The Chemicals of Plant-Derived Medicines
Plants synthesize a wide array of secondary compounds that play a role in the physiology of plants but do not usually constitute an important part of the basic metabolism of plants. Secondary compounds enable plants to attract animals and also help plants to avoid or overcome their natural enemies of infection, parasitism, and predation. These secondary compounds are the main chemicals in plants that humans use as medicinal herbs. Fatty acids, essential oils, gums, resins, alkaloids, and steroids are the most common secondary compounds in plants.
Humans use oils and gums as purgatives and as carriers or emulsifiers in many drug preparations. Volatile oils and resins are often used to help processes that seek to penetrate tissues of the body and are also used as antiseptics. Alkaloids and steroids are the two major classes of plant-derived compounds used in human medicine today. These chemical compounds can occur in different forms that have one or more sugar molecules attached. Such forms, called "glycosides," are often the medicinally active form of a compound.
All forms of steroids are complex chemical compounds that have the same fundamental structure of four carbon rings, called the "backbone." When different chemical groups are added at different places on the backbone, a variety of steroidal compounds are produced as a result. For example, when sugar molecules are added to the carbon rings, steroidal glycosides are produced. Various cardiac glycosides and steroid hormones are produced by the addition of specific side chains or extra rings to the steroid backbone.
The second major group of medicinally important chemicals synthesized in plants is the alkaloids. They contain nitrogen and usually exhibit an alkaline reaction. Alkaloids were formerly considered secondary products, but unlike steroids, they have recently been shown occasionally to enter into the primary metabolism of plants. Some alkaloids are extremely poisonous to humans, and many have been used as poisons in various cultures around the world. Several of these plant chemicals possess antimicrobial properties and are used to kill harmful microorganisms that are pathogenic. A number of them are used as dietary supplements for balanced human nutrition and good health.
Medicinal Plants of Importance
In the past, some natural chemicals and oils were of tremendous medicinal use in treating diseases. Quinine was used for the treatment of malaria, cocaine was used as a stimulant and a local anesthetic, and chaulmoogra oil was employed for the treatment of leprosy. Although these herbal medicines are rarely used today, many plants are still of great importance as sources of medicinal compounds. Both steroids and glycosides occur in many angiosperms.
To cite a few examples, certain members of the yam genus
Dioscorea contain particular kinds of steroids in their tubers called "saponins" that are similar to human sex hormones. The chemical diosgenin can be extracted from the tubers, which is a good starting point for the chemical synthesis of saponins. The majority of the hormones synthesized from diosgenin are used in birth control pills, for the production of hormones that regulate the menstrual cycle, or as a component of fertility drugs. Cortisone and hydrocortisone are two other important hormones that are synthesized from diosgenin. They are used for the treatment of severe allergic reactions, arthritis, and Addison disease, which is caused by malfunctioning of the adrenal glands.
Opium poppy, Papaver somniferum, is one of the oldest and still predominant sources of analgesics, which relieve pain. More than twenty-six different alkaloids have been isolated from opium, but only three of them—morphine, codeine, and papaverine—are used extensively. Morphine and codeine are used as painkillers, and papaverine is used primarily in drugs for the treatment of internal spasms, particularly those of the intestinal tract.
Another group of alkaloids is obtained from members of the potato family. They are considered analgesics and are used for controlling a variety of muscle spasms and in psychiatry. Alkaloids from Atropa belladonna are prescribed for stomach and bladder cramps and to prevent nausea and vomiting caused by motion sickness. They are prescribed for victims of Parkinson’s disease to decrease stiffness and tremors and are often given to patients before surgery as a relaxant and to reduce salivation. These alkaloids, especially atropine, may be helpful in cases of nerve gas (organophosphate insecticide) or mushroom poisoning. Alkaloids of the North American lily such as American false hellebore, Veratrum viride, have hypotensive properties and are used to treat high blood pressure.
Cinchona, the genus known primarily as a source of quinine, produces about thirty other alkaloids, including the compound quinidine, which is useful in treating heart disease. Quinidine inhibits abnormal rapid contractions of the upper right chamber of the heart and corrects improper heart rhythms.
Rauwolfia serpentina was used in the past for treating snakebites. It was later found to be useful in the treatment of hypertension by relaxing the heart muscle and thus lowering blood pressure; however, it produced the side effects of depression and tremors, and its use was therefore discontinued. In 1952, in Switzerland, the important alkaloid reserpine was isolated from the root where it is concentrated, although it occurs everywhere in the plant. The dramatic effects of reserpine completely altered practices in mental institutions because of its pronounced calming effect on schizophrenics without producing undesirable side effects. A relatively New World species, R. tetraphylla, is also a source of the alkaloid.
