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. 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. 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 β-D-Galp residues, highly branched at O-6 to (1→6)-β-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 strong reduction of ML I binding. Testing of a (1→6) linked galactose nona-saccharide, 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.

We conclude that there is strong binding of ML I to galactose-oligosaccharides, consisting predominantly of 1,3- and 1,6-linked galactopyranosyl residues.