Hemlock alkaloids from Socrates to poison aloes

Hemlock (Conium maculatum L. Umbelliferae) has long been known as a poisonous plant. Toxicity is due to a group of piperidine alkaloids of which the representative members are coniine and gamma-coniceine. The latter is the more toxic and is the first formed biosynthetically. Its levels in relation to coniine vary widely according to environmental conditions and to provenance of the plants. Surprisingly, these piperidine alkaloids have turned up in quite unrelated species in the monocotyledons as well as the dicotyledons. Aloes, for instance, important medicinal plants, are not regarded as poisonous although some species are very bitter. Nevertheless a small number of mostly local species contain the alkaloids, especially gamma-coniceine and there have been records of human poisoning. The compounds are recognized by their characteristic mousy smell. Both acute and chronic symptoms have been described. The compounds are neurotoxins and death results from respiratory failure, recalling the effects of curare. Chronic non-lethal ingestion by pregnant livestock leads to foetal malformation. Both acute and chronic toxicity are seen with stock in damp meadows and have been recorded as problems especially in North America. The alkaloids derive biosynthetically from acetate units via the polyketide pathway in contrast to other piperidine alkaloids which derive from lysine.