Biochemistry of Sterols, Cardiac Glycosides, Brassinosteroids, Phytoecdysteroids and Steroid Saponins

Abstract

Phytosterols are synthesized via the mevalonate pathway of terpenoid formation and arise from the initial cyclization of 3 S ‐squalene‐2,3‐epoxide. Plant steroids are derived from sterols and comprise steroid saponins, steroid alkaloids, pregnanes, androstanes, estranes, ecdysteroids, withanolides and cardiac glycosides. The typical route of sterol and steroid biosynthesis follows the cycloartenol pathway, whereas the lanosterol route seems to be operative mainly in fungi and animals. It was demonstrated, however, that both sterol pathways can be operative in higher plants. Crucial steps in the conversion of cycloartenol/lanosterol to sterols are the events leading to the removal of the methyl groups at C‐4 and C‐14. Meanwhile, all steps in the sterol pathway have been elucidated and the respective enzymes/genes characterized. The biosynthetic pathway leading from phytosterol precursors to the cardiac glycosides – important compounds in the treatment of cardiac insufficiency in humans – was basically deduced from studies using radiolabelled precursors. The more recent identification and characterization of several enzymes/genes involved in pregnane and cardenolide metabolism, such as 3β‐hydroxysteroid dehydrogenase and progesterone 5β‐reductase, have further clarified the pathway. Brassinosteroids (BRs) are hydroxylated derivatives of cholestane and they are specific plant steroid hormones that are essential for normal plant development. The biosynthesis of BRs has mainly been studied in Arabidopsis thaliana . Many of the genes encoding biosynthetic enzymes have been cloned using mutants of Arabidopsis thaliana , pea, tomato and rice which revert to a wild‐type phenotype following treatment with exogenous BRs. Phytoecdysteroids are related in structure to the invertebrate steroid hormones. Their biological significance in plants is still under discussion. The understanding of the biosynthetic pathway(s) for phytoecdysteroids is very limited. Steroid saponins constitute a vast group of glycosides present almost exclusively in the monocotyledonous angiosperms, and occurring in only a few dicotyledonous families. As far as enzymatic and genetic aspects are concerned, the biosynthesis of steroid saponins (including the steroid alkaloids) has not been studied extensively. The withanolides are C 28 ‐steroids and biogenetically related to the steroid saponins in that they are derived from ergostane‐type sterols. These compounds appear to be specific for the Solanaceae and their biosynthesis has not yet been studied at the enzyme/gene level.

Medically Useful Plant Terpenoids: Biosynthesis, Occurrence, and Mechanism of Action

“…Due to the large number of plant terpenoids described in the literature in various health contexts, we have excluded those of a purely nutritional, nutraceutical, or anti-oxidant nature and instead focused on compounds with specific pharmacological activity. Cardenolides such as digoxin, which inhibit cardiac Na + /K + ATPase and improve cardiac muscle contraction and stroke volume, have been reviewed recently [16,17] and therefore were not covered here.…”

Pharmacologic activities of phytosteroids in inflammatory diseases: Mechanism of action and therapeutic potentials

“…The oxidation states of these tetracyclic carbon cores and methyl groups and their structure chain exhibit immense chemical diversity of steroids (Gunaherath & Gunatilaka, 2020). Based on taxonomic considerations and biological structures, plant steroids can be classified into seven major classes (Kreis & Müller-Uri, 2018) (Table 1). The cyclization of triterpene squalene into cycloartenol via the acetate-mevalonate pathway leads to plant steroids biosynthesis (Figure 1).…”

The Use of Plant Steroids in Viral Disease Treatments: Current Status and Future Perspectives

“…PSs also act as precursors in the synthesis of main bioactive compounds such as steroidal glycoalkaloids, steroidal saponins, phytoecdysteroids, and brassinosteroids. 2,56,57 As a new functional food group, extensive research has been conducted on the anticancer, antidiabetes, anti-obesity, cytotoxic, insecticidal/larvicidal, antiatherosclerosis, anti-Alzheimer and hepatoprotective effects of PSs related to the management of human health and metabolic disorders. 57 Plant sterols have been proven to increase mitochondrial ATP content, reduce dyslipidemia, insulin resistance and dysfunction of cells, improve gut microbiota dysbiosis and barrier dysfunction, increase adipose inflammatory signal, reduce oxidative stress, and modulate inflammatory signals.…”

“…The addition of distinct chemical groups at different positions to the nucleus leads to the formation of many types of steroids categorized into seven main groups based on their biological functions, structures, and taxonomic considerations such as brassinosteroids, bufadienolides, cardenolides, cucurbitacins, ecdysteroids, saponins, steroidal alkaloids, and withanolides (Table 1). 2,3 Along with their important physiological functions within plants, they exhibit pharmacological activities helpful to humankind. Due to the knowledge about the pharmacological activities of these herbs, they are gaining increasing attention all over the world as herbal medicine supplements.…”