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Easily confused with : Irish Yew and Japanese Yew , neither of which are native but still widely planted. There are
Yew is widely planted in parks and especially in churchyards and in the grounds of other very old buildings, going back centuries.
The arils are not berries, but contain the female seed cone. The tree uses the aril to attract birds which feed on the pulpy red surround, depositing the in-edible seed cone elsewhere to propagate the species. There are also male cones, and these are inconspicuous and look like miniature brussel sprouts.
The leaves and stems are poisonous to most (if not all) mammals, including humans, but not the fleshy red arils (although the seed cone contained within it may be very poisonous). Yew contains a varied cocktail of toxic diterpene Taxanes and Baccatins, the Toxanes block Na+ and Ca2+ channels in heart cells, whilst Taxol and Decetaxel are spindle poisons, inhibiting cell division by preventing microtubules from de-polymerising. Symptoms of poisoning include mydriasis, nausea, vomiting, dizziness, diarrhoea, kidney damage. At first breathing is stimulated and the victim may hyperventilate, leading to acidosis; later breathing is suppressed. Pulmonary spasms follows, then coma and death from respiratory and circulatory failure in just 2 to 24 hours. Treatment is possible.
Taxol, or Paclitaxel, was first discovered in the bark of the rare Pacific Yew Tree (which is not native to the UK) from where it was commercially extracted until a method of laboratory synthesis was devised. It is also present in our Yew. It was found to be a very effective treatment for some cancers, but not all; cancer is not one disease, but many. Taxol stabilises microtubules against disassembly, inhibiting cell division (in both normal growing, and in cancer). It is now marketed under the Generic name Taxol and brand name Paclitaxel. Taxol consists of three condensed rings, one of 4, five of 6 and the other of 8 members. The eight-membered ring, oxetane (shown in blue), is crucial to the drugs activity, as is the benzoyl group (shown in red).
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Taxol is just one of possibly seven Taxines and Taxoids (which include Taxine B, Isotaxine B and Cephalomannine) which are the main toxic pseudo-alkaloids and deadly poisonous constituents of Yew trees.
10-deacetyl Baccatin III was later discovered in the leaves of both the rare Pacific Yew Tree and the much more common Yew. As can be seen, it is more of a basic sub-block, still containing the requisite 8-membered ring (in blue) and the O-benzoyl group (in red). It is now used as a starting compound (from Taxus Baccata only) in the manufacture of both Paclitaxel (Taxol) and Taxotere (Docetaxel), thus saving the rare Pacific Yew tree from assured extinction by over-exploitation.
Cephalomannine, aka Taxol B, is almost identical to Paclitaxel, but part of a phenyl group is missing, shown in green. Cephalomannin was probably first discovered in Cephalotaxus fortunei a species of Yew Tree which in the UK is found only in Pembrokeshire.
Taxine does not have an 8-membered ring as do Taxol, Docetaxel, Baccatin derivatives and Cephalomannine, but a 9-membered ring, depicted in magenta. Also, the long side-chain containing the nitrogen atom has swapped allegiance to the opposite side of the multi-membered ring. Several other features are absent too.
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Docetaxel, which does not occur naturally, is related to Taxol which does. The only difference between Taxol and Docetaxel is depicted in green, where some further atoms are also missing. Again it has the 8-membered ring rather than the 9-membered ring of Taxine. It is manufactured by chemically esterifying 10-deacetyl baccatin III, which is obtained from the leaves of the much more abundant Taxus Baccata (Yew), shown on this page. Docetaxel is marketed under the brand name of Taxotere as a drug to treat breast, prostrate, ovarian, melanomas and lung cancers. The modus operanus is much the same as for Paclitaxel (Taxol) although it appears to be more effective, increasing survival periods by several more months. Thus neither appear to actually cure cancer.
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TOXINS and PATHOGENS, the never-ending war.
From these, it is immediately apparent that synthesis of toxins by plants is not always a well targeted affair, for not only are several intermediate compounds fabricated (as would be expected anyway) but a whole plethora of 'wrongly' assembled molecules are generated in, what must be, a stochastic generation process. A scattergun approach. Whilst this may seem counter-productive to us, to the plant trying to protect itself from invading pathogens and hungry beasts it matters not as long as all or most of these compounds are poisonous. In fact, it can be of huge benefit to the plant not to selectively manufacture just one toxin, but instead a bizarre selection. The attacking microorganisms or famished creature could, by natural selection, develop resistance to one certain toxin, but is most unlikely to develop resistance to a huge arsenal of haphazardly assembled toxins.
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