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alkaloid

alkaloid, any of a class of organic compounds composed of carbon, hydrogen, nitrogen, and usually oxygen that are often derived from plants. Although the name means alkalilike, some alkaloids do not exhibit alkaline properties. Many alkaloids, though poisons, have physiological effects that render them valuable as medicines. For example, curarine, found in the deadly extract curare, is a powerful muscle relaxant; atropine is used to dilate the pupils of the eyes; and physostigmine is a specific for certain muscular diseases. Narcotic alkaloids used in medicine include morphine and codeine for the relief of pain and cocaine as a local anesthetic. Other common alkaloids include quinine, caffeine, nicotine, strychnine, serotonin, and LSD. Aconitine is the alkaloid of aconite. Cinchonine and quinine are derived from cinchona, coniine is found in poison hemlock, and reserpine is an extract of rauwolfia roots. Emetine is an alkaloid of ipecac.

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Alkaloids

Alkaloids

Alkaloids are natural, organic substances that are predominantly found in plants and normally contain at least one nitrogen atom in their chemical structure. Their basic (alkaline) nature has led to the term alkaloids. Since the identification of the first alkaloid, morphine, from the opium poppy (Papaver somniferum ) in 1806, more than ten thousand alkaloids have been isolated from plants. Alkaloids are the active components of numerous medicinal plants or plant-derived drugs and poisons, and their structural diversity and different physiological activities are unmatched by any other group of natural products.

Although alkaloids have been detected in some animals (e.g., in the toxic secretions of fire ants, ladybugs, and toads), their major occurrence is in the flowering plants. Alkaloids are relatively stable compounds that accumulate as end products of different biosynthetic pathways, mostly starting from common amino acids such as lysine, ornithine, tyrosine, tryptophan, and others. Their classification is usually based on the formed heterocyclic ring system (e.g., piperidine in coniine, pyridine in nicotine, and quinoline in quinine). Some structures are relatively simple, whereas others are quite complex.

Alkaloids can occur in all parts of the plant but frequently, depending on the plant species, they accumulate only in particular organs (e.g., in barks, roots, leaves, and fruits), whereas at the same time other organs are alkaloid-free. In potato plants, the edible tubers are devoid of alkaloids, whereas the green parts contain the poisonous solanine. The organ in which alkaloids accumulate is not always the site of their synthesis. In tobacco, nicotine is produced in the roots and translocated to the leaves where it accumulates.

The functions of alkaloids in plants are mostly unknown, and their importance in plant metabolism has been much debated. A single plant species may contain over one hundred different alkaloids, and the concentration can vary from a small fraction to as much as 10 percent of the dry weight. Breeding for plants devoid of alkaloids has also demonstrated that alkaloids are apparently not vital. Why does a plant invest so much nitrogen and energy in synthesizing such a large number and quantity of compounds? Most alkaloids are very toxic and, therefore, have the potential to function in the chemical defense arsenal of plants against attack by herbivores and micro-organisms. For example, the nicotine present in tobacco leaves inhibits the growth of tobacco hornworm larvae; the purified compound is also applied as an effective insecticide in greenhouses. In addition, alkaloids have been suggested to serve as a storage form of nitrogen or as protectants against damage by ultraviolet light.

Alkaloids have traditionally been of great interest to humans because of their pronounced physiological and medicinal properties. From the beginning of civilization, alkaloid-containing plant extracts have been used in all cultures as potions, medicines, and poisons. Greek philosopher Socrates died in 399 B.C.E. by consumption of coniine-containing hemlock (Conium maculatum ), and Egyptian queen Cleopatra (69-30 B.C.E.) used atropine-containing plant extracts (such as belladonna ) to dilate her pupils. In modern times, the stimulants caffeine in coffee, tea, and cacao and nicotine in cigarettes are consumed worldwide. Alkaloids with hallucinogenic , narcotic, or analgesic properties have found medical application as pure compounds (e.g., morphine, atropine, and quinine) or served as model compounds for modern synthetic drugs, while several are abused as illegal drugs (e.g., cocaine). Other alkaloids are too toxic for any therapeutic use (e.g., coniine and strychnine), but plant constituents are still screened for new, biologically active compounds. An example is the discovery of taxol, which has cytostatic properties and is applied as an anticancer drug.

see also Cacao; Coca; Coffee; Defenses, Chemical; Medicinal Plants; Opium Poppy; Poisonous Plants; Potato; Tobacco.

Erich Kombrink

Bibliography

Harborne, Jeffrey B., and Herbert Baxter, eds. Phytochemical Dictionary: A Handbook of Bioactive Compounds from Plants. Bristol: Taylor & Francis, 1995.

