Cardenolides are a type of steroids. Many plants contain cardenolides in the form of cardenolide glycosides (i.e. cardenolides that contain structural groups derived from sugars; see Glycoside). Cardenolides are toxic (specifically, heart-arresting).
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Supposedly, the term derives from Greek kardiā, heart. It shouldn't be confused with cardanolides. Cardanolides 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).
Cardenolides are C(23)-steroids with methyl groups at C-10 and C-13 and a five-membered lactone 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:
Some plant and animal species use cardenolides as a defense mechanism, most notably the monarch butterflies. Adult monarch butterflies store the cardenolides they have built-up as larvae feeding mostly on milkweeds (Asclepias). The cardenolide content in butterflies deters vertebrate predators, with some exceptions of cardenolide-tolerant predators like 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.
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