Chemical defence in chrysomelid larvae and adults

Astragalus L. is the largest genus in the family Leguminosae (subfamily Papilionideae, tribe Galegeae). Widely distributed throughout the temperate region of the world, is principally located in Europe, Asia and North America, but also on mountains in Africa and South America. They are annual and perennial herbs or small shrubs. Astragalus species are divided in two main groups: the medicinal plants and the poisonous species. “Astragali radix” (dried roots of A. Membranaceous Bunge and other Astragalus spp.) represents a very old and well knowndrug in traditional Chinese medicine. They are officially listed in the Chinese Pharmacopoeia and prescribed mainly as an antiperspirant, a diuretic and a tonic, but also for their hepatoprotective, antioxidative, immunostimulant and antiviral properties. The other most common use of Astragalus is as forage for liverstock and wild animals; however a number of species are toxic for cattle and in many cases this toxicity could be transferred to humans through meat and milk. From a chemical point of view the biologically active principles of Astragalus species consist of saponins, polysaccharides and phenolics, while the toxiccompounds include imidazoline alkaloids, nitro toxins and selenium derivatives. This review deals with the chemistry, purification procedures, structure elucidation methods and biological activity of triterpenoidic sapogenins and saponins, the most widely studied secondary metabolites from Astragalus . However the other important metabolites, such as phenolics, polysaccharides, alkaloids, nitro compounds and seleniferous derivatives, have been considered and a brief summary of their important biological properties has also been included.

Secondary metabolites of genus Astragalus: Structure and biological activity

Glycosides of 3-nitro-1-propanol (nitropropanol) and glucose esters of 3-nitro-1-propanoic acid (nitropropionic acid) occur in many forages distributed throughout the world. Systemically, nitropropionic acid irreversibly inactivates succinate dehydrogenase, thereby blocking ATP formation. Nitropropanol is not toxic per se in mammals but is converted to nitropropionic acid by hepatic alcohol dehydrogenase. Nitrotoxins can be metabolized by rumen microbes, which may provide a mechanism for detoxification. At least 20 different ruminal bacteria are known to metabolize the nitrotoxins, but most appear to play a minor role in detoxification. Evidence suggests that an obligate anaerobic nitro-respiring bacterium, Denitrobacterium detoxificans, may be particularly important in conferring protection to animals consuming the nitrotoxins as this bacterium metabolizes the toxins at rates near those by mixed ruminal populations. Rates of ruminal nitrotoxin metabolism can be enhanced by modifying the rumen environment through dietary manipulations, which suggests in vivo enrichment of competent nitrotoxin-metabolizing bacteria such as D. detoxificans.

E. Ubben

Papers

1,6-Di-3-nitropropanoyl-β-d-glucopyranoside from Astragalus cibarius