Secondary Metabolites of Genus Astragalus: Structure and Biological Activity

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