Cardenolide

Etymology

The term derives from card- "heart" (from Greek καρδία kardiā) and the suffix -enolide, referring to the lactone ring with double bond at C17. Cardenolides are a class of steroids (or aglycones if viewed as cardiac glycoside constituents), and cardenolides are a subtype of this class (see MeSH D codes list).

Structure

Cardenolides are C(23)-steroids with methyl groups at C-10 and C-13 and a five-membered lactone (specifically a butenolide) at C-17. They are aglycone constituents of cardiac glycosides and must have at least one double bond in the molecule. The class includes cardadienolides and cardatrienolides. Members include:

• acetyldigitoxins
• acetyldigoxins
• cymarine
• digitoxin
• digitoxigenin
• digoxigenin
• digoxin
• medigoxin
• neoconvalloside
• ouabain
• strophanthidin

Bufadienolide and marinobufagenin are similar in structure and function.

Cardanolide is the same core structure, but has a saturated lactone ring instead of one containing an alkene.

As defense mechanism

Some plant and animal species use cardenolides as defense mechanisms, notably the milkweed butterflies. Species such as the monarch, queen, and plain tiger ingest the cardenolides contained in the milkweeds (Asclepias) that they mostly feed on and sequester as larvae for defense as adults. The cardenolide content in butterflies deters most vertebrate predators, except a few which have evolved to become cardenolide-tolerant, such as the black-backed orioles (Icterus abeillei Lesson) and black-headed grosbeaks (Pheucticus melanocephalus Swainson) that account for 60% of monarch butterfly mortalities in the overwintering sites in central Mexico. In addition to milkweeds and other members of the Apocynaceae, plants in at least 12 botanical families have convergently evolved cardenolides, used as a chemical defense mechanism against herbivores. Herbivorous insects in six different orders have evolved resistance to the toxic effects of cardenolides in the food sources that they use. These cardenolide-resistant insect species convergently evolved this resistance through similar amino-acid substitutions in the alpha subunit of the enzyme Na+/K+-ATPase.

References

References

1. Naudé, T. W.. (1977). "The occurrence and significance of South African cardiac glycosides". Journal of the South African Biological Society.
2. "Interactions with Milkweed | Breeding / Life Cycle | Biology & Natural History | Biology & Research | Monarch Lab".
3. (April 2012). "Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions". The New Phytologist.
4. (1976-12-01). "Pyrrolizidine alkaloids inDanaus plexippus L. andDanaus chrysippus L.". Experientia.
5. (2012). "Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions". New Phytologist.
6. (2012-09-28). "Parallel Molecular Evolution in an Herbivore Community". Science.
7. Dobler, S., Dalla, S., Wagschal, V., & Agrawal, A. A. (2012). Community-wide convergent evolution in insect adaptation to toxic cardenolides by substitutions in the Na,K-ATPase. Proceedings of the National Academy of Sciences, 109(32), 13040–13045. https://doi.org/10.1073/pnas.1202111109
8. (2019). "Predictability in the evolution of Orthopteran cardenolide insensitivity". Philosophical Transactions of the Royal Society of London, Series B.