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Pyrrolizidine Alkaloids

Most alkaloids are amino acid derivatives, and have no certain role in plant metabolism except for serving to repel insects and herbivore predators due to their bitter taste. Pyrrilozidine alkaloids are very common in plants used in livestock feeds and in rangeland weeds such as tansy ragwort, Senecio jacobea, and Heliotropium spp. and Crotolaria spp.. Pyrrolozidine alkaloids contain nitrogen in a heterocyclic ring forming a definite structure called the pyrrolizidine nucleus. It appears that the pyrrilozine alkaloids themselves are not toxic, rather, some of their metabolites, primarily their "pyrrolic" derivatives are highly toxic. Toxic alcohols can also be produced as secondary metabolites. The common PAs, Heliotrine and lasciocarpine, are partially reduced to the non-toxic 1-methylene and 7-hydroxy-1 methyl derivatives in the rumen. However, these can be activated by oxidases in the animal's liver to pyrrolic derivatives and thus have pathological effects in the heart, liver, kidney and respiratory system. The basis for alkaloid regulation and its effects in the ruminal ecosystem are not yet fully understood, but is a well-known fact that its degradation is directly proportional to its concentration in the rumen. Heliotrine degrading bacteria have been isolated and identified in the rumen. Although these bacteria appear to get very little useful energy from the clevage of heliotrine, this characteristic may well improve their ability to successfully compete in the rumen of an animal exposed to this type of alkaloid.

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