41. Interaction of lectin from Viscum album L. with arabinogalactan-proteins from Echinacea purpurea L. Moench.

Today mistletoe extracts play an important role in adjuvant cancer treatment because of their immunostimulating properties. The main active components are mistletoe lectins, ribosome-inactivating proteins of type II consisting of two protein chains linked by an intermolecular disulphide bridge. Clinical studies have shown that mistletoe lectin I enhances the secretion of cytokines and interleukins and increases the number of natural killer cells. As recognition and binding of mistletoe lectins to sugar chains present on the cell surface of immunogenic cells is the first step in biological activity, knowledge on sugar-binding properties of lectins is significant in determining their medicinal potential. Since many years, it is known that ML I binds to galactose, but current investigations propose a specific-binding site for ML I consisting of N-acetylglucosamin, galactose and terminal N-acetylneuraminic acid residues.

It has already been shown, that mistletoe polysaccharides, especially acidic rhamno-arabinogalactans interact with ML I. In order to enhance knowledge on possible binding sites of mistletoe lectins, we examined interaction of ML I with an arabinogalactan-protein (AGP) from Echinacea purpurea. The polysaccharide moiety of the AGP is composed of a backbone of 3-linked [beta]-D-Galp residues, highly branched at O-6 to (1 [right arrow] 6)-[beta]-D-galactopyranosyl side chains, terminating predominantly in Araf mono- or oligosaccharides. Using biomolecular interaction analysis (BIACORE), the influence of AGP on the binding of ML I to asialofetuin, immobilized on a biosensor chip, was measured. Asialofetuin has strong affinity to ML I, but addition of AGP led to a reduction of ML I binding of up to 70%. Testing of a (1 [right arrow] 6) linked galactose nonasaccharide, carrying three arabinose residues in position 3, revealed nearly similar binding capacities. Chemical modification of AGP with loss of terminal arabinose residues (GP) resulted in enhanced interaction, leading to complete loss of binding of ML I to asialofetuin. We conclude that there is strong binding of ML I to branched galactose-oligosaccharides, consisting of 1,3-, 1,6- and 1,3,6-linked galactopyranosyl residues.

Keywords: Viscum album; Lectin; Echinacea purpurea; Arabinogalactan-protein; BIACORE

B. Classen (a), B. Herbst (a), W. Blaschek (a), K. Pfuller (b), U. Pfuller (b,*)

(a) Pharmaceutical Institute, Christian-Albrechts-University of Kiel, Gutenbergstrasse 76, 24118 Kiel, Germany

(b) Institute of Phytochemistry, Private University Witten/Herdecke gGmbH, Stockumer Strasse 10, 58453 Witten, Germany

*Corresponding author. Tel.: [phone], fax: [phone].

E-mail address: [e-mail] (U. Pfuller).