The Castor Bean

It is interesting to trace the origin of the name "castor." Castor is the generic name of the North American beaver (Castor canadensis) and one of the brightest double stars in the constellation Gemini. In Greek and Roman legend, Castor was one of the twin sons of Jupiter and Leda. According to E. A. Weiss, writing in Castor, Sesame and Safflower (1971), the name "castor" has nothing to do with beavers, luminous stars, or offspring of Greek and Roman Gods. Castor was apparently coined by English traders who confused it with the oil of another shrub, Vitex agnus-castus, which the Spanish and Portuguese in Jamaica called "agno-casto." Although it is commonly known as the castor bean plant, the seed is really not a true bean and it is not related to the Bean or Legume Family (Fabaceae). There are many other examples of "beans" that are technically not beans, such as Mexican Jumping "beans" and coffee "beans."

The scientific name for the castor plant, Ricinus communis, has a much more logical derivation. Communis means common in Latin, and castor plants were already commonly naturalized in many parts of the world when the eighteenth century Swedish naturalist Carolus Linnaeus (Karl von Linné) was giving scientific first and last names to plants and animals over 200 years ago. Ricinus is the Latin word for tick and is the specific epithet for the Mediterranean sheep tick (Ixodes ricinus). Apparently Linnaeus thought the seeds looked like ticks, particularly large ticks engorged with blood.

The castor plant has many uses, particularly the thick, yellowish or almost colorless oil obtained from the seeds. There are an astonishing number of industrial applications for castor oil and its derivatives, and new ones are continually being discovered. When dehydrated, castor oil is converted into a quick-drying oil used extensively in paints and varnishes. In fact, one of the largest single markets for castor oil in the United States is in the paint and varnish industry. Some experts say that dehydrated castor oil has qualities superior to linseed oil and tung oil, two of the most important drying oils. Castor oil is one of the world's most versatile natural products. Its water resistant qualities make it ideal for coating fabrics and for protective coverings, insulation, food containers, and guns.

Castor oil is the primary raw material for the production of sebacic acid, which is the basic ingredient in the production of nylon and other synthetic resins and fibers. Approximately three tons of castor oil are necessary to produce one ton of nylon. Sebacic acid is a 10-carbon dicarboxylic acid with a carboxylic group (C-OOH) at each end of the molecule. It is reacted with 1,6-hexanediamine, a 6-carbon molecule with an amino group (C-NH2) at each end. The free carboxylic and amino ends of these molecules begin bonding together in a chain reaction called condensation polymerization, in which a water molecule is produced at each link. The resulting nylon polymer is called Nylon 6,10 to denote the 6-carbon diamine and 10-carbon sebacic acid.

The superior "oiliness" of castor oil and its ability to "cling" to very hot moving parts make it an outstanding racing oil for high performance engines. In fact, it is the basic ingredient of Castrol-R racing motor oil for high speed automobile and motorcycle engines. Castor oil is a popular fuel additive for two cycle engines, and imparts a distinctive aroma to the exhaust of these engines. Castor wax, a hard wax produced by the hydrogenation (chemical combination with hydrogen) of pure castor oil, is used in polishes, electrical condensers, carbon paper, and as a solid lubricant.

Although castor oil is rather malodorous and distasteful, it is the source of several synthetic flower scents and fruit flavors (esters), such as jasmine, apricot, peach, plum, rose, banana, and lemon. The chemicals (esters) responsible for these flavors and aromas are obtained from ricinoleic acid, one of the important ingredients of natural castor oil. In fact, ricinoleic acid comprises about 90% of the total triglyceride fatty acids of castor oil. Castor oil is also used in making soap, inks, and plastics; for preserving leather; as an illuminant; in Turkey red oil for dyeing and finishing textiles; and in brake fluids and certain insecticidal oils. Even after the oil has been removed, the poisonous crushed seeds or oil cake (pomace) makes an excellent fertilizer.

