Secondary Metabolites: Secondary Metabolic Products Consisting of C and H; C, H, and O; N, S, and P Elements; and O/N Heterocycles

Abstract

Terpenes and terpenoids, steroids and sterols, volatile oils, miscellaneous isoprenoids, phenols and phenyl propanoids, alkaloids, glycosides, bitter principles, resins, saponins, cardioactive compounds, etc., are important groups of secondary metabolites of plant origin. Terpenes and terpenoids are naturally occurring hydrocarbons, and ~2000 plant species of 60 families produce more than 55,000 terpenes and their derivatives. Terpenes are built from isoprene monomer (C₅H₈), and (C₅H₈)_n. is the basic molecular formula. The oxygen-containing terpenes are called terpenoids or isoprenoids while steroids are cyclic terpenoids, and sterols are steroid alcohols. Terpenoids have significant importance in food, pharmaceutical, and cosmetic industry. Terpenoids contribute to plant essential oils (eucalyptus, lavender, thyme, and mint), flavors (cinnamon, cloves, and ginger), color (yellow—sunflowers, red—tomatoes), etc. They protect plant against predators and pests (e.g., from herbivores, insects, fungi, microorganisms, etc.), aid to pollination and dispersal of spores, and in living organisms function range from pigments and fragrances to vitamins and precursors of sex hormones. Plant sterols, including campesterol, inhibit the absorption of cholesterol in the intestines and thereby reduce LDLs or cholesterol level. Phenols or phenolics are a class of chemical compounds with a benzene nucleus supporting a hydroxyl group and range from simple substances like phenolic acids or phenols, cumarines, flavonoids, and quinines to very complex ones such as lignins and tannins. Phenol and its chemical derivatives are used in the production of detergents, phenoxy herbicides, numerous pharmaceutical drugs, and many industrial synthetic goods. Alkaloids are cyclic bitter organic compounds containing nitrogen in a negative state of oxidation having a marked physiological action on man and other animals. A large variety of organisms produce alkaloids, including bacteria, fungi, plants, and animals. Alkaloids like caffeine, ephedrine, codeine, colchicine, nicotine, pilocarpine, opium, quinine, reserpine, cocaine, psilocin morphine, atropine, berberine, vincristine, yohimbine, etc., are some common examples of drugs principles of pharmaceutical importance and often are used as recreational drugs, or in entheogenic rituals. A glycoside is a heteromolecule consisting of a non-sugar (aglycone) and a sugar part (glycone) components. The glycone may be monosaccharide or oligosaccharide, and the aglycone may be an alcohol, anthraquinone derivative, phenol, aldehyde, acid, ester, or another compound. Glycosides play numerous important roles in living organisms, and many such plant glycosides are used as medications, e.g., the active principles of digitalis, strophanthus, cascara, willow, and poplar barks are being among the most valued remedies. The bitter principles are mostly terpenoid, especially the sesquiterpene lactones, monoterpene iridoids, and the secoiridoids. Diterpene bitters are found in columbo root and white horehound, and triterpenoids are the cause of bitterness in Cucurbitaceous plants, which is due to cucurbitacins. Plant lignans are diphenolic compounds (phenylpropanoids dimers) whose structure is the union of two units of phenylpropane. Tannins are non-nitrogenous bitter plant polyphenolic compounds having a molecular weight between 500 and 3000 (gallic acid esters) and up to 20,000 (proanthocyanidins). They are non-crystallisable colloidal compounds and may be (i) hydrolyzable tannins, which consist of gallic acid or related polyhydric compounds esterified with glucose, and they are readily hydrolysed to yield the phenolic acids and the sugar; and (ii) non-hydrolyzable or condensed tannins contain only phenolic nuclei and most of such tannins are formed by the condensation of two or more flavanols, such as catechin. Pharmaceutically, tannins have antibacterial, antiviral, antiparasitic, astringent, and antiseptic properties, and may be used in the treatment of hemorrhages (constrict of blood vessels), burns (cicatrizing), diarrhea, and as an antidote for alkaloid poisoning because of their ability to precipitate alkaloids; effective against 6-hydroxydopamine-induced toxicity and also have anti-inflammatory and antiulcer activity. Quinones are cyclic organic compounds (aromatic diketones) and are found in bacteria, in certain fungi, in various higher plant forms, and in a few animals (e.g., sea urchins, aphids, lac insects, and certain scale insects). It is highly active anti-microbacterial, antifungal agent and highly toxic and fatal if swallowed, inhaled, or absorbed through the skin and widely used in medicine, herbicides, chemical reagents, dyes, and tanning agents. Saponins are amphiphilic glucoside molecules composed of hydrophilic glycoside glycone and lipophilic triterpene or steroid aglycone. Saponins have been used in medicine, foaming agents, in fire extinguishers, and fish poisons. Drugs that influence heart or drugs having an influence on the heart are cardioactive drugs. (i) Beta-adrenoceptor antagonists, (ii) calcium channel blocking drugs, and (iii) cardiac glycosides are three major classes of cardioactive drugs. Cardiac glycosides are also important in the pathogenesis and therapy of different human diseases (e.g., stroke, diabetes, neurological diseases, cancer, etc.). Cardioactive steroids are a class of animal and plant-derived compounds with a steroid nucleus and a specific inotropic, chronotropic, and dromotropic effect. Cardioactive steroids (CAS) became the mainstay of treatment for congestive heart failure and to control the ventricular response rate in atrial tachydysrhythmias. Antibiotics are produced by different groups of microorganisms like bacteria, fungi, and actinomycetes and in many cases by higher plants. Antibiotics in low concentration are capable of inhibiting the growth of microorganisms through an antimetabolic mechanism. They differ from antiseptics and disinfectants in their mode of action, chemical, and physical properties. The development of resistance among the microorganisms on prolonged contact with the drug is the present-day problems in the field of antibiotics. The microbial and plant sources from the terrestrial and marine environments are now providing natural products with antitumor activity.