ML amounts to not more than 2% of total polypeptide and protein content of mistletoe. While polypetides of viscotoxin type (see below) are well characterised, there are only few information on structure and biological properties of other proteins present in the plant. An impression of number and quantity of that proteins was given by Schink (1990) following results of two-dimensional SDS-PAGE. Vester (1977) described the isolation of a protein complex (VP-16) with 14-125 kDa, exhibiting anti-tumour activity in vivo and strong cytotoxicity in vitro (for review see Luther and Becker, 1987). Franz (1985) classifies these proteins according to their molecular weights at least to mono- and dimeric ML and their subunits. However, we and others were unable to reproduce the results described by Vester (unpublished).
Starting from commercial mistletoe preparations, Kuttan and co-workers (Kuttan and Kuttan, 1992a,b) isolated polypeptide fractions with immunomodulating activities. Similarly, these polypetides remain to be characterised.
Sakurai and Okumura (Okumura and Sakurai, 1973; Sakurai and Okumura, 1979) isolated a cyclic amphiphilic pentapeptide from Viscum album var. coloratum, termed viscumamide. This amide or related substances has not yet been described for the European mistletoe. However, there is no report on its biological activity. The cyclopeptide (Figure 7) has pronounced membrane binding properties as demonstrated by interaction with artificial membranes using Surface Plasmon Resonance Spectroscopy and BIA-technique (Becker and Pfüller, unpublished) and isolated lymphocytes (Büssing et al., unpublished results).
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