PLANTS IN MEDICINE
Ethnobotany is the science of the relationship between the chemical constituents and properties of native species of plants and their uses by indigenous peoples. As plants contain a wide range of different, biologically active secondary metabolites, the discovery of compounds new to western medicine is an important aspect of this science.
The cardiac glycosides or carnolides based on digitoxin and digoxin inhibit the heart Na+/K+-pump. Applied at appropriate dose, they stabilize the heart rhythm and are used to treat heart failure and similar conditions.
Alkaloids are a wide range of compounds with many applications. The quinoline alkaloid quinine, the isoquinolines morphine and codeine, and the indole alkaloids vinblastine and vincristine, are all widely used medicinals.
The terpene taxol, from the bark of the Pacific yew, stabilizes microtubules and thereby inhibits cell division in tumor cells. It is therefore a potent chemotherapy agent in cancer treatment.
Historically, botany and medicine were closely allied subjects as most medicines were herbal, using plant materials to supply medicinally active drugs. The earliest botanic gardens were dedicated to the production of plants with medicinal properties. Human beings have used plants as medicines for millennia and there is a rapidly growing interest in identifying medicinally active plant products. This involves everything from learning about traditional medicines to the assessment of crops and native floras for the production of new medicines. The study of ethnobotany, the study of plant species and their uses in indigenous societies, has been inspired by awareness of rapid loss of species diversity and of the importance of plant derived drugs. Ethnobotany seeks to record the uses of plants by societies and wherever possible, to conserve those species for future use.
Medicines based on plant products include many different compounds with many different uses. The cardiac glycosides, based on the compounddigitonin from foxglove (Digitalis purpurea) are widely used to treat heart disease. Drugs based on the plant alkaloids, the opiates, derived from the opium poppy (Papaver somniferum) are amongst the most effective pain-relievers known. Many anti-cancer agents such as taxol (derived from the pacific yew, Taxus brevifolia)and vincristine and vinblastine (derived from the periwinkle, Vinca rosea) are also plant products.
Plant-derived medicinal compounds are plant secondary products.
Glycosides are a very diverse group of compounds. The cardiac glycosides (or cardenolides) contain sugar residues bonded to sterols; in one example (digitoxin, from the foxglove Digitalis purpurea), the sugar residues are one glucose molecule, two digitoxose molecules and one molecule of 1-acetyl digitoxose. Digitoxose is a rare 6-carbon sugar. A second cardiac glycoside, digoxin is alsopresent in the foxglove.
Cardiac glycosides are extremely toxic compounds that inhibit the heart Na+/K+-pump. At a suitable dose, they slow and strengthen the rhythm of the heart and are very effective in the treatment of heart failure and other heart conditions.
Alkaloids are also a diverse group of compounds, all of which contain nitrogen, usually as part of a heterocyclic ring. They are synthesized from amino acids. Many alkaloids are extremely poisonous; others, including codeine and morphine are very effective pain killers, while others are addictive drugs (nicotine, cocaine). Table 1 presents some of the alkaloids and their effects.
Terpenes are synthesized either from acetyl coenzyme A (CoA) or from 3-phosphoglycerate and pyruvate and are based on the 5-carbon isoprene unit . Complex terpenes are generally believed to be anti-herbivory defenses in plants; however, amongst the very diverse range of terpenes are essential oils such as the menthols (used as insect repellants and in proprietry remedies for colds and coughs). The most significant medicinal terpene of recent years has been the diterpene taxol, isolated initially from the bark of the Pacific yew, Taxus brevifiolia. Taxol causes a blockage of cell division in tumor cells by stabilizing and polymerizing microtubules. Taxol is effective against solid tumors. Its initial discovery led to a considerable effort to secure sufficient supplies of the compound, as its production (like many plant secondary products) is limited to one tissue (the bark) of a single species. Application of plant tissue culture was less successful than attempts at organic (chemical) synthesis and taxol-based drugs are now produced by this means.