Iridoid

Iridoids are phytochemicals that have been found in more than 50 plant families. Meanwhile, more than 2,500 different iridoids are known. The iridoids belong to the large group of terpenes or isoprenoids. The biosynthesis of isoprenoids takes place predominantly in the plastids. Iridoids are monoterpenes, that is, they are composed of two isoprene units - C5 - bodies - synthesized and thus have ten carbon atoms (C10 - body ). In general, are iridoids as bicyclic monoterpenes. The name of the base body and Iridodial of Iridomyrmecin derived from the ant genus Iridomyrmex; in the defensive secretions of the species Iridomyrmex purpureus Iridodial was first isolated in 1956.

Typical iridoids

Typical iridoids include, for example Aucubin and Catalpol of plantain ( Plantago lanceolata ) and Loganin from buckbean ( Menyanthes trifoliata ). Also in valerian (Valeriana officinalis) and Devil's claw ( Harpagophytum procumbens ) are iridoids and iridoid glycosides contain.

Function of Iridoidglycosiden

Iridoids are used to ward off predators of plants, but they also exhibit antimicrobial activity and provide the plant protection from microorganisms (bacteria and fungi). Iridoids are characterized among other things by an unusually bitter taste. So herbivore insects and vertebrates ( vertebrates ) are prevented from seizing.

Iridoid glycosides and glycosidases

Iridoids are in the plant cell often as glycosides before, that is, they are often via an O -glycosidic bond to a monosaccharide - often D -glucose - bound. As glycoside they unfold for the plant itself not toxic, as they are likely to be stored as a readily water-soluble substances such as many other secondary plant substances in the cell sap vacuole. By compartmentalization within the plant cell, it is avoided that the glycosides are connected with enzymes which can cleave the covalently bound sugar and thus enable the iridoids. In fact, possess plants which synthesize toxic glycosides, often a "suitable " enzyme, which converts the corresponding glycosides with a high affinity. If the plant is now attacked by predators ( herbivores ), the compartmentalization is canceled within plant cells by the seizure by the plant tissue is destroyed. Glycosidases, which are present often bound in the plasma membrane, can now come into contact with the released from the vacuole Iridoidglycosiden. The monoterpene is hydrolytically cleaved from the sugar molecule by enzymatic catalysis and the toxic aglycone is released. The aglycone is biologically very reactive. With the release of the aglycone, the proteins are denatured in the immediate vicinity and thus lose their nutritional value.

In addition to the deterrent effect of iridoids as bitter flavors and the consumption of iridoidhaltigem plant material has negative effects on the organism of herbivorous animals.

Many animals possess digestive enzymes, which are as well as the plant enzymes capable of cleaving the glycosidic bond between the Glukosemonomer and iridoid. Thus, the toxic iridoid from the digested plant tissue is released in the intestine and in the intestinal proteins present as well as protein molecules are denatured in the chyme. On the one hand decreases the usefulness of the proteins in the chyme as food, but on the other hand can also membrane proteins in the intestinal wall and other animal proteins are damaged in the intestine. If not adapted insect larvae eat iridoidhaltige plants, it affects in the form of lower growth of up to increased mortality.

There are different evolutionary strategies on how animals cope in the short term and long term in their food with toxic substances. A short-term strategy is the following: Many animals avoid it easy to eat plants that taste bitter. If the environment allows, they just change the food plant. Another possibility is a long-term adaptation to the toxic components in the forage. Here, the toxic plant compounds exert a strong negative selection pressure on individuals and populations. Only those individuals can survive, have the efficient detoxification mechanisms. Of course, also depends on the long-term survival of populations.

Synthesis of Iridoidglycosiden in plants

The biosynthesis of Iridoidglycosiden is investigated with the aid of labeled iridoid precursors since the late 1970s. It varies in different genera easily. Generally it can be assumed that iridoid glycosides are synthesized in plants starting from 8 -epi- Iridodial and 8 -epi- Desoxyloganinsäure (see Sampaio - Santos and Kaplan, 2001).

Synthesis and function of iridoids in animal organisms

Not only in plants, iridoids and their derivatives are formed to a lesser extent iridoids and iridoid glycosides are also in a few species of insects and arachnids synthesized as a chemical defense substance ( for example, actinidin, chrysomelidial, Dolichodial, Iridodial, nepetalactone, Plagiodial ). From various leaf beetles ( Chrysomelidae ) iridoids de novo synthesized and stored in defensive glands as defensive secretions. Other leaf beetle take isoprenoid precursors on from their food plants and use them for the synthesis of specific iridoids, which are also stored in the defensive glands. Here, the isoprenoid precursors on the midgut wall of the beetles are recorded. So that they pass specific transport molecules (carriers) and get into the hemolymph. From the hemolymph, they are also transferred over Carrier in the defensive glands. In the defensive glands even then, the synthesis of the defense secretion takes place. When threatened, droplets of military secretion are excreted dorsal. Once a predator coming into contact with, he is held by the strong bitter taste of the food. Eats the people interested in the same animal beetle spite of the bitter taste, then place in the intestine also the above described enzymatic cleavage of the iridoid glycosides instead, which in turn has protein denaturing effects.