Plants are a regular cause of medical problems, usually due to the chemicals they contain. The 250,000 different flowering plant species differ not only in shape but also in countless biochemical properties. In all plant species only a handful of photosynthetic pigments are found. Since all plants need the same basic materials such as light, carbon dioxide and water, the biochemical variety of plants has little to do with the mechanism of photosynthesis. Why then should plants produce special chemical substances? The answer lies in their immobility. They cannot move themselves to reproduce or to escape from enemies. The chemical substances they produce serve not only to repay animal pollinators and seed distributors, but also to protect them from animals which form a threat. However, these substances may cause problems in humans. The problems vary widely, from skin irritation (e.g. phytodermatitis) to thyroid problems (goitrogenic plants) and neurological syndromes (e.g. konzo and buckthorn paralysis). Below are some examples, but it is not possible here to provide a complete survey of this material.

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Plants have always occupied an important place in medicine, chiefly as a source of drugs. Plant products are still used today, from nose drops containing ephedrine to the psoralens used in PUVA therapy, from podophyllotoxin for condylomata acuminata to papaverine as an antispasmodic. Other examples are taxol (paclitaxel, initially obtained from the bark of the yew tree Taxus brevifolia), morphine (from the opium poppy or Papaver somniferum), digitalis (from the foxglove or Digitalis lanata), quinine (from the bark of the cinchona tree, Cinchona ledgeriana), artemisinin (from sweet wormwood or Artemisia annua), pilocarpine (from the Calabar bean Pilocarpus jaborandi), reserpine (from Rauwolfia serpentina), vincristine (from pink periwinkle, Vinca or Cantharanthus roseus), colchicine (from autumn crocus or Colchicum autumnale) and atropine (from deadly nightshade or Atropa belladonna). It is now cheaper to produce some of these synthetically. Further active substances have also been discovered, in some cases via the analysis of plants which have been used in traditional medicine in various geographical regions (ethnobotany). Some plants contain chemicals which may exhibit interactions with drugs. Consider the ingestion of grapefruit juice or St John's wort and the changing blood levels of certain anti-HIV medications.

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Many plants contain proteins which have a very strong affinity for particular sugar groups. These carbohydrate-binding proteins are called lectins. There can be several binding sites on each lectin. Concanavalin A from the jack bean, binds to alpha-mannose residues. Pokeweed mitogen is a lectin which encourages cell division. Pokeweed or Phytolacca americana comes originally from the Eastern USA. The word "Poke" is taken from an Indian language, the Algonquian word "pakon" or "puccoon," which refers to the use of the plant in coloured paints. The name Phytolacca comes from the Greek word phyton (plant) and the French lac ("lake": a dark red pigment), which refers to the deep red berries. The pokeweed mitogen stimulates chiefly B lymphocytes and is used in the lymphocyte transformation test.

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Phytohaemagglutinin is a lectin from red kidney beans (Phaseolus vulgaris). This lectin agglutinates red blood cells via cross-linking, because it binds well-determined oligosaccharides which are found on the erythrocytic membrane. Red kidney bean poisoning, also known as Kinkoti bean poisoning, follows consumption of raw or undercooked red kidney beans. Symptoms appear after 1 to 3 hours after ingestion. The patient will develop extreme nausea and vomiting. A short lasting diarrhea will occur one to a few hours later. Some persons report abdominal pain. As a general rule, there is a spontaneous and rapid recovery (3-4 h).

In the past steroid hormones had to be prepared from adrenal cortex or ox bile, both very expensive methods of preparation. Another, cheaper solution was needed. Many plants contain saponins, chemicals which have a steroid skeleton. This non-polar steroid part (sapogenin) is bound to a water soluble sugar side chain. "Saponin" (L. sapo = soap) refers to the fact that these substances produce a soap-like foam when they are shaken with water, an illustration of their chemical structure mixed polar/non-polar structure. In the years 1936-40 it was discovered that some species of yams of the genus Dioscorea (fam. Diascoreaceae) contained large amounts of saponins. Two Central American species, Dioscorea floribunda and D. composita contained considerable quantities of diosgenin, a sapogenin. This was a good starting product for chemical synthesis. It was an unusual application of these plants that are used in many tropical countries as food. They have large thickened underground tubers. They should not be confused with cassava (Manihot esculenta) from the family of spurges (Euphorbiaceae) or with sweet potatoes (Ipomoea batatas) from the family of Convulvulaceae. The saponin is extracted from the tubers. The sugar chains are then split off and micro-organisms are used to remove other chemical groups and to add new groups. In this way semi-synthetic steroid hormones are obtained. The production process proved to be inexpensive. Various anabolica, cortisone and hydrocortisone were produced in this way and brought about a revolution in medicine. The majority of hormones prepared from diosgenin are at present used for contraceptive pills. The impact which inexpensive contraceptive pills has had on the growth of the population and on the position of women in many communities, should not be underestimated. One alternative source of steroid precursors is Solanum aviculare.