Pyrrolizidine alkaloids

Definition

A class of plant-derived bicyclic alkaloids characterized by a pyrrolizidine nucleus, produced primarily by the Asteraceae, Boraginaceae, and Fabaceae families. These compounds function as potent hepatotoxins and genotoxins in vertebrates, but are sequestered and utilized as chemical defenses by specialist herbivorous insects. Many insects have evolved tolerance mechanisms including cytochrome P450-mediated detoxification, and some species actively seek PA-containing plants; the alkaloids are often retained through metamorphosis and deployed in adult defense, pheromone precursors, or nuptial gifts.

Etymology

From pyrrolizidine, the fused bicyclic amine core (two fused five-membered rings with a shared nitrogen), + alkaloid, nitrogen-containing basic compounds of plant origin

Example

Male arctiine moths (Arctiinae) such as Utetheisa ornatrix sequester pyrrolizidine alkaloids from Crotalaria host plants as larvae; adults use these compounds for chemical defense against predators and as precursors for hydroxydanaidal, a pheromone component transferred to females during courtship and subsequently incorporated into eggs as protective alkaloids.

Synonyms

• necine bases

Related Terms

• Sequestration
• chemical defense
• aposematism
• Batesian mimicry
• pharmacophagy
• host plant specialization
• cytochrome P450
• hydroxydanaidal
• cardenolides
• iridoid glycosides

Usage Notes

Often abbreviated PA or PAs in ecological literature. The term 'necine base' refers specifically to the amino alcohol core structure without the esterifying acids; in strict usage, 'pyrrolizidine alkaloid' encompasses the complete esterified compounds. Not all PAs are equally toxic to all organisms—specialist insects may tolerate or prefer specific structural subclasses (retronecine vs. heliotridine bases). In medical entomology, PAs are relevant as contaminants of pollen and honey, posing risks to apiary products and pollinator health.