Brazil and India are two of the major world producers of castor oil, although the plants are grown commercially in many other countries, including the United States (New Mexico, Texas, and the midwestern United States). It has been estimated that the world production of castor seeds was nearly one million tons in 1970. In terms of total production, castor oil is one of the world's most important industrial vegetable oils.

The active poison in castor seeds is ricin (RYE-sin), a very deadly protein called a lectin. Ricin is found in the meal or cake after the oil has been extracted. Those who occasionally take castor oil may be assured that ricin does not occur in the pure oil. When a gram of ricin is compared with equivalent weights of other toxic substances, it turns out to be one of our deadliest natural poisons. It has been estimated that, gram for gram, ricin is 6,000 times more poisonous than cyanide and 12,000 times more poisonous than rattlesnake venom. Ricin mixed with food and used as bait is highly toxic to certain pest animals, such as some rodents and insects. E. A. Weiss (1971) states that a dose of 0.035 milligram (approximately one millionth of an ounce) may kill a man, and even small particles in open sores and in the eyes may prove fatal. According to the Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals (1997), a dose of ricin weighing only 70 micrograms or two millionths of an ounce (roughly equivalent to the weight of a single grain of table salt from a salt shaker) is enough to kill a 160 pound person. As few as four ingested seeds can cause death in an adult human, and lesser amounts may result in symptoms of poisoning, such as vomiting, severe abdominal pain, diarrhea, and convulsions. Of course the degree of poisoning depends upon the amount ingested and the age and general health of the individual. There are numerous documented cases of ricin poisoning and death when horses, livestock, and poultry accidentally ate castor seeds or meal.

With the exception of certain pathogenic bacteria, lectins include some of the most insidious plant toxins affecting people. In addition to ricin from castor bean seeds, another very poisonous lectin called abrin occurs in the seeds of rosary bean (Abrus precatorius), a common tropical vine in the Legume Family (Fabaceae). According to the Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 1983), one thoroughly masticated seed of rosary bean can cause fatal poisoning. Brightly colored rosary beans are commonly strung for seed jewelry in Mexico and Central America. Sometimes the seeds are boiled in order to facilitate the piercing of their hard seed coats, and this heating would undoubtedly denature the toxic proteinaceous lectins inside. Of course, the undisputed record for the deadliest natural toxin goes to the anaerobic bacterium of spoiled food (Clostridium botulinum). A fascinating article on botulism appeared in Scientific American, April, 1968. So deadly is the toxin (even deadlier than strychnine, arsenic and snake venoms), that an amount equal to the weight of ink in a printed period in a textbook is enough to kill 30 adult humans. One ounce could theoretically kill 30 million tons of living matter and one pound could kill the entire human population. However, even an innocuous coconut can be a lethal weapon if you stand under a heavily laden palm. The odds of this unfortunate event is considerably greater than winning the jackpot in the California State Lottery. [In fact, the chance of being struck by lightning is actually greater than winning the California Lottery.]

The poisoning mechanism of castor beans is very complicated. It causes clumping (agglutination) and breakdown (hemolysis) of red blood cells, hemorrhaging in the digestive tract, and irreparable damage to vital organs such as the liver and kidneys. It is most toxic when taken intravenously or inhaled as fine particles. In fact, the possibilities of ricin dust in chemical warfare are horrendous. There are even documented cases of castor bean poisoning in murders by paid assassins.

At the cellular level, ricin is known as a "ribosome-inactivating protein" or Type 2 RIP, one of the most potent cytotoxins in nature. It is synthesized in the endosperm cells of maturing seeds. Ricin is composed of two complex 3-dimensional molecular subunits, a 267 amino acid globular protein called the A chain (RTA) and a 262 amino acid protein called the B chain (RTB). The two polypeptides are held together by a single disulphide bond. The RTA and RTB proteins do not become deadly until the disulsulfide bond is broken and they are separated from each other. Once inside the cell, RTA becomes a deadly enzyme that catalyzes the depurination (loss of purine bases) from ribosomes. When ribosomes lose specific purine bases, such as adenine, they no longer function and protein synthesis shuts down.

