According to Cowan (1999) and Upadhyay et al. (2014) SM are classified into phenolics and polyphenols, alkaloids, terpenes, lectins and polypeptides.
2.6.1 Phenolics and polyphenols
Phenolics and polyphenols consist of a variety of aromatic SM comprising flavonoids, quinones, tannins and coumarins (Cowan, 1999; Upadhyay et al., 2014).
Flavonoids consist of different groups of SM which include flavones, flavonols, anthocyanidins (Upadhyay et al., 2014; Croteau et al., 2000, p. 1304), chalcones, aurones, isoflavonoids, catechins and anthocyanins (Croteau et al., 2000, p. 1304).
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Flavonoids promote pollination in plants by acting as chemoattractants for insects, modifying plant physiology by signalling to beneficial microbiota, defending plants against herbivores, bacteria and fungi (Upadhyay et al., 2014). Croteau et al. (2000, p. 1304) highlighted that the dietary isoflavonoids inhibit incidence of cancer. Isoflavonoids daidzein and genistein in soybeans are believed to suppress the development of breast and prostate cancers in human (Croteau et al., 2000, p. 1304).
Quinones are organic compounds comprising of two ketone substitution (Upadhyay et al., 2014; Cowan, 1999). Some quinones such as vitamin K, a complex naphthoquinone has been utilised by man as an antihaemorrhage. Hypericin, an anthraquinone has reportedly been used as an antidepressant (Cowan, 1999).
According to Upadhyay et al. (2014) tannins are a group of water soluble oligomeric and polymeric polyphenolic compounds with remarkable properties. The molecular weight for tannins range from 500 to 3,000 (Cowan, 1999) and they exist in most of the plant parts including bark, leaves, fruits and roots (Upadhyay et al., 2014; Cowan, 1999). Tannins are divided into two groups, non-hydrolysable and hydrolysable tannins (Wink, 2015; Cowan, 1999). Non-hydrolysable tannins may be formed when catechins, a special class of flavonoids, dimerise or polymerise to procyanidins and oligomeric procyanidins. The phenolic hydroxyl groups can react with proteins to yield hydrogen and ionic bonds and probably covalent bonds. These compounds may be refered to as tannins if they contain more than 10 hydroxyl groups (Wink, 2015). Hydrolysable tannins constitutes gallic acid between esters and sugar (Wink, 2015; Cowan, 1999). Tannins are used as antioxidants, anti-inflammatory, antidiarrhoeal, antiparasitic, antibacterial, antifungal and antiviral (Wink, 2015) and also in leather industry (Upadhyay et al., 2014; Cowan, 1999) and food industry (Upadhyay et al., 2014).
Coumarins are a group of phenolic benzopyrone substances made up of fused benzene and α-pyrone rings (Upadhyay et al., 2014; Wink, 2015). Coumarins are often found to a concentration of up to 2% in most genera of Apiaceae, Fabaceae, Poaceae, Rubiaceae. They have been used in traditional medicine as anti-inflammatory, anti-edemic and antimicrobial agents. Coumarins have also reportedly been utilised in cosmetics and in beverages (Wink, 2015).
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2.6.2 Alkaloids
Alkaloids are SM comprised of one or several nitrogen atoms in a ring structure (true alkaloids). Besides true alkaloids, other alkaloids consist of nitrogen atoms in a side chain (Wink, 2015). Examples of well known alkaloids are morphine, codeine and atropine (Croteau et al., 2000, p. 1271). Alkaloids defend plants against herbivores, carnivores due to their bitter taste (Wink, 2015; Croteau et al., 2000, p. 1274) and to a lesser degree against bacteria, fungi and viruses (Wink, 2015). Alkaloids are classified into several groups depending on the ring structure.
2.6.2.1 Amaryllidaceae alkaloids
Classical alkaloids in this group are ambelline, galanthamine, haemanthamine, lycorine and narciclasine synthesised by several genera of the Amaryllidaceae. Galanthamine from Galanthus woronowi, Leucojum aestivum, Narcissus pseudonarcissus and N. nivalis is used as an analgesic and a therapeutic agent for Alzheimer’s (Wink, 2015).
2.6.2.2 Pyrrolizidine alkaloids
Pyrrolizidine alkaloids are synthesized from almost all members of the Boraginaceae, a number of Asteraceae (Scenecioninae) and Fabaceae (Crotalarieae). A number of pyrrolizidine alkaloid containing plants especially Crotalaria, Heliotropium, Petasites and Senecio are used to arrest bleeding, treat diabetes and consumed as herbal tea (Wink, 2015). Additionally, pyrrolizidine alkaloids from Symphytum officinale and some Boraginaceae are utilised in wound and fracture therapies.
2.6.2.3 Pyrrolidine alkaloids
A typical example of pyrrolidine alkaloid is nicotine from Nicotiana tabacum. Previously nicotine was used as natural insecticide in agriculture before synthetic insecticides. Currently nicotine is utilised in cigarette manufacturing (Wink, 2015).
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2.6.2.4 Steroid alkaloids
Steroid alkaloids are mostly made up of a lipophylic steroid moiety and a hydrophilic oligosaccharide chain. Steroid alkaloids are synthesised by four unrelated plant families; Apocynaceae, Buxaceae, Liliaceae and Solanacea. They are distributed in the genus Solanum that comprises potato and tomato (Wink, 2015). Other dietary plants have reportedly been producing steroid alkaloid of the spirosolane type; with soladulcidine and tomatidine and solanidane type with solanine and chaconine (Wink, 2015). Steroid alkaloids of Solanum species including Solanium dulcamara are used in phytomedicine as anti-inflammatory drugs. Solanium alkaloids have also been employed in agriculture as an insecticide. Furthermore, steroidal alkaloids such as cyclobuxine D and buxamine E from the genus Buxus have been utilised as purgative (Wink, 2015).
