Pyrrolizidine Alkaloids

The pyrrolizidine alkaloids such as seneciphylline from ragwort, Senecio jacobaea, and monocrotaline from Crotolaria species (Figure 10.4) are well-studied examples of metabolized plant substances converted to insect use. The subject of pyrrolizidine alkaloids and their presence in insects is too detailed to consider fully here. Taxonomically unrelated insect groups feed on unrelated plants, chiefly in the Asteraceae and Boraginaceae which contain these alkaloids and store the compounds in their bodies. The alkaloids consist of two parts, the basic pyrrolizidine (the necine part), and various hydroxy and branched acids esterified to it (the necic acid part). A feature of pyrrolizidine alkaloids is that they exist in two interchangeable forms, the non-toxic N-oxides, and the pro-toxic free bases. They only become toxic when the free bases are metabolized to highly reactive pyrroles by cytochrome P450 oxidases. In most plant taxa the alkaloids are stored as the N-oxides. On ingestion by many insects they seem to be reduced in the gut to free bases and then re-oxidized for storage. The origin in the plant of the nitrogen-containing necine part is putrescine, which by exchange with spermidine gives first homospermi-dine (Figure 10.4), while the carboxylic acid portion of seneciphylline comes from amino acids, with many changes. In monocrotaline, the dicarboxylic acid comes from two molecules of 2-methylbutanoic acid.


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