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"Secondary metabolites that can be created from primary ones are not directly involved in the normal growth, development, and reproduction of the organism. They are molecules which are primarily involved in the overall maintenance/homeostasis of the organism. Secondary metabolites specifically modulate health-maintaining processes, including excretion of waste and toxic products from the body. That means, sustaining the overall health and functional status of the cells within organ systems of the body, the principal function of secondary metabolites."
Plants produce secondary metabolites to aid in self-defense.
Secondary metabolites support plant survival and species propagation because they act as physical defenses against predators. For example, lactucin in chicory leaves is toxic to slugs. Its presence in leaves deters slugs from consuming and harming the species. Some secondary metabolites act as warning signals to other plants of the same species to inform them that danger is present. Other metabolites attract pollinators with their aesthetic hue.
Secondary metabolites have been found to have interesting applications over and above their well-known medical uses, e.g., as antimicrobials, etc.
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These alternative applications include antitumor, cholesterol-lowering, immunosuppressant, antiprotozoal , antihelminth , antiviral and anti-ageing activities.
Secondary metabolites, including antibiotics, are produced in nature and serve survival functions for the organisms producing them. The antibiotics are a heterogeneous group, the functions of some being related to and others being unrelated to their antimicrobial activities. Secondary metabolites serve:
"Bioactive compounds in plants are compounds produced by plants having pharmacological or toxicological effects in man and animals.
Although nutrients elicit pharmacological or toxicological effects when ingested at high dosages (e.g. vitamins and minerals), nutrients in plants are generally not included in the term bioactive plant compound. The typical bioactive compounds in plants are produced as secondary metabolites. Thus, a definition of bioactive compounds in plants is: secondary plant metabolites eliciting pharmacological or toxicological effects in man and animals."
Secondary Metabolites role as Bioactive Component:
The following is a brief presentation of the main chemical groups of bioactive compounds in plants:
The glycosides consist of various categories of secondary metabolites bound to a mono- or oligosaccharide or to uronic acid. The saccharide or uronic acid part is called glycone, and the other part the aglycone. The main groups of glycosides are cardiac glycosides, cyanogenic glycosides, glucosinolates, saponins and anthraquinone glycosides. Furthermore, flavonoids frequently occur as glycosides. Following ingestion the glycosides usually hydrolyse in the colon, and the more hydrophobic aglycone might be absorbed.
The aglycones of cardiac glycosides have a steroidal structure. Their effect is inhibition of Na+/K+-ATPase-pumps in the cell membranes. These pumps are concentrated in and critical for the functioning of the cardiac cells and the effects from these compounds are very pronounced in the heart, resulting in increased contractility and reduced rate. The cardiac glycosides are present in plants of Scrophulariaceae (figwort family) particularly Digitalis purpura (foxglove) and in Convallariaceae (convall family) with Convallaria majalis (lily of the valley) as a typical example.
The cyanogenic glycosides have aglycones derived from amino acids. Several of these compounds can interfere with the iodine utilisation and result in hypothyroidism. The other important effect is via their release of hydrogen cyanide, which is very toxic being lethal at high dosages. Cyanogenic glycosides are present in species of Rosaceae (rose family) in particular in Prunus spp.
The glucosinolates contain sulphur-containing, pungent amino acid-derived aglycones. The compounds show a complex set of effects on cytochrome P450 isoforms in various cells and tend to decrease hepatic bioactivation of environmental procarcinogens. The glucosinolates can be skin irritating and also induce hypothyroidism and goitre. The Brassicaceae (brassica family) is the family mainly associated with glucosinolate production.
Most saponins - "soap forming compound"- occur as glycosides. The aglycones consist of either pentacyclic triterpenoids or tetracyclic steroids. They are structurally distinct, but have main functional properties in common. The saponin glycosides are large molecules with a hydrophilic glycone and a hydrophobic aglycone, which give emulsifying properties and can be used as detergents. Saponins show immune modulating and antineoplastic effects. A common in vitro effect is haemolysis of red blood cells. However, this effect does not seem to be an in vivo problem. Some saponins induce photosensitisation and jaundice. Saponins are present in a range of plant families. Among them is Liliaceae (lily family) with the important sheep toxic plant Narthesium ossifragum (bog asphodel).
Anthraquinone glycosides show a relatively limited distribution within the plant kingdom. In Polygonaceae (dock family) they are present for instance in Rumex crispus (curly dock) and Rheum spp (rhubarbs). Their primary effect is induction of water and electrolyte secretion as well as peristalisis in colon.
Flavonoids consist of a central three-ring structure. Proanthocyanidins are oligomers of flavonoids. Both groups of compounds can occur as glycosides. All compounds contain phenol-groups involved in an effect as general antioxidant. Other actions are diverse - several structures reduce inflammation or carcinogenicity. The group isoflavones are primarily known as phytoestrogens. Flavonoids and proanthocyanidins are all pigments occurring in a long range of plant families. Isoflavones are produced by species of Fabaeceae (bean family).
There are two distinct types of tannins. Condensed tannins which are large polymers of flavonoids and hydrolysable tannins which are polymers composed of a monosaccharide core (most often glucose) with several catechin derivatives attached. The two types of tannins have most properties in common, but hydrolysable tannins are less stable and have greater potential to cause toxicity. The water solubility is restricted and decrease in general with the size of the tannin molecule. Tannins indiscriminately bind to proteins and larger tannins are used as astringents in cases of diarrhoea, skin bleedings and transudates. Tannins are very widely distributed in the plant kingdom. Examples of plant families associated with precence of tannins are Fagaceae (beech familiy) and Polygonaceae (knotweed family).
