Constituents of mistletoe extracts

The biological and pharmacological effects of the mistletoe constituents have been widely analysed  (overview see [174, 175]), and it has been possible – in some cases as early as the middle of the last century – to isolate many different pharmacologically active components, such as mistletoe lectins (ML I, II, III) [186], viscotoxins [187], Kuttan’s peptides [188], oligo- und polysaccharides [189, 190, 191], lipids [192], flavonoids [191], thiols [193], plant acids, phytosterols und sterols, phenylpropanes, lignans, alkaloids, minerals, trace elements and various other proteins [174, 175, 194, 195, 196] and partly triterpenes [192, 197]. The content of the different constituents varies depending on season, stage of development of the plant, time of harvest, habitat and host tree [198].

Consequently, mistletoe extracts contain a large number of different constituents with different modes of action which can only develop their complex and cross-system effects with agonistic/antagonistic and synergistic/costimulatory effects when used together. Thus, the effects cannot be only reduced to individual constituents, but only the total extract can unfold its maximum effect.

To take account of the seasonal differences in concentration of the various constituents, anthroposophical mistletoe preparations – strictly separated according to their host tree – are usually harvested twice a year at summer and winter solstice, and the extracts are then mixed together in a specific machinery process.

 

Last update: June 10th, 2020/AB

Mistletoe lectins

At present, the structure, active profile and modes of action of mistletoe lectins, in particular ML I, are the best studied. Mistletoe lectins are glycoproteins with a carbohydrate content of 4 – 12 percent depending on the host tree and time of harvest with the specific characteristic to agglutinate cells and specifically recognise and bind certain sugars. As monomers they have a molecular weight between 50 and 63 kDa. They consist of a toxic A-chain (29 kDa, 254 amino acids) with enzymatic properties and a carbohydrate-binding B-chain (34 kDa, 264 amino acids), which are connected by a disulphide bridge.

The three different mistletoe lectins ML I, ML II and ML III can be identified by their sugar specificity and molecular weight. ML I binds specifically to D-galactose, ML II to D-galactose and N-acetyl-galactosamine and ML III to N-acetyl-galactosamine. Within the cell the disulphide bridge connecting the A and B subunits is reduced during molecular transport, so that the two subunits dissociate. The free A subunit of the mistletoe lectin becomes a potent ribosome-inactivating protein in the cytosol, irreversibly inhibiting protein biosynthesis and thus initiating apoptosis. Mistletoe lectins I, II and III thus belong to the type 2 ribosome-inhibiting proteins [175, 193, 199, 200, 201, 202, 203].

Only about 1% of mistletoe proteins are mistletoe lectins. The lectin content is subject to a wide range of variations and is particularly dependent on the host tree and season. Oak, poplar and apple tree mistletoe are particularly rich in lectins, with ML I clearly predominant here. Pine mistletoe contains considerably less lectins (predominantly ML III) and almost no ML I. Furthermore, mistletoe contains considerably larger quantities of mistletoe lectin in winter than in summer, which are mainly located in the centre of the mistletoe bush and in the sinker. The structure of mistletoe lectins is very similar to those contained in the castor oil plant [177, 204].

Although the lectins are only present in mistletoe extracts in low concentrations, their pharmacological activity is an important factor in the efficacy of mistletoe preparations. The anti-tumour effects of mistletoe lectins were already demonstrated in the 1970s and have been investigated in numerous in vitro experiments since then. It was demonstrated that mistletoe lectins act by directly damaging tumour cells and indirectly by stimulating immunological processes.

Direct cytotoxicity is mainly based on the inhibition of protein synthesis and the induction of programmed cell death (apoptosis). The activation of the immune system is achieved, among other things, by increasing the number and activity of natural killer cells (NK cells) and T-helper cells (Th cells), releasing beta-endorphin, reducing progression and alleviating the side effects of conventional cancer therapies. Treatment with mistletoe lectins has also been shown to increase the activity of lymphatic cells, significantly augment cytokines IL-1, IL-6, IL-10, IL-12, IFN-gamma and TNF-alpha levels within the serum, increase phagocytosis activity and amplify oxidative burst. Protection of cellular DNA against methylation was also observed [175, 201, 202].

 

Last update: June 10th, 2020/AB

Viscotoxins

Besides lectins, viscotoxins represent the other pharmacologically important and typical mistletoe constituents. They are low-molecular, heat-resistant, strongly alkaline polypeptides consisting of 46 amino acids and having a molecular weight of about 5 kDa. Three disulphide bridges are responsible for the high stability of viscotoxins.

