All terpenoids are made up of C5 isopentane- isoprene units and thus are known as Isoprenoids or isopentanoids and formed by mevalonic acid pathway. If 2 isopentane units combain they form one C10 monoterpene (very volatile, volatile oils), 3 isoprenes make C15 sesquiterpene (less volatile volatile oils), 4 isoprenes join to make a C20 diterpene (resins), 6 isoprenes form a C30 triterpene (steroids, saponins), 8 isoprenes make a C40 tetreterpenoids (carotenoids and xanthophylls) and many isoprenoids unite to form Cn –polyterpenoids (Rubber and Gutta). Here we look into a larger group of isopreniods the Triterpenoids.
The triterpenoids are C30 compounds composed of three monoterpene (3x10) or of two sesquiterpene (2 x 15) units linked tail to tail. More than 5000 triterpenoids are isolated from plants. Nearly hundred types of skeletons are known in this group. Most of them have 4 rings (tetracyclic) or 5 rings (pentacyclic). Steroids also possess the cyclopentanoperhydrophenanthrene nucleus (6-6-6-5) and 8-carbon side chain at C17. But they differ from the tetracyclic triterpenes by having only two methyl groups, at positions 10 and 13. These compounds, are extensively located in higher plants now, where they occur free, as esters or as other derivatives. The typical “animal sterols” like cholesterol and hormones like estrone and ecdysones are reported from many plants, but their role in plants is still very ambiguous. Among the phytosterols, ergosterol is confined to fungi whereas sitosterol, stigmasterol and campesterol are widely distributed in the plant kingdom. More than 300 plant sterols have been identified. There are 8 groups of triterpenoids such as phytosterols, ecdysteroids, cucurbitacins, withanolides, limonoids and quassinoids, saponins, cardiac glycosides and brassinosteroids
1. Phytosterols (plant sterols)
Sitosterol, campesterol and stigmasterol are the three common plant sterols present in almost all plant parts, tough they are particularly abundant in seeds, oils, waxes, fruits etc. These steroids many a times replace cholesterol and thus the foods rich in them are considered good for heart. They are converted to saturated Stanols which are added to rape seed (mustard oil) oil based margarine or other foods for a reduction in LDL cholesterol of about 10%. Eg. Benecol- a dosage of 2gm of plant stenol incorporated in 25gm margarine is found to reduce the risk of heart disease by about 25%.
2. Ecdysteroids.
Ecdysteroids are insect moulting hormones used efficiently by plants to protect themselves by disturbing the reproduction of insects feeding them. 400 members are found both in plants and animals. They are most commonly used in medicine for increasing muscle mass and improve athletic performance. As these compounds are not in the list of banned drugs by sports authorities, sports persons prefer them to increase their performance in field. They are present in many members of Amaranthaceae, Chenopodiaceae and related families. The seeds of these families used as foods (Buckwheat, Quinoa) and grain amaranths (seeds of Amaranthus cruentus, A. hypochondriacus and A. caudatus- commonly known as Ramdana or Rajgira are rich in these compounds and are real health foods.
3. Cucurbitacins
Cucurbitacins, the bitter principles of bitter guard and related cucurbitaceous plants are a group of highly oxygenated C30 triterpenoids about 50 in number. They are bitter to ward of the insects and herbivorous animals. These compounds in bitter guard are found to be antidiabetic, reduces cholesterol and antiulcerogenic and cancer preventive. Coccinia (Bimbi-Giloda) is well known for the antidiabetic potential and antitumour properties. Picrorhiza kuroa (Katukka) with its cucurbitacins is highly effective against jaundice and is hepatoprotective.
4. Withanolides
These are C28 steroidal lactones common in Solanaceae. More than 200 withanolides are isolated, majority as aglycones (Anjanayelu et al., 1998). Withanolides are restricted to a few genera within Solanaceae. Withanolides have shown a wide range of pharmacological activities including hypno-sedative, immunomodulatory, anti-inflammatory, antiarthritic, angiogenesis inhibitor, anticholinesterase, antioxidant,antibacterial and above all, antitumour. Withania somnifera is the best studied medicinal plant with withanolides.
The remaining triterpenoids, limonoids and quassinoids, saponins, cardiac glycosides and brassinosteroids will be discussed in Second part of this article.
Sources. Analytical Methods for Medicinal Plants and Economic Botany, M. Daniel & Denni Mammen (2016) Scientific Publishers, Jodhpur
Mammen Daniel
