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Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Cardenolide is a type of steroid. Many plants contain derivatives, collectively known as cardenolides, including many in the form of cardenolide glycosides (cardenolides that contain structural groups derived from sugars). Cardenolide glycosides are often toxic; specifically, they are heart-arresting.
Supposedly, the term derives from Greek καρδία kardiā, "heart" and the suffix -enolide. It should not 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 (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:
Marinobufagenin is similar in structure and function.
As defense mechanism
Some plant and animal species use cardenolides as defense mechanisms, most notably the milkweed butterflies. Species such as the Monarch and the Queen 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 such as 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.