Human galectin binds to multivalent arabinogalactan-protein isolated from aerial parts of Echinacea purpurea

Abstract

Pressed juices from the aerial parts of Echinacea purpurea are used as unspecific immunostimulants, e.g. against common cold. For an arabinogalactan-protein (AGP) from this plant material, immunomodulating activities have been shown in vitro, such as activation of the human complement system and binding to human leucocytes (1, 2). The question arises, how high-molecular-weight arabinogalactan-proteins interact with the human immune system when taken orally. One possibility is uptake of AGPs in the area of Peyer's patches of the intestinal immune system (3). Another possible mode of action might be binding of AGPs to galectins present at the brush border membrane of the intestine. Galectins are a family of multifunctional proteins, located in several cells and tissues and characterized by a conserved carbohydrate-binding domain with affinity for β-Gal containing glycoconjugates. Using different ELISA techniques, we proved binding of AGP isolated from aerial parts of E. purpurea to human recombinant galectin 3. Galectin 3 is expressed widely in immune cells and also epithelial cells, including gastric and colon mucosa (4). Enhanced binding affinity was found after partial acid hydrolysis of the AGP, which might also happen in the human stomach.