(Photos: Barleria, Morinda, Vitex and Valeriana)
Iridoids, known as pseudoindicans because they react with acid giving blue-colored derivatives are cyclopentanoid skeleton mostly along with a pyran ring. Blackening of leaves (or herbarium specimens) on drying is a sign of the presence of iridoids. Altogether more than 650 iridoids are recognized. Many of the iridoids occur as glycosides though aglycones like nepetalactone (in catnip/catmint, Nepeta cataria- the domestic cats are attracted by these compounds) occur in volatile oils.
Iridoids are mostly bitter in taste and contribute much to the bitterness of many medicines. Both these groups of compounds are found to exhibit a wide range of medicinal properties such as neuroprotective, hepatoprotective, anti-inflammatory, antitumor, hypoglycemic, and hypolipidemic activities. For example, all of them are found antimicrobial in nature, while allamandin from Allamanda cathartica is antileukemic, oleuropin from olive as well as valepotriates (a group of non-glycosidic compounds) from Valeriana spp. are found to be hypotensive and sedative; herpagide from Herpagophytum procumbens is analgesic and geniposide from Gardenia jasminoides is found to be a laxative. Amarogentin and amaroswerin, two secoiridoid glycosides from gentian root, are among the bitterest substances. Some of the iridoids are found to be anti-inflammatory in that they interfere with the production of inflammation promoters like prostaglandins and leukotrienes.
Though there are six groups (Dinda et al., 2009) of Iridoids, three are most common. They are 1. Iridoid glycosides eg. Asperuloside, loganin etc. 2. Seco-iridoids, formed by the cleavage of the cyclopentane ring of iridoids (gentiopicroside, oleuropin and swertiamarin etc.). 3. bis- and tetrakis-iridoid glycosides. bis- iridoids normally contain an iridoid and a seco-iridoid moieties linked by an ester linkage Eg. Sylvestroside. Dipsanoside C is a tetrakis-iridoid glycoside.
Iridoids are almost restricted to advanced families of Dicots. They are common in Bignoniaceae, Gentianaceae, Lamiaceae, Loganiaceae, Oleaceae, Pedaliaceae, Plantaginaceae, Rubiaceae, Scrophulariaceae and verbenaceae. A number of medicinal plants like Gentian, sahacharah (Barleria prionitis), Nirgudi (Vitex ngundo) Picrorhiza, Emetic nut (Catunaregam spinosa) contain these compounds. Loganin, one of the iridoids, is presumed to be an intermediate in the biogenesis of strychnine type alkaloids, occurs in high concentrations (upto 5%) in the fruit pulp. They are bitter as well as pesticidal in nature. Iridoids also occur combined with coumarins, lignans, monoterpenes and alkaloids. In fact Hagnauer, R (1980) included all these compounds in a broader group of Iridoids containing more than 1500 members.
Weiffering (1966) recommends a simple procedure for surveying plant material for some of the common iridoids. Fresh plant material (or herbarium specimen) is cut into small pieces (or pulverized) and placed in a test tube with 5-10 ml of 1% aqueous HCl. After 3-6 hours, 3-5 drops of the extract is decanted into another test tube containing 1 ml of Trim-Hill reagent (made of 10 ml acetic acid, 1 ml of 0.2 CuSO4.5H2O in water and 0.5 ml con. HCl) and heated on a flame. Asperuloside, aucubin and monotropein give blue/Green colour while herpagide gives a red-violet color.
The important medicinal plants containing them are the following
1. Sahacharah (Barleria prionitis Linn.- Acanthaceae)
The leaves and stems are found to contain five iridoid glucosides such as barlerin, acetyl barlerin, shanzhiside methylester etc. Sahacharah is widely used against neurological disorders like paraplegia, sciatica etc.
B.cristata L. containing iridoids and anthraquinones, barlacristone and cristabarlone is used as sahacharah in many places in India. Barleria strigosa Willd and B.courtallica Nees also are used as substitutes of B. prionitis.
2. Nirgundi (Vitex negundo Linn.- Verbenaceae)
The leaves contain, aucubin, agnuside; alkaloids nishindine (C15H21ON) and hydrocotylene(C22H33O8N), glycoflavonoids, orientin and isoorientin and 5-hydroxy, 3,6,7,3’,4’-penta methoxy flavone.
The leaves, as a paste, are used for inflammatory swellings of the joints formed due to rheumatism, hydrocele and splenic enlargement. They are also used in skin diseases, nervous disorders, leprosy and as a vermifuge. Oil prepared with leaves is useful for growth of hair and increases the functions of brain. Roots also are useful in rheumatism, dyspepsia, piles and as anthelmintics.
