Glycosides are organic or synthetic substances that arise from the reversible condensation of two or more ring-shaped sugars or from the condensation of a sugar with various alcohols via a so-called glycosidic bond, wherein in each case a H2O molecule is cleaved off. Glycosides are synthesized by many plants in almost unmanageable variety, and some of them are of high medicinal importance to humans, for example as cardiac glycosides or as so-called aminoglycoside antibiotics.

What are glycosides?

Some of the glycosides, such as certain flavonoids and so-called cardiac glycosides, are of importance as medicinal agents in the manufacture of medicines.

Glycosides or glycosides are organic compounds in which at least one alcohol radical (R-OH) is linked via a glycosidic bond to the anomeric C atom of at least one sugar radical. The glycosidic bond is formed by condensation of a cyclic sugar with an alcohol residue or another sugar. The sugar may have a five- or a six-membered ring, each with a different name.

At each glycosidic bond, one molecule of water (H2O) is split off. The glycosidic linkage is so widespread in plants and so diverse that it can be compared in importance with the peptide bond in the construction of polypeptides and proteins.

There is a barely manageable variety of glycosides, which is reinforced due to the different three-dimensional structure of the molecules with the same chemical empirical formula. Glycosides can not only be synthesized via metabolic processes in plant cells, but a few are also produced industrially using enzymatic or nonenzymatic methods. Some of the glycosides, such as certain flavonoids and so-called cardiac glycosides, are of importance as medicinal agents in the manufacture of medicines.

Pharmacological action

Some glycosides are used by the plants to ward off pathogenic germs or to protect against predators. Required toxins can be stored in the form of a glycoside as an inactive toxin in small cell inclusions (vacuoles or lysosomes). If necessary, the inactivated toxin can be released and activated by a specific glycoside appropriate for the glycoside.

Some glycosides are used by the pharmaceutical industry because of their pharmacological effects. These are mainly some flavonoids, cardiac glycosides and phenol glycosides. Of the approximately 6, 500 known flavonoids, a few show a vasodilator effect. They can reduce inflammation by inhibiting histamine.

Also anticonvulsant properties and antiviral effects of certain flavonoids are known. Likewise, the antioxidant effect of flavonoids is estimated. However, it should be noted that the antioxidant effect of quercetin, which is found in many plants as a secondary ingredient, is offset by the consumption of very little milk.

Some flavonoids can lower total cholesterol without affecting HDL fractions, so that the important ratio of LDL cholesterol and HDL cholesterol actually improves. The target value of the quotient is three or less.

Due to their antioxidant properties some naturally occurring flavonoids (apples, green tea, blueberries, cranberries, onions) are also given a certain protective effect against cancer.

Of particular importance is the so-called cardiac glycosides which, in the case of digitalis glycosides, increase the contraction force of the cardiac muscle, reduce the heart rate and slow down the electrical excitation conduction. Heart glycosides are found not only in the well-known thimble (Digitalis), but also in many other plants such as Adonis, bishop, lily of the valley and some others.

The skin of some poisonous toads contains cardiac glycosides in the form of bufadienolides, which were used as arrow poisons. From the group of phenol glycosides or phenyl glycosides, the arbutin and various anthraquinones have a special pharmacological significance due to their laxative effect.

Medical application & use

In addition to the antioxidant effects of certain flavonoids, a long-term study from California has shown that fisetin, a flavonoid found in the willow shrub, has positive long-term memory effects.

Digitalis glycosides, which are obtained from woolly and red foxgloves, are used for heart failure and sinus or AV nodal tachycardia (heart tachycardia) with a stroke rate of more than 100 beats per minute due to their pharmacological effects. Cardiac glycosides such as digoxin and digitoxin are often given in combination with an ACE inhibitor and / or a beta-blocker and a diuretic.

Arbutin and some anthraquinones, both substances belonging to the phenol glycosides, are used pharmacologically because of their laxative effect. Both agents are often components of laxatives. It should be noted that hydroquinone, an arbutin remodeling product, should not be taken more than five times a year for no longer than one week as it can damage the liver at higher doses.

Risks & Side Effects

For a drug therapy containing the cardiac glycosides digoxin and digitoxin, it must be considered that the half-life of digoxin is 1.5 days and that of Digitoxin is even 7 days because of the enterohepatic circulation. This means that gastric lavage and the administration of activated charcoal is necessary in the event of an overdose, which can lead to serious cardiac arrhythmias as well as to visual disturbances and discomfort and vomiting. In addition, the enterohepatic circulation can be interrupted with colestyramine, resulting in increased excretion of the digitoxin.

Cyanogenic glycosides often occur in plants in their green leaves. They serve the protection against predators. The damaged leaves release both the cyanogenic glycoside, as well as the corresponding glycosidase, which splits the glycoside and leads to the release of hydrogen cyanide (cyanide). Cyanide (HCN) is highly toxic to almost all organisms because it paralyzes the respiratory chain and causes poisoning within seconds. However, the concentration of cyanogenic glycosides in the leaves of plants is usually so low that there is no danger to humans.

Widely known is the cyanogenic glycoside amygdalin, which is present in the cores of bitter almonds and apricots and may lead to their excessive consumption to symptoms. Tags: 

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