Triterpenoid and Steroidal Alkaloids
Steroidal alkaloids
Steroidal alkaloids are most common in the Solanaceae, Apocynaceae, and Liliaceae.
Much of the recent work on this group of alkaloids was done by the group of Klaus Schreiber.
Solanum alkaloids
Many of these alkaloids are glycosides that occur in aereal parts of Solanum spp. About 1400 alkaloids of this group are known. Many of the plants that contain these alkaloids are of economic importance, e.g., potatoes, tomatoes, tomatillos, and eggplant. Species of this genus are often involved in animal poisoning problems.
There are 5 major structural types. These are the spirosolanes, solanidanes, 22,26-epiminocholestanes, a-epiminocyclohemiacetals, and 3-aminospirostanes. Most of these alkaloids occur as glycosides.
The steroid part of the pathway is as in other plants. The sterols of the plants and the alkaloids are often similar structurally. Cycloartenol and cholesterol are frequent precursors. The glycoalkaloids appear to be synthesized in the above-ground part of the plants, not in the roots as is the case for tropane alkaloids.
Spirosolanes: Compounds such as tomatidine and solasodine have the basic nitrogen included into an oxaazaspirane unit that includes the original steroidal side chain.
Solanidine alkaloids: Solanidine biosynthesis involves the intermediates verazine and etioline. These alkaloids also are usually glycosides.
22,26-Epiminocholestanes: Alkaloids with this structure are intermediates in the biosynthesis of spirosolane, solanidine, a-epiminocyclohemiacetal, and 3-aminospirostane alkaloids.
3-Aminospirostane alkaloids: Alkaloids of this group, such as juribidine, are probably synthesized
analogously to the corresponding spirostanols, e.g., tigogenin. The 3-amino group is probably introduced
late in the biosynthesis by transamination.
Biological activity of Solanum alkaloids
As mentioned above, alkaloids of this series are toxic and occur in many common plants. Many of them have antifeeding effects against insects.
a-Tomatine complexes 3-b-hydroxysteroids, which alters membrane permeability.
Solanine is a mitotic poison, whereas a-chaconine is a choline esterase inhibitor. This alkaloid also disrupts membranes.
Several Solanum alkaloids have antifungal activity.
Potatoes contain a number of alkaloids of these types and when allowed to "green" become reasonbly
toxic. There is some debate about whether some of the alkaloids are teratogenic.
Veratrum alkaloids
Many Veratrum alkaloids are identical or similar in structure to those of the previous group. However, there are also a number of structurally distinct ones. Veratrum alkaloids cause repetitive discharges of nerve cells.
There are two main types of Veratrum alkaloids, the jerveratrum and ceveratrum types (protoverine A and B). 12-Hydroxysolanidine is a major precursor.
Some Veratrum alkaloids depolarize membranes and affect Na+ channels.
Plant material of Veratrum and related genera is extremely poisonous to people and livestock. "Cyclopism", or the development of malformed lambs with only one eye, occurs when the female sheep eats Veratrum species. These alkaloids are often responsible for teratogenic effects. Jervine, cyclopamine, cyclopsine are the most potent.
Many Veratrum alkaloids are used for the treatment of hypertension. Medicinally, the most important are protoverine polyesters.
Steroidal alkaloids in Apocynaceae
Many of these alkaloids are aglycones. Many of them are based on an a-pregnane skeleton and appear to be derived from pregnenolone or cholesterol. A number of them are relatively poisonous. Holarrhena antidysenterica is used to treat dysentery. Conessine is antiamebic.
Triterpene alkaloids
Triterpene alkaloids are relatively uncommon. The best known ones are found in species of Buxus.
Another type of triterpene alkaloids is found in the genus Daphniphyllum (Daphniphyllaceae). Although these are 20 carbon compounds, squalene is incorporated.
Frog and toad alkaloids
Samandarine and batrachotoxin are found in certain frogs and toads. These compounds are highly toxic and are used to poison arrows and darts. They hold Na+ channels open.
Lecture Slides
Plants with Steroidal and
Triterpenoid Alkaloids
© David S. Seigler, Plant Biology 363, Plant Secondary Metabolism, Department of Plant Biology,
265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA.
217-333-7577. seigler@life.uiuc.edu.