2.6.3 Terpenes
Terpenes are synthesised from C5-units building block (isoprene units) in a “head to tail” fashion. Terpenes are classified into monoterpenes (C10), sesquiterpenes (C15), diterpenes (20), triterpenes (30), tetraterpenes (C40) and polyterpenes (Croteau et al., 2000, p. 1252; Cowan, 1999; Wink, 2015). Steroid terpenes (C27) are synthesised from triterpenes. When additional elements such as oxygen are incorporated in the terpenes, the compound is termed as terpenoid (Cowan, 1999).
Monoterpenes have reportedly been found in Asteraceae, Apiaceae, Burseraceae, Dipterocarpaceae, Lamiaceae, Myricaceae, Myristicaceae, Poaceae, Rutaceae, Verbenaceae and resin of conifers (Wink, 2015). Monoterpenes play a significant role in plants by attracting insect pollinating flowers. Monoterpenes have been utilised in aroma therapy and ethnomedicine as a remedy for rheumatism, bacterial and fungal infections, cold, unrest, flatulence, intestinal spasms and as a stimulant for an appetite (Wink, 2015). In addition, monoterpenes possessing phenolic hydroxyl or an aldehyde functional groups are utilised as antiseptics (Wink, 2015).
According to studies conducted by Wink (2015), sesquiterpenes are widely distributed in Asteraceae and a few other families such as Apiaceae, Magnoliaceae, Menispermaceae,
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Lauraceae. Sesquiterpenes possess various antimicrobial properties such as cytotoxic, antihelminthic, phytotoxic, insecticidal, antibacterial and antifungal properties. Furthermore, sesquiterpenes have also been known to possess expectorant and anti- inflammatory properties (Wink, 2015).
Diterpenes are natural compounds found in Euphorbiaceae and Thymelaeceae (Wink, 2015; Vasas & Hohmann, 2014). They possess different moities such as jatrophane, ingenane, daphnane, tigliane, lathyrane. The different moities are as a result of different macrocyclic and polycyclic skeletons. Diterpenes are regarded as significant taxonomic markers due to their limited occurrence in Euphorbiaceae and Thymelaeceae (Vasas & Hohmann, 2014). Diterpenes existing in plants of the genus Euphorbia are the main target of natural product drug discovery due to their broad range of biological properties (Vasas & Hohmann, 2014). Diterpenes have been used in phytomedicine due to their biological properties such as antitumour, cytotoxic, antiviral, multidrug-resistance-reversi ng, different vascular effects, anti-inflammatory (Vasas & Hohmann, 2014).
Triterpenes are one of the subclasses of terpenes occurring in large quantities in plant natural products (Thimmappa, Geisler, Louveau, O'Maille & Osbourne, 2014).Triterpenes are formed by cyclisation of 2,3-oxidosqualene (plants, animals and fungi) by oxidosqualene cyclases giving rise to more than 100 different triterpene scaffolds. The scaffords are further transformed by oxidation, reduction, isomerisation, acylation to impart functional properties of the triterpenoids. Currently, more than 20,000 triterpenes have been identified (Thimmappa et al., 2014). Simple triterpenes (sterols) have been reported to perform as signalling molecules while complex glycosylated triterpenes (saponins) shield plants from potential enemies such as bacteria, fungi and predators. Sterols such as Bryoma cucurbitacins have been used as a remedy for rheumatism and muscle pain (Wink, 2015). Additionally, glycosylated triterpenes have been employed as anti-inflammatory (glycyrrhizic acid from Glycyrrhiza glabra) agents. Furthermore, triterpenes have also been used as detergents for washing clothes (Wink, 2015). Since glycosylated triterpenes are very toxic for fish as they hinder respiration, they are traditionally used in fishing . They have also been utilised in elimination of water snails that tansmit Schistosoma that cause schistosomiasis (Wink, 2015).
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2.6.4 Lectin and polypeptide
Lectins have been reported in seeds of several plants such as abrin in Abrus precatorius, phasin in Phaseolus vulgaris, ricin in R. communis. Like majority of proteins, the information for lectin biosynthesis is encoded in the genes. The mRNA transmits the information to ribosomes to synthesise polypeptides in the endoplasmic reticulum (Sharon & Lis, 2012, p. 65). The polypeptides serve as precursors for lectin biosynthesis. The polypeptides go through co- and post-translational modification to yield mature lectins. A co-translational modification involves proteolytic removal of a 20-30 amino acid signal sequence from the amino terminal end (Sharon & Lis, 2012, p. 65). Post- translational modification involves; N-glycosylation and additional modification of the carbohydrate units such as O-glycosylation of hydroxyproline (Sharon & Lis, 2012, p. 65). According to Wink (2015) lectins defend plants against herbivores. Lectins have been reported to possess a wide range of activities such as antitumor, antifungal, immunomodulatory activities, anti-insecticide activities (Lam & Ng, 2011), HIV-1 reverse transcriptase inhibitor (Lam & Ng, 2011; Cowan, 1999). A minority of lectins have shown antibacterial and antinematode activities. Lectins from seeds of Vicia cracca have been utilised in human ABO erythrocyte grouping (Lam & Ng, 2011).