The terpenoids are synthesized via the five-carbon building block isoprene. Monoterpenoids consist of two isoprene units and sesquiterpenoids of three units. They are referred to as low-molecular-weight terpenoids and represent the most diversely category of plant constituents with more than 25,000 individual compounds identified. The less diverse phenylpropanoid are based on a ninecarbon skeleton and are synthesised via another pathway. Compounds of all three groups are lipophilic and tend also to volatilise readily. They have strong odours and flavours. Their actions vary greatly, a range of which have been utilised in herbal remedies. Of particular importance are antineoplastic, antibacterial, antiviral effects as well as gastrointestinal stimulation. However, they are not associated with toxicity unless they are concentrated as volatile oils. The plant family best known for these compounds is Lamiaceae (thyme family) but are also present in a range of other families.
Diterpenoids are composed of 4 isoprene units (20 carbons). They are very lipophilic and tend to have strong flavours, but are not volatile and thus, odourless. Much less toxicological information is available on the diterpenoids than on the lower molecular terpenoids. Several of the compounds possess antineoplastic activity. Diterpenoids are found in several plants, among them Coffea arabica (coffee). Diterpenoids are also typically present in resins (see below).
The resins are complex lipid-soluble mixtures - usually both non-volatile and volatile compounds. The non-volatile fraction may consist of diterpenoid and triterpenoid compounds, and mono- and sequiterpenoids predominate in the volatile fraction. Most typical are resins secreted by wood structures, but resins are also present in herbaceous plants. They are all sticky and the fluidity depends on their contents of volatile compounds. When exposed to air they harden. Most resins are antimicrobial and wound healing, but their actions depend on the composition of the chemical mixture. Resins are generally safe, but contact allergy may occur.
Lignans Lignans are composed of two phenylpropanoid units to form an 18-carbon skeleton, with various functional groups connected. They are generally lipophilic and have structural functions within the plant cell membranes. Lignans are present at highest concentrations in oil seeds, but are also found in other parts of a long range of plants of different families. Several lignans show clinical activity as phytoestrogenic, cathartic or antineoplastic effects.
The alkaloids are heterocyclic, nitrogen containing compounds, usually with potent activity and bitter taste. They are of limited distribution in the plant kingdom. The various groups have diverse clinical properties.
Tropane alkaloids are present in Solanaceae (nightshade family) for instance Atropa belladonna (deadly nightshade) Datura spp (thorn apples). and Hyoscyamus niger (henbane). The compounds have anticholinergic activity (muscarine receptor antagonists) and are used medically to reduce smooth muscle spasms, hypersecretion and pain.
Pyrrolizidine alkaloids are produced in Asteraceae (daisy family), particularly in Senecio spp. (Ragworts) and in Boraginaceae (borage family). Their adverse effect in man and animals are hepatotoxicity after bioactivation.
Papaveraceae (poppy family) and Berberidaceae (barberry family) produce isoquinoline alkaloids which have a range of biochemical effects relevant for medical use, as inhibition of various conditions as pain, cancer cells and bacteria, and stimulation of bone marrow leucocytes as well as myocardial contractility.
The main producers of methylxanthine alkaloids are Coffea arabica (coffee) and Theobroma cacao (cacao). Methylxanthines to a various extent bind to adenosine receptor and elicit neurological effects in man and animals which may be regarded stimulating at low to moderate intake. In rodents high intakes of methylxanthines show reduced sperm production and testicular atrophy.
Compounds called pseudoalkaloids have chemical properties close to alkaloids and are produced by species in Apiaceae (carrot family) for instance Cicuta virosa (cowbane) and Conium maculatum (hemlock), and in Taxaceae (yew family) for instance Taxus baccata (yew). The pseudoalkaloids in Cicuta virosa and Conium maculatum have effects on the central nervous system and taxine in yews like T. baccata inhibits the ion transport of the hearth.
Furocoumarines in Apiaceae (carrot family) particularly in Heracleum spp (cow parsnips) have photosensitizing properties. The naphthodianthrones for instance in Hypericum spp (St. John's-worts) of Clusiaceae (garcinia family) and in Polygonaceae (dock family) e.g. Fagopyrum esculentum (buckwheat) have similar effects. The compounds in Hypericum spp. have an antidepressant effect.
Proteins from plants are an important source in food and feed. Amino acids thereof are absorbed from the intestine of man and animals and are built up into adapted proteins. Nevertheless, there are also plant proteins and peptides with bioactivity. They are often not hydrolysed in the digestive tract, but may to a certain extent be absorbed and exert their specific action in the body. Euphorbiaceae (spurge family) include plants producing such proteins, for instance ricin in seeds of Ricinus communis (castor bean). The very potent little protein (lectin) ricin inhibits protein synthesis and induce systemic effects in animals and humans, with gastrointestinal symptoms dominating. Far less potent lectins are also present in seeds of several species of Fabaceae (bean family). Colic and other gastrointestinal symptoms may occur if seeds are eaten without sufficient heat treatment, which inactivates many lectins.
Exploring the Diversity and Roles of Secondary Metabolites in Plants. (2019, Dec 07). Retrieved from https://studymoose.com/secondary-metabolites-essay
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