Due to their high cysteine content, they are classified into the group of thionines, of which the isoforms viscotoxin A1, A2, A3, B and 1-PS are still known today. In their chemical structure viscotoxins resemble the cardiotoxins of snakes, especially those that are highly concentrated in the cobra toxin [174, 200, 202, 204, 205, 206, 207, 208].

As is the case with lectins, viscotoxin content of the mistletoe depends on the time of harvest and the type of host tree. For example, mistletoe from different host trees contain different amounts of viscotoxins, which are mainly found in very young leaves, stems and flower-bearing short shoots (including berries), i.e. in the periphery of the plant. The sinker contains no viscotoxins. The viscotoxin content is highest in June. Thus the viscotoxins behave in exactly the opposite way to the mistletoe lectins [198].

The effectof viscotoxins is less well researched than that of lectins. It is known that they have an immunogenic effect just like the mistletoe lectins and that they induce anti-viscotoxin antibodies after repeated exposure. A cytotoxic effect has also been demonstrated, but in contrast to mistletoe lectins this effect is caused by the rapid lysis of the cell membrane, i.e. necrosis [174, 204]. Their toxic effects are presumably caused by the binding of thionines to membrane phospholipids and subsequent pore formation and cell wall damage.

The various viscotoxins differ considerably in their cytotoxicity. Viscotoxins A3 and 1-PS have the strongest cytotoxicity, in contrast to viscotoxin B, which has only about 1/15 of the cytotoxic effect of viscotoxins A3 and 1-PS.

Viscotoxins also increase the activity of the cytotoxic T-cells and granulocytes (respiratory burst, phagocytosis), so that bacteria and probably also tumour cells can be destroyed more effectively. They inhibit RNA, DNA and protein synthesis, increase the NK-cell-mediated cytotoxicity towards tumour cells and cause a release of IL-6 [174, 175, 202, 209].

 

Last update: June 10th, 2020/AB

Kuttan's peptides

Kuttan's peptides are low-molecular, heat-stable peptides with a molecular weight of about 5 kDa, which exhibit some of the properties of viscotoxins. The molecules show cytotoxic and immunostimulatory effects. In animal experiments, an anti-tumoural effect could also be observed when applied locally [188].

 

Last update: June 10th, 2020/AB

Oligo- and polysaccharides

The mistletoe plant also contains oligo- and polysaccharides, the contents being subject to seasonal fluctuations. Polysaccharides are high-molecular sugar polymers formed from monosaccharides to which proteins can bind.

Both poly- and oligosaccharides contained in mistletoe are immunologically and anti-tumourally active substances. In animal experiments mistletoe polysaccharides protected against radiation damage and significantly improved survival time of gamma-irradiated mice. It is assumed that the binding of polysaccharides to mistletoe lectins modulates their effect and may possibly lead to a stabilisation of the lectins in the mistletoe extract [175, 191, 202]. 

 

Last update: June 10th, 2020/AB

Flavonoids

Numerous different flavonoid derivatives can be detected in the mistletoe plant, in particular quercetin and quercetin methyl ester, which are predominantly present in glycosylated form. These induce apoptosis in various cell culture models and exhibit scavenger properties [175, 191, 202].

 

Last update: June 10th, 2020/AB

Thiols

Mistletoe extracts contain a high concentration of thiols such as glutathione, which also exhibit good scavenger properties.

In addition, the activity of poly-ADP-ribose polymerase (PARP), the key enzyme in DNA repair processes, is significantly influenced by the thiol content. The anti-oxidative potential of these substances could contribute to the mistletoe's general effectiveness [193].

 

Last update: June 10th, 2020/AB

Triterpenes

Very early on, triterpenes were identified as constituents of mistletoe, but did not receive increasing attention until the beginning of the 21st century when their anti-tumour properties were recognized [192]. Among seven triterpenes identified in mistletoe, oleanolic acid is the predominant representative with concentrations of up to 3 percent of dry weight. Anti-proliferative and apoptosis-inducing effects have been demonstrated both for oleanolic acid and for other pentacyclic triterpenes isolated from mistletoe, such as betulinic and ursolic acid [192, 197].

In aqueous mistletoe extracts, previously available as pharmaceuticals, only small amounts of triterpenes can be detected. There are, however, new pharmaceutical developments using these glue-like substances as an ointment for the topical treatment of lesions. They have been described as being well tolerated by the skin and having favourable effects on eczema, ulcers, burns and a granulation-promoting effect [210].

 

Last update: June 10th, 2020/AB

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