What are Pyrrolizidine Alkaloids?
Pyrrolizidine alkaloids (PAs) are a large class of naturally occurring chemical compounds found in numerous plant species across the globe. Produced by plants as a defense mechanism against herbivores, PAs are esters derived from a core pyrrolizidine nucleus. For many PAs to become toxic, they must first be metabolized in the liver, where specific forms are converted into highly reactive pyrroles. The toxicity is largely dependent on the chemical structure, specifically the presence of a double bond at the 1,2-position in the necine base. This metabolic activation makes PAs hepatotoxic, meaning they can cause significant liver damage. Human and animal exposure can occur through the consumption of PA-containing plants or contaminated food products, including grains, milk, and honey.
Examples of Pyrrolizidine Alkaloid-Containing Plants
PAs are most famously found in three major plant families: Asteraceae, Boraginaceae, and Fabaceae. Below are some notable examples from these families and the specific alkaloids they contain.
Boraginaceae Family (Borage Family)
This family is a significant source of PAs, with several genera known to be toxic. Prominent examples include:
- Symphytum spp. (Comfrey): Used widely in traditional herbal medicine, comfrey is a well-known source of PAs, including symphytine and echimidine. Health authorities advise against internal consumption due to its hepatotoxicity.
- Heliotropium spp. (Heliotrope): Responsible for numerous mass poisonings due to contamination of grain crops, especially during droughts. Heliotropium lasiocarpum contains alkaloids like heliotrine and lasiocarpine.
- Echium spp. (Viper's Bugloss): A common weed that can contaminate honey, particularly in regions like Australia, leading to human exposure. Echium plantagineum is a key species.
Asteraceae Family (Daisy or Composite Family)
This family includes several species known for their PA content, many of which are common weeds.
- Senecio spp. (Ragworts and Groundsels): This is one of the most important sources of PAs, producing toxic compounds like senecionine and jacobine. Species like tansy ragwort (Senecio jacobaea) are notorious for poisoning livestock.
- Tussilago farfara (Coltsfoot): Used traditionally in herbal teas for cough relief, coltsfoot contains PAs like senkirkine. Its use is now cautioned due to toxicity risks.
- Petasites spp. (Butterbur): Another plant used in herbal medicine, some species contain PAs such as senkirkine, and their consumption is linked to liver damage.
Fabaceae Family (Legume or Pea Family)
Within this family, the genus Crotalaria is a major concern for PA toxicity.
- Crotalaria spp. (Rattlebox): These plants produce toxic PAs like monocrotaline. Contamination of grain with Crotalaria seeds has been a cause of large-scale poisoning outbreaks.
Indirect Sources of Pyrrolizidine Alkaloids
Exposure to PAs is not limited to direct consumption of toxic plants. Contamination of everyday food and animal feed is a significant pathway for human and animal poisoning. Bees visiting PA-producing flowers can transfer the alkaloids to honey and pollen. Milk from livestock that have grazed on PA-containing plants can also contain the toxins. Accidental co-harvesting of PA-containing weeds with cereal crops, herbs, and spices is another common route of exposure.
Comparison of Major PA-Containing Plant Families
| Feature | Boraginaceae | Asteraceae (Senecio) | Fabaceae (Crotalaria) |
|---|---|---|---|
| Common Plant Examples | Comfrey (Symphytum spp.), Heliotrope (Heliotropium spp.), Viper's Bugloss (Echium spp.) | Ragworts, Groundsels (Senecio spp.), Coltsfoot (Tussilago farfara) | Rattlebox (Crotalaria spp.) |
| Representative Alkaloids | Symphytine, Lasiocarpine, Heliotrine, Echimidine | Senecionine, Jacobine, Seneciphylline, Retrorsine | Monocrotaline |
| Primary Toxicity | Hepatotoxic, can cause hepatic veno-occlusive disease (VOD) | Strong hepatotoxins, causes VOD and cirrhosis | Pulmonary toxicity (in some species), hepatotoxic |
| Major Exposure Route | Herbal remedies, teas, contaminated grains, and honey | Contaminated hay or silage, contaminated grain, honey | Contaminated grain and seeds |
| Risk Level | High, especially with prolonged use of herbal products | High, particularly for livestock and through food chain contamination | High, especially from seed contamination in food sources |
Conclusion
Numerous examples of pyrrolizidine alkaloids exist in a wide variety of plants, posing significant health risks due to their hepatotoxicity, genotoxicity, and carcinogenicity. While these compounds serve as natural defenses for plants, their presence in common weeds and some medicinal herbs leads to human and animal exposure, often through contaminated food products. Recognizing the primary plant sources from the Boraginaceae, Asteraceae, and Fabaceae families is critical for minimizing risks. Awareness and adherence to advisories, such as those published by the World Health Organization regarding herbal preparations, are essential to preventing exposure and associated illnesses, including hepatic veno-occlusive disease.
- Authoritative Source: A comprehensive safety guide on pyrrolizidine alkaloids is available from the World Health Organization (WHO) at: PYRROLIZIDINE ALKALOIDS HEALTH AND SAFETY GUIDE.