3. Picrorhiza (Picrorhiza kurroa Linn.- Scrophulariaceae)
The major constituents are two iridoid glycosides, picroside 1 and kutkoside. Also present are other iridoids picroside III, veronicoside, minecoside; phenol glycosides picein and androsin, a number of cucurbitacin glycosides and 4-hydroxy-3-methoxy acetophenone (Stuppner and Wagner, 1989). Kutkin as well as the whole extract show hepatoprotective activity. It is also an immunostimulant of both cell-mediated and humoral immunity (Ansari et al.,1988).
4. Noni ( Morinda citrifolia Linn.- Rubiaceae)
Contains a large number of iridoid glucosides like 6α-hydroxyadoxoside (1) and 6β,7β-epoxy-8-epi-splendoside (2), as well as 17 known compounds, americanin A (3), narcissoside (4), asperuloside, asperulosidic acid,etc. Now widely used as health drink having antimicrobial, antioxidant, anti-arthritic, anticancer properties.
5. Gentian (Gentiana lutea Linn.- Gentianaceae)
The fermented rhizome and roots yield more than 2% of a bitter iridoid principle known as gentiopicrin (gentiopicroside, gentiamarin) which on hydrolysis yields a lactone gentiogenin and glucose, along with related glycosides amaropanin, amarogentin and amaroswerin. The other components of the drug are 0.6-1% of monoterpenoid alkaloids gentianine and gentialutine, swertiamarin and sweroside, flavones, glycoflavones isoorientin and isovitexin and xanthones. Gentian is extensively used as a bitter tonic. Root infusion is used for lack of appetite and stomach disorders and as a week febrifuge.
6. Emetic nut, Madana (Catunaregam spinosa Tirveng.- Rubiaceae)
The bark contains saponins based on randialic acid A & B, mannitol (up to 6%), 10-methyl lixoside (an iridoid) and scopoletin. Stems yield iridoids such as randinoside, galioside, deacetylasperulosidic acid methyl ester, scandoside methyl ester, geniposide and gardenoside. Roots also contain mannitol, scopoletin and a yellow dye. The fresh fruits contain 2-3% saponins (10% dry wt.) which mostly are concentrated in pulp. The bark is used in diarrhoea, dysentery and is abortifacient, anthelminitic, antipyretic and emetic. It is also considered sedative and hypoglycemic.
7. Valerian (Valeriana officinalis Linn. Valerianaceae)
A native of Europe, valerian is a rhizomatous glabrous perennial. Its rhizome, the drug, contains over 150 chemical constituents and many ofthem are physiologically active, mainly valepotriates, (esterified iridoids) pyridine alkaloids, some organic acids and terpene. Valerian is used to treat sleeping disorders, restlessness and anxiety, but it seems only to work when taken overlong periods (several weeks). Some studies have demonstrated that valerian extracts interact with the GABAand benzodiazepine receptors. Valerian is also used traditionally to treat gastrointestinal pain and spasticcolitis.
8. Psyllium, Isabgol (Plantago ovata Forsk. - Plantaginaceae)
Seeds contain mucilage (20-30%), a fatty oil (2.5%), proteins and iridoids. Psyllium is a bulk forming laxative useful in constipation and diarrhoea. It is also found to be hypocholesteremic.
Plantago major is an important medicinal plant with different pharmacological properties including anti-inflammation, analgesic, wound healing, antipyretic, antitussive, anti-infective, anti-hemorrhagic, laxative, astringent, hemostatic activity and diuretic
Natural Dyes
1. Gardenia spp. Rubiaceae
Gardenia grandiflora Lour. a Chinese tree, the fruit of which is employed in dyeing the yellow robes of the Mandarins, contain crocin. Gardenia campanulata fruits also are used to dye clothes. Gardenia jasminoides J. Ellis (Cape jasmine) a native if China, containing a number of iridoids such as genipin, its glycosides, geniposide, acetyl geniposide, etc. is used as a food coloring and natural dye of long shelf life. A blue dye is produced by hydrolyzing geniposide and reacting the resultant genipin with a primary amine.
2. Catunaregam spinosa
The leaves and fruits give a blue dye.
3. Holarrhena pubescens
Leaves yield a dye similar to indigo.
4. Vitex negundo Linn.
Oil prepared with leaves is useful for the blackening and growth of hair and increases the functions of brain.
Biopesticides
1. Vitex negundo
The leaves are used to repel insects in grain stores. Extracts of the leaves have insecticidal activity and used against armyworm, diamondback moth, hairy caterpillar, rice leaf folder, rice stem borer, semi-looper. The fresh leaves are burnt with grass as a fumigant against mosquitoes. The leaves are laid over stored grain to ward of insects. It was found that either all plant extracts in aqueous suspensions or cold alcoholic extracts were found to possess insecticidal properties against insects Spodoptera litura F and Lipaphis erysimi K. in the laboratory. The petroleum ether extract is found to have promising insecticidal property with more than 50 percent mortality. The plant is an antifeedant and repellant also.
Source:
Useful Herbs of the Planet Earth, By M. Daniel, (2012), Scientific Publishers, Jodhpur,
Mammen Daniel
