The term “pharmacognosy”, formed from the Greek words pharmakon (drug) and gnosis (knowledge), was first used in 1811 in Vienna to describe the study of medicinal plants and their properties. By the mid-twentieth century, the study of medicinal plants underwent a significant shift as the medical and pharmaceutical communities migrated from botanicals to synthetic drugs. As a result, the field of pharmacognosy faded to the background and medicinal chemistry became the hook upon which the medical community chose to hang its hat. Renewed interest in herbal medicine has brought medicinal plants to the forefront again, yet many of the terms associated with plant activities are not well-known. But familiarity with the eight categories of plant-based chemical constituents can begin to unravel the mystery.
Because plants are immobile and lack the classic immune response that animals possess, they need the following compounds either to ward off predators and infectious microbes or to attract pollinators.
Complex polysaccharides are groups of sugars linked together. Starch, inulin, and celluloses are all complex polysaccharides. When energy is needed, plants convert starch to glucose. Inulin yields fructose when it reacts with water. Both starch and inulin are used extensively in food products and pharmaceuticals. Cellulose constitutes the primary part of the cell wall in plants and is the most abundant organic compound on earth. Raw cotton is 91 percent cellulose.
Glycosides are compounds that yield one or more sugars plus a nonsugar component when broken down. They play an important role in the life of the plant and are involved in its regulatory and protective functions. Glycosides encompass a variety of compounds, including therapeutically active agents. Plants that act as purgatives, such as senna, aloe, rhubarb, and cascara sagrada, contain glycosides. Glycosides from the digitalis plant are used as a cardiac stimulant.
Lipids, which include fats, oils, and waxes, are esters of long-chain fatty acids and alcohols and are insoluble in water but soluble in common organic solvents. Fats and fixed (nonvolatile) oils are obtained from either plants (for example, olive and peanut oils) or animals (for example, lard). Lipids, together with proteins and carbohydrates, are the main structural material of living cells and function primarily to store food for the plant. They are used in pharmaceuticals, industry, and food. Fixed oils and fats of vegetable origin may occur in any part of the plant, but generally, seeds contain more than the rest of the plant. Sunflower seeds, corn, almonds, and sesame seeds all contain fixed oils. Waxes may also be of plant or animal origin. In plants, waxes protect against water loss. They are used in pharmaceuticals to harden ointments and creams and in other products as protective coatings.
Steroids are a class of lipids found widely in nature. Many steroids are biologically active sex hormones, including progesterone, testosterone, and estrogen. Other steroids, such as the corticosteroids, several forms of vitamin D, cholesterol, cardiac glycosides, and bile acids exhibit a wide range of therapeutic applications. They are used as anti-inflammatories, cardiac medications, oral contraceptives, and as building blocks for other biological chemicals.
Terpenoids (or terpenes) are defined as hydrocarbons found especially in essential oils, resins, and balsams. They are widely distributed in nature and are abundant in higher plants. Fungi and marine animals also produce a large variety of terpenoids. Scientists have isolated approximately 20,000 terpenoids, an amount that far exceeds any other group of natural constituents. They are involved in almost every possible interaction between plant and animal, plant and plant, or plant and microorganism—as insect repellents, pollination attractants, defense mechanisms against herbivores, and plant hormones. Terpenoids include vitamin E, vitamin K, camphor, menthol, thymol, and taxol, to name a few.
Phenylpropanoids are derived from aromatic amino acids. Also known as plant phenolics, these constituents combine with or react to other phenylpropanoids and compounds to form flavonoids, lignans, coumarins, and tannins. Flavonoids are abundant in the plant kingdom and number more than 2,000. The flavonoid quercetin, found in onions, is reputed to help treat symptoms of the common cold. Lignans play an important role in plant defense as antimicrobial and antifungal agents. Mayapple contains the powerful ligan podophyllotoxin, a compound with strong purgative action that also prevents cell division. Coumarins are fragrant crystalline compounds with a bitter taste. Extracted from plants such as sweet clover and tonka beans, coumarins were first used as flavoring agents but are now used for their anticoagulant and antispasmodic properties. Tannins are a large group of astringent complex substances; those found in witch hazel leaves give them their astringent properties.
Alkaloids represent a varied group of compounds that come chiefly from vascular plants and some fungi. Many alkaloids have potent physiological effects on mammals. The alkaloids atropine, morphine, quinine, and vincristine are used to treat a gamut of diseases ranging from malaria to cancer. Other alkaloids, such as caffeine, nicotine, and cocaine, have strong psychotropic effects. Except for the fact that they are organic nitrogenous compounds with a limited distribution in nature, alkaloids are difficult to define because they do not represent a homogeneous group of compounds in terms of chemical, biological, or physiological properties.
Proteins and peptides were used as drugs long before the biotechnology revolution. Proteins, the principal constituents of the protoplasm of all cells, are combinations of amino acids joined together by peptide bonds. Proteins include many substances, such as enzymes, structural elements, hormones, and immunoglobulins (antibodies), and are involved in oxygen transport, muscle contractions, electron transport, and photosynthesis. The protein diastase, found in barley, is a digestive enzyme, and the protein somatropin acts as a growth hormone. A peptide is a compound that yields two or more amino acids when broken down.
Almost thirty years ago, a detailed survey of the use of natural products showed that about one-fourth of all prescription drugs contained plant components. Today, plant-based steroids alone account for 15 percent ($22 billion) of the annual parmaceuticals market. Based on these figures, drugs from higher plants have at least maintained their market share and, in all probability, increased their share of the total drug market.
This primer was compiled by the Herbs for Health staff from Pharmacognosy and Pharmacobiotechnolgy (Williams & Wilkins, 1996) by James E. Robbers, Marilyn K. Speedie, and Varro E. Tyler.
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