Kutchan, Toni M. "Alkaloid Biosynthesis: The Basis for Metabolic Engineering of Medicinal Plants." Plant Cell 7 (1995): 1095-1070.

Mann, John, R. Stephen Davidson, John B. Hobbs, Derek V. Banthrope, and Jeffrey B. Harborne. Natural Products: Their Chemistry and Biological Significance. Essex: Longman Group, 1994.

Wink, Michael, ed. Biochemistry of Plant Secondary Metabolism. Sheffield, UK, and Boca Raton, FL: Sheffield Academic Press and CRC Press, 1999.

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Alkaloids

ALKALOIDS

This is the general term for any number of complex organic bases that are found in nature in seed-bearing plants. These substances are usually colorless but bitter to the taste. Alkaloids often contain nitrogen and oxygen and possess important physiological properties.

Examples of alkaloids include not only quinine, atropine, and strychnine but also Caffeine, Nicotine, Morphine, Codeine, and Cocaine. Therefore, many drugs that are used by humans for both medical and nonmedical purposes are produced in nature in the form of alkaloids. Naturally occurring receptors for many alkaloids have also been identified in humans and other animals, suggesting an evolutionary role for the alkaloids in physiological processes.

Nick E. Goeders

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alkaloid

alkaloid One of a group of basic, nitrogenous, normally heterocyclic compounds of a complex nature. Alkaloids are derived from plants, and have powerful pharmacological effects. More than 1000 alkaloids are known, from 1200 plant species. Their function is uncertain, but in some species they may confer a degree of protection against insect attack. Pharmacologically powerful alkaloids derived from plants include cocaine, morphine, and strychnine.

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alkaloid

alkaloid One of a group of basic, nitrogenous, normally heterocyclic compounds of a complex nature. Alkaloids are derived from plants and have powerful pharmacological effects. More than 1000 alkaloids are known from 1200 plant species. Their function is uncertain but, in some species, they may confer a degree of protection against insect attack. Pharmacologically powerful alkaloids derived from plants include cocaine, morphine, and strychnine.

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alkaloid

alkaloid Member of a class of complex nitrogen-containing organic compounds usually found in certain plants. They are sometimes bitter and highly poisonous substances, used as drugs. Examples include codeine, morphine, nicotine, and quinine. Epibatidine, a new class of alkaloid that is an organochlorine compound, was first extracted in 1992 from the skin of a frog. It is used as a powerful painkiller.

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alkaloid

alkaloid One of a group of nitrogenous organic compounds derived from plants and having diverse pharmacological properties. Alkaloids include morphine, cocaine, atropine, quinine, and caffeine, most of which are used in medicine as analgesics or anaesthetics. Some alkaloids are poisonous, e.g. strychnine and coniine, and colchicine inhibits cell division.

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alkaloids

alkaloids Naturally occurring organic bases which have marked pharmacological actions in man and other animals. Many are found in plant foods, including potatoes and tomatoes (the Solanum alkaloids), or as the products of fungal action (e.g. ergot), although they also occur in animal foods (e.g. tetrodotoxin in puffer fish, tetramine in shellfish).

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alkaloid

al·ka·loid / ˈalkəˌloid/ • n. Chem. any of a class of nitrogenous organic compounds of plant origin that have pronounced physiological actions on humans. They include many drugs (morphine, quinine) and poisons (atropine, strychnine).

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alkaloid

alkaloid (al-kă-loid) n. one of a diverse group of nitrogen-containing substances that are produced by plants and have potent effects on body function. Many alkaloids are important drugs, including morphine, quinine, atropine, and codeine.

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alkaloid

alkaloid One of a group of basic, nitrogenous, normally heterocyclic compounds of a complex nature. Alkaloids are derived from plants, and have powerful pharmacological effects.

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alkaloid

alkaloidavoid, Boyd, Coed, droid, Floyd, Freud, Lloyd, overjoyed, self-employed, unalloyed, underemployed, unemployed, void •geoid • amoeboid (US ameboid) •globoid • cuboid • gadoid • typhoid •fungoid • discoid • tabloid • colloid •celluloid • mongoloid • alkaloid •coralloid • crystalloid • prismoid •arachnoid • sphenoid • hominoid •crinoid, echinoid •solenoid • humanoid • paranoid •hypoid • anthropoid • gabbroid •android • steroid • thyroid • hydroid •spheroid • meteoroid • Murgatroyd •Polaroid •haemorrhoid (US hemorrhoid) •asteroid • schizoid • factoid • mastoid •deltoid • planetoid • ovoid • trapezoid •rhizoid

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