The iridoids and secoiridoids form a large group of plant constituents that are found usually, but not invariably, as glycosides. For example, harpago-side, an active constituent of Harpagophytum procumbens, is an iridoid glycoside. Plant families, e.g. Lamiaceae (especially genera Phlomis, Stachys and Eremostachys), Gentianaceae, Valerianaceae and Oleaceae, are good sources of these glycosides.
In most natural iridoids and secoiridoids, there is an additional oxygenation (hydroxy) at C-1, which is generally involved in the glycoside formation.
It is also extremely common amongst natural iridoids and secoiridoids, to have a double bond between C-3 and C-4, and a carboxylation at C-11. Changes in functionalities at various other carbons in iridoid and secoiridoid skeletons are also found in nature, as shown below.
Some examples of plants that produce irirdoid or secoiridoid glycosides, and their medicinal uses, are summarized below.
Devil's claw (Harpagophytum procumbens) Harpagophytum procum-bens is native to South Africa, Namibia and Madagascar, and traditionally used in the treatment of osteoarthritis, rheumatoid arthritis, indigestion and low back pain. This plant contains 0.5-3 per cent iridoid glycosides, harpagoside, harpagide and procumbine being the major active iridoid glycosides present.
The toxicity of H. procumbens is considered extremely low. To date, there have been no reported side-effects following its use. However, this plant is said to have oxytocic properties and should be avoided in pregnancy. In addition, due to its reflex effect on the digestive system, it should be avoided in patients with gastric or duodenal ulcers.
Picrorhiza (Picrorhiza kurroa) Picrorhiza kurroa is a small perennial herb that grows in hilly parts of India, particularly in the Himalayas between 3000 and 5000 m. The bitter rhizomes of this plant have been used for thousands of years in Ayurvedic traditional medicine to treat indigestion, dyspepsia, constipation, liver dysfunction, bronchial problems and fever. It is, in combination with various metals, useful in the treatment of acute viral hepatitis. The active constituents of picrorhiza are a group of iridoid glycosides known as picrosides I-IV and kutkoside.
Picrorhiza has been used widely in India, and no significant adverse reactions have been reported to date. The oral LD50 of Picrorhiza iridoid glycosides (known as 'kutkin') is greater than 2600mg/kg in rats.
Oleuropein, a secoiridoid glycoside Fraxinus excelsior (ash tree), Olea europaea (olive tree) and Ligustrum obtusifolium from the family Oleaceae are the major sources of oleuropein. This compound has hypotensive, antioxidant, antiviral and antimicrobial properties. There is no known toxicity or contraindications for this compound.
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