Introduction

Extracts from the European mistletoe (Viscum album L.) exhibit both, cytotoxic properties and pronounced immunomodulating abilities (see Büssing, this book). Mistletoe extracts containing carbohydrate-binding proteins (lectins), thionins (viscotoxins), flavonoids and many other components, but also the mistletoe lectins (ML) by themselves, are targets of clinical studies in cancer treatment and other diseases.

The evergreen plant is growing on host trees such as apple tree, acer, robinia, poplar, wallow, and oak. The host tree and other natural factors like sex, local climate, harvesting time, parts and age of plant used etc. have a major influence on the content of proteins, polypeptides and carbohydrates (Hincha et al, 1997).

CHEMICAL CONSTITUENTS OF EUROPEAN MISTLETOE Lectins—Common Introduction

Lectins are sugar-binding proteins derived from plants, animals or bacteria which do not change chemically the recognised carbohydrate structures and do not exert antibody function (Sharon and Lis, 1989; Pusztai and Bardocz, 1995; Rhodes and Milton, 1998). The lectins show specificity for terminal and/or subterminal carbohydrate residues and have at least two sugar-combining sites. Therefore, lectins will specifically recognise and bind to carbohydrate residues on cell surface, to cytoplasmic and nuclear structures, and to components of extracellular matrix. The binding ability of lectins is usually described in terms of simple sugars inhibiting lectin binding to its targets. This is an oversimplification because the complex sugar sequences and structures recognised by a lectin are in most cases unknown. Lectin binding sites of a glycanor glycoconjugate consist usually of 1 to 4 saccharide moieties. Further, non-covalent interactions with hydrophobic or other residues of the target molecule may enhance the binding.

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