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Taxine alkaloids, which are often named under the collective title of taxines, are the toxic chemicals that can be isolated from the yew plant. The amount of taxine alkaloids depend on the species of yew with Taxus Baccata and Taxus Cuspidata containing the most. The major taxine alkaloids are taxine A and taxine B although there are at least 10 different alkaloids. Until 1956, it was believed that all the taxine alkaloids were one single compound named taxine.

The taxine alkaloids are cardiotoxins with taxine B being the most active. Taxine alkaloids have no medical uses but Paclitaxel and other taxanes that can be isolated from yews have been used as chemotherapy drugs.

History
The toxic nature of yew trees has been known for millennia. There are a number of early recorded examples of poisonings by Greek and Roman writers including Julius Caesar’s account of Cativolcus, king of Eburones, who committed to suicide using the “juice of the yew”. The first attempt to extract the poisonous substance in the yew tree was in 1828 by Piero Peretti who isolated a bitter substance. In 1856, H. Lucas, a pharmacist in Arnstadt, prepared a white, alkaloid powder from the foliage of Taxus Baccata L. which he named taxine. The crystalline form of the substance was isolated in 1876 by W. Marmé, a French chemist. A. Hilger and F. Brande used elemental combustion analysis in 1890 to suggest the first molecular formula of.

For the next 60 years, it was generally accepted that taxine was made of a single compound and it was well known enough for Agatha Christie to use it as a poison in A Pocket Full of Rye. However, in 1956, Graf and Boeddeker discovered that Taxine was actually a complex mixture of alkaloids rather than a single alkaloid. Using electrophoresis, they were able to isolate the two major components, Taxine A and Taxine B. Taxine A was the fastest moving band and accounted for 1.3% of the alkaloid mixture and Taxine B was the slowest moving band and accounted for 30% of the mixture. The full structure of Taxine A was reported in 1982 and of Taxine B in 1991.

Toxicity (for taxus baccata page)
All parts of a yew plant are toxic to humans with the exception of the yew berries (however, their seeds are toxic); additionally, male and monoecious yews in this genus release cytotoxic pollen, which can cause headaches, lethargy, aching joints, itching, and skin rashes; it is also a trigger for asthma. These pollen grains are only 15 microns in size, and can easily pass through most window screens.

Taxines A and B the major taxine alkaloids found in the yew plant are cardiotoxic. The taxines act as calcium and sodium channel antagonists, causing an increase in cytoplasmic calcium. Taxine B is a worse cardiotoxin than taxine A.

The foliage itself remains toxic even when wilted, and toxicity increases in potency when dried. Ingestion and subsequent excretion by birds whose beaks and digestive systems do not break down the seed's coating are the primary means of yew dispersal. Horses have a relatively low tolerance to taxine, with a lethal dose of 200–400 mg/kg body weight; cattle, pigs, and other livestock are only slightly less vulnerable. Several studies have found taxine values under 20 mg/kg in mice and rats.

Symptoms of yew poisoning include an accelerated heart rate, muscle tremors, convulsions, collapse, difficulty breathing, circulation impairment and eventually cardiac arrest. However, there may be no symptoms, and if poisoning remains undetected death may occur within hours. Fatal poisoning in humans is very rare, usually occurring after consuming yew foliage. The leaves are more toxic than the seed.

Toxicity (for taxus cuspidata page)
The entire yew bush, except for the fleshy berry surrounding the seed, is toxic due to a group of chemicals called taxine alkaloids. Yew poisonings are relatively common in both domestic and wild animals who consume the plant accidentally. Taxine B, the most toxic of the taxine alkaloids, is a cardiotoxin which works by disrupting the calcium and sodium currents of the myocardial cells. The taxine alkaloids are absorbed quickly from the intestine and can cause death in high enough quantities due to cardiac arrest or respiratory failure. Ingesting yew causes symptoms such as dizziness, dilation of pupils, abdominal pain, nausea and an irregular heartbeat.

The minimum lethal dose (LDmin) of yew leaves is 3.0-6.5 mg/kg body weight for humans, 1-2 mg/kg for horses and 82.5 mg/kg for chickens. There is currently no known antidotes for yew poisoning but drugs such as atropine have been used to treat the symptoms.