In summary, ricin is a potent cytotoxic protein is that is lethal to eukaryotic cells by inactivating the organelle sites of protein synthesis called ribosomes. Just one single ricin molecule that enters the cytosol of a cell (the semifluid medium between the nucleus and plasma membrane) can inactivate over 1,500 ribosomes per minute and kill the cell.

The agglutination of red blood cells has been atributed to a powerful hemagglutinin in castor beans called ricinus communis agglutinin (RCA) and not ricin. Ricin is a potent cytotoxin but a weak hemagglutinin. Poisoning by the ingestion of castor beans is due to ricin, not RCA, because RCA does not penetrate the intestinal wall, and does not affect red blood cells unless given intravenously. If RCA is injected into the blood, it will cause the red blood cells to agglutinate and burst due to hemolysis.

Although it is a very potent poison, ricin has been shown to possess antitumor qualities and has been used in cancer research and chemotherapy during recent years. One of the most promising uses of ricin is in the production of immunotoxins, where the protein ricin is joined to monoclonal antibodies. The antibodies are produced in a test tube (in vitro), and have protein receptor sites that recognize the specific target cells of a tumor. The resulting ricin-antibody conjugate is called an immunotoxin. By arming these antibodies with ricin, the deadly toxin can be carried directly to the site of the tumor in a cancer patient. Thus, ricin can destroy the tumor cells, without damaging other cells in the patient.

To produce monoclonal antibodies, B-lyphocytes (plasma cells) from mice are exposed to a specific tumor antigen so that they can produce antibodies against the tumor. The plasma cells are then fused with myeloma (cancer) cells, producing rapidly-dividing hybridoma cells that are essentially "immortal." The hybridoma cells divide rapidly in a test tube and continue to produce the same type of anti-tumor antibodies, called monoclonal antibodies.

Castor oil has been used medicinally in the United States since the days of the pioneers. As Americans moved west after the Civil War, settlers were very attracted to Indian medicines and popular "cure-all" remedies. The stronger smelling and the more vile tasting the concoction, the better, and some medical historians have described the latter part of the 1800's as the "age of heroic cures." Traveling medicine men peddled their elixirs throughout towns of the west, and often their products contained up to 40 percent (80 proof) alcohol. Castor oil was one of the old-fashioned remedies for everything from constipation to heartburn. It is indeed a very effective cathartic or purgative (laxative) and is still used to this day; however, there are milder, less drastic methods of inducing regularity. Castor oil is also used as personal lubricant: It is sometimes applied externally as a soothing emollient for dry skin, dermatitis, other skin diseases, sunburn, open sores, and it is the primary ingredient of several brand name medications. Several additional little-known uses for castor oil include hair tonics, ointments, cosmetics, and contraception creams and jellies. One remarkable old remedy mentions administering castor oil to induce labor during pregnancy.

One of the reasons castor plants have become so successful is their extremely viable seed that germinates readily in a variety of soils. In fact, desperate vegetable gardeners have been known to place the poisonous seeds in the burrows of gophers and moles, thus propagating and dispersing the plant. Because of its ability to grow wild and reseed itself, castor plants are recognized by botanists as naturalized weeds throughout the southwestern United States.

Since many towns in the southwestern United States have wild castor plants growing near residential areas, children should be taught to recognize its very dangerous seeds. It is doubtful that any large scale campaign to eradicate this hardy plant will ever be completely successful. Unless the southwest experiences another ice age, castor plants will continue ejecting their beautiful but deadly seeds by the millions, and will be with us for many generations to come. Although it is usually classified as a weed in the southwestern United States, one can't help admiring this valuable oil-producing plant in an energy-hungry world.