The few known substances that are able to arrest cancer
cells are plant alkaloids. The common periwinkle, Catharanthus reseus, has been used in its native range in Europe for hundreds of years as a folk treatment for diabetes. It is now used effectively to treat some cancers and leukemias, especially those that commonly afflict children. The two active alkaloids are vinblastine and vincristine. Mayapple, Podophylum peltatum, contains antitumor alkaloids. Today, mayapple alkaloids are used as the basis of VM-26 (teniposide), a drug used to treat testicular tumors and, with other agents, breast and lung cancer. The alkaloid colchicine is extracted from the corms of the autumn crocus, Colchicum autumnale. Colchicine is primarily used for the reduction of inflammation and pain caused by gout but is also used in the treatment of cancer. Taxol, a compound most abundant in the bark of the Pacific yew, Taxus brevifolia, has been a major success in the treatment of breast and uterine cancer.
Several mucilaginous compounds of plant origin are used in soothing ointments and as carriers for other medicines. Species of Aloe, primarily A. barbadensis, have been used for their soothing gels. Chymopapain, an enzyme that exhibits specificity in its dissolution of proteins, is obtained from papaya, Carica papaya. It is injected by doctors into the soft central area of a deformed spinal disk in humans to dissolve a large part of it and relieve the pressure on adjacent nerves.
Perspective and Prospects
The medicinal uses of plants by humans have been known since ancient times and can be said to predate written history. Every culture on earth has used plants to cure disease, ease pain, and heal the ills and discomforts of the human body. People first started to keep records of herbal medicine about five thousand to seven thousand years ago in China and Mesopotamia. Sumerian drawings of opium poppy capsules from 2500 b.c.e. suggest that considerable knowledge of medicinal plants was in place. A substantial record of the use of herbs in medicine comes from the Code of Hammurabi, a series of tablets carved under the direction of the king of Babylon in about 1770 b.c.e. These tablets mention plants, such as henbane, licorice, and mint, that are still used in medicines. The Egyptians later recorded their knowledge of illnesses and cures on temple walls and in the Ebers papyrus (1550 b.c.e.), which contains more than seven hundred medicinal formulas.
In Greece, Hippocrates (ca. 460–ca. 370 b.c.e.) prescribed sound nutrition, purgatives, and botanical drugs for humans. He consequently earned the reputation as the founder of medicine by being the first person to document illnesses and their treatments in a rational and orderly fashion. The most significant contribution made by the Greeks toward the documentation of plants with healing properties was made by Dioscorides in his five-volume work entitled De Materia Medica. His encyclopedia described the preparation of about one thousand simple drugs. For several centuries afterward, it was the foundation text for practitioners of herbal medicine throughout Europe.
A stronger link was established between the studies of botany and medicine during the Middle Ages, and printed herbals became more available with the invention of the printing press in 1439. More herbs were added to the list from the New World when Europeans arrived in North America and learned American Indian herbal uses. In the fourteenth century, the Renaissance led Europe into the determination of the medicinal uses of plants.
In the seventeenth and eighteenth centuries, science and philosophy advanced to the stage of experiments and hypothesis testing. It was not until the early nineteenth century, however, that scientists first began to isolate and extract healing compounds from plants. This experimental approach to medicine led to an improved understanding of physiology and provided a framework for the careful testing of medical treatments, including medicinal herbs.
The first half of the twentieth century saw tremendous advancements in medicine as more causes of diseases were discovered and new effective drugs were produced. Several modern medicines were produced as isolated and purified products of traditional plant-derived extracts, including morphine, quinine, and ephedrine. Medical chemists then began to determine the structures of these compounds and the possibility of their synthesis. In addition, they explored the chances of using the knowledge of the active ingredients of a natural healing herb to synthesize chemically related compounds that were potentially better medicines than the original one.
In the latter part of the twentieth century and the beginning of the twenty-first century, the prevalence of some diseases in some parts of the world and the emergence of diseases such as acquired immunodeficiency syndrome (AIDS) and severe acute respiratory syndrome (SARS) have challenged scientists, botanists, and doctors to explore plant sources for drugs that will offer possible or better cures.
Medicinal herbs are central to alternative therapies, which are gaining popularity in the twenty-first century. This trend is partly attributable to modern research into plant medicine and the remarkable healing results of herbal application to some diseases. For example, years of studies have shown that garlic can help control blood pressure and cholesterol. Yet few mainstream doctors recommend it, even though garlic is cheaper than pharmaceuticals and causes fewer side effects. This situation is beginning to change, however, because of a growing interest in natural sources of medicine. An estimated 80 percent of the world’s population still rely on traditional medicine, and particularly herbs, for treating and preventing disease. Native American herbs are still used by North American doctors in the twenty-first century. Many people do not realize that medical herbs are a key link between alternative therapies and mainstream medicine. Scientists around the world depend on herbs to develop new, more potent medications, and the search continues for plants with healing properties.
According to the World Health Organization, herbal medicines were regulated in over 100 countries in 2008. However, in the United States, herbal remedies do not require testing before use by the public, and the 1994 Dietary Supplement Health and Education Act (DSHEA) does not require proof of safety or efficacy. Since the Federal Food and Drug Administration (FDA) issued good manufacturing practices for the industry in 2007, US herbal supplement manufacturers have been required to ensure the "identity, purity, strength, and composition" of their ingredients.
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