Glycoside#Cyanogenic glycosides
Cyanogenic glycosides are widespread plant toxins from the group of glycosides. They consist of a glycoside, i.e., a compound selected from an alcohol and a carbohydrate (sugar), which additionally carries a nitrile group -CN. In the enzymatic cleavage of it arises, inter alia, the toxic hydrogen cyanide HCN, also called hydrocyanic acid (hence the term cyanogen ).
- 3.1 Polymorphisms in the white clover and birdsfoot trefoil
Representative
- Amygdalin is found in many Prunus species such as plum, apricot and almond.
- Dhurrin found in sorghum and other millets.
- Linamarin occurs in plants such as cassava, lima bean or flax.
- Lotaustralin can be extracted from legumes, cassava, lima bean, the golden root, white clover and other plants.
- Sambunigrin found in elderberry and apple seeds.
- Simmondsin occurs in jojoba.
- Vicianin occur in tropical Goldtüpfelfarn
- Linustatin and neolinustatin are found in flaxseed.
Effect
Cyanogenic glycosides which have no toxic effect, only by cleavage of the molecule there is release of hydrogen cyanide ( HCN), which is the really toxic substance. The reduction is first enzymatically. By a more or less specific β -glucosidase of the sugar fraction ( mostly glucose) cleaved. The result is the free cyanohydrin. This spontaneously breaks or enzyme catalyzed by a hydroxy nitrile lyase in a ketone or aldehyde and HCN.
Critical to this process is referred to as cyanogenesis the initial cleavage of the sugar moiety. For example, the enzyme linamarase is required for cleavage of the glycoside linamarin. In the intact plant, both substances are located in different organelles before separated and the glycoside is maintained ( compartmentalization ). This has the advantage that the toxins are only released when it (such as insects ) will cause damage to the plant cells and so the poison is produced directly ( Dekompartimentierung, Plant defense against herbivores ).
Toxicity of cyanide
The cyanogenic glycosides released by cyanide is highly toxic to almost all animals. The reason for this is that the hydrocyanic acid intervenes in the metabolism. The photosynthesis of plants is not affected. The energy metabolism of the animals on the other hand is paralyzed in a very short time.
HCN binds to the central iron ion (oxygen - binding site ) of the enzyme cytochrome c oxidase, which can run a basic redox reaction of the respiratory chain. Wherein the enzyme is irreversibly inactivated. If a certain proportion of cytochrome c oxidase is bound by HCN, falls from the respiratory chain, and the organism does not get more energy.
Cyanogenic plants
Examples of plants whose toxicity is due to the release of hydrogen cyanide from cyanogenic glycosides, many rosaceous plants such as plum (Prunus domestica), blackthorn (Prunus spinosa), apricot (Prunus armeniaca ), almond (Prunus dulcis ), peach (Prunus persica ) sour cherry (Prunus cerasus), ferns such as the Goldtüpfelfarn ( Phlebodium aureum ), sorghum millet and cassava (Manihot esculenta ).
Polymorphisms in the white clover and birdsfoot trefoil
A polymorphism of cyanogenic glycosides occurs in Europe at various clover populations. Through breeding experiments can be concluded that two independent genes are responsible.
One of the genes controlling the synthesis of the cyanogenic glycosides. Only if this is dominant, the plant may even produce these substances.
The second gene is responsible for the enzyme that is required to cleave the glycosides and thus making dangerous. There are therefore four different types of clover in relation to their toxicity by cyanogenic glycosides. Only if both genes are dominant, it comes to the toxic effect. Otherwise, the plants can indeed synthesize glycosides, but can not develop it. Or enzymes are present, however, no glycoside that can be digested. In the fourth case neither glycosides nor enzymes are present.
Which genes the plant bears in itself, can be studied by chemical tests: water is fresh leaves into a test tube, they crushed shortly with a glass rod with a drop of chloroform and closes the tube with a stopper from which a piece in picric acid solution soaked filter paper hanging. A color within one hour indicates that hydrogen cyanide is present, and so that both genes are dominant.