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Ricin and RCA

Castor Bean Toxins

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Ricin is made of two protein chains linked by a disulfide bond.

Ricin is made of two protein chains linked by a disulfide bond. The A chain (blue) is an N-glycoside hydrolase that inhibits protein synthesis. The B chain (orange) is a lectin that helps ricin bind to a cell.

AzaToth, Wikipedia Commons Castor beans are the source of ricin, a deadly toxin.

Castor beans (seeds from Ricinus communis) are the source of ricin, a deadly toxin.

USDA-ARS
The castor bean plant, Ricinus communis, contains two toxins that are poisonous to people, animals, and insects. The main toxic protein, ricin, is so potent that a single milligram may be sufficient to kill a human adult. Ricin is considered both a chemical and biological weapon and is explicitly prohibited by the Biological and Toxin Weapons Convention (BTWC) and Schedule 1 of the Chemical Weapons Convention (CWC). Ricin works its harm by destroying small parts of cells, called ribosomes. Ribosomes produce all of the proteins needed by a cell. If the proteins cannot be produced, the cell dies. Although the effects of ricin ingestion may be felt within a few hours (abdominal pain, diarrhea, vomiting), it is a slow-acting poison, with death occurring after 1-3 days. A victim who survives the severe dehydration and lowered blood pressure of advanced ricin poisoning will generally recover.

The other toxic protein in the castor bean, RCA (Ricinus communis agglutinin), agglutinates red blood cells. In other words, injection of RCA into the bloodstream essentially causes a person's blood to coagulate. Ingestion of a castor bean or its products will release ricin, but the RCA cannot cross the intestinal wall.

Castor oil and products made from castor oil contain very little ricin or RCA. However, castor beans are grown for ornamental purposes, too. The seeds from the garden plant present a poisoning hazard to children and pets. Dehydration and vomiting are more dangerous for children than adults, so ingestion of a single castor bean seed may be fatal for a child. However, if the seed is ingested whole, there is a chance that it may pass through the gastrointestinal system without releasing its ricin.

Purified ricin and RCA are of considerable concern as weapons for several reasons. First, castor bean seeds are readily obtainable. Second, several routes of exposure are possible (for ricin: inhalation, injection, or ingestion). Once the proteins are purified, the powdered toxin can be used to contaminate food or beverages. Ricin is heat-stable, so it can be applied to shrapnel within an explosive device. Possibly the greatest concern about ricin used as a weapon is that symptoms of poisoning can readily misdiagnosed.

At present, therapy for ricin poisoning consists of replacing fluids and treating the symptoms of poisoning, but research is underway to develop a vaccine for the toxin. Also, testing is underway for a new drug, using an inactivated form of the ricin protein, to treat individuals following exposure.

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