The alkaloids represent a large group of chemical compounds found in many plants which are mainly identified by their nitrogen containing, heterocyclic rings. As may be inferred by their name, alkaloids are usually somewhat basic, with the nitrogen usually accepting the protons. At physiological pH found in the cytosol and the vacuoles, the nitrogen is protonated making the alkaloid water soluble.

          As defensive compounds, alkaloids "advertise" their presence as severely bitter tasting. For example, quinine is an alkaloid found in cinhona bark which tastes bitter to humans in concentrations as low as 1X10-6 M. Actual toxicity, however, is dependant on the dosage involved. Solanine is present in tiny amounts in all eating potatos, but the concentration only reaches hazardous amounts in tubers which have grown and been exposed above the ground.


Some alkaloids can prove to be fatal to some organisms but useful to others. Pyrrolizidine alkaloids, from weeds of the genus Senecio exemplify this role in mammals and insects.


                                        

          The above image is the metabolic path of pyrrolizidine alkaloids in various animals. It appears that this class of alkaloids are not in themselves toxic, but their subsequent metabolites become toxic. When these alkaloids are metabolized in most mammals, the final product becomes toxic when it binds to the liver. In some moths, these chemicals can be maintained in the body without any modification and without any harmful effects. Some butterflies, such as Danaid butterflies, use these alkaloids as materials for producing courtship chemicals (pheromones). After the male has synthesized the alkaloid into the subsequent components, they are stored in the hair pencils, and later play an important role in the male's courtship display (Harborne 1982).

          An example of a moth which is able to sequester alkaloids for its own protection is the arctiid moth, Utetheisa ornatrix (Hare and Eisner 1993). Larvae are born on some members of the Crotalaria genus of legumes. The larvae feed mostly on the plant's seeds which are heavily laden with alkaloids. The injested alkaloids are sequestered throughout the larval and adult stages and serve to protect the animals from predators such as spiders and birds (Hare and Eisner 1993). The eggs of this moth are also protected by the same compounds which protect the other life stages. This protecting agent is transfered to the egg by the mother, which has the chemicals sequestered from her larval stage. The female's alkaloid reserves are supplemented by a courtship nuptual gift from the male (Hare and Eisner 1993).

          Around the world, the pyrrolizidine alkaloids are responsible for many death of domestic animals (Bell 1981). In the United States, large numbers of livestock die from injestion of plants which produce alkaloids: lupines, larkspur, and grounsel (Taiz and Zeiger 1991).

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