilitates the transfer of the A chain across a membrane into the cytoplasm [49]. Separated A and B chains are essentially non-toxic, the toxic A chain lacking the ability to bind to and enter cells in the absence of the B chain. The toxicity of ricin therefore results from three sequential steps (1) binding of the whole molecule to the cell surface via the B chain (2) penetration of at least the A chain into the cytosol, and (3) inhibition of protein synthesis caused by the interaction of the A chain with the 60 S ribosomal subunit. 

Stripe, F. Ribosome-Inactivating Proteins. Protein Toxins and Their Use in Cell Biology. Rappuoli, R. Montecucco, C. Edsl997. Oxfod University Press, Oxford, U.K. p 57-58. 

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