Ricin: Biomedical Mechanisms
Ricin, the lethal poison distilled from the mash of processing caster beans, holds two toxins: ricin and RCA (Ricinus communis agglutinin).
The ricin toxin is a cytotoxin which inhibits protein synthesis by inactivating the eukaryotic ribosomes. These proteins are known as RIPs (ribosome-inactivating proteins). Typically, RIPs cannot breach the cell membranes to inactivate the ribosomes (the protein producing organelle), and RIPs in wheat and barley are not poisonous. In castor beans, the RIPs are accompanied by galactose-binding, 30 kDa lectins that bind to the cell surface. Lectins are attached to the RIP cytotoxin by disulfides. The cytotoxin is now able to enter the cell membrane, inhibit protein synthesis in the ribosomes, and to cause cell death.
RCAs are hemagglutinins that cause the red blood cells to agglutinate or clump and burst. The RCA cannot penetrate the intestinal walls and are not toxic if ingested. These proteins are only toxic if ricin poison is injected into the blood stream.
Ricin and RCA toxins are stored in the endosperm cells of maturing caster bean seeds in a protein body. The toxins are destroyed when the seeds germinated.
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