Glycosides

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Chapter 24 Glycosides

Chapter contents

Anthraquinones 182

Saponins 182

Cardiac glycosides 183

Cyanogenic glycosides 183

Isothiocyanates (glucosinolates) 184

Many of the plant secondary metabolites are found naturally attached to sugars (glycosides). Mostly, the sugars are monosaccharides (Figure 24.1), such as glucose, but they can be more complicated. The sugars link to a non-sugar part called an aglycone and can be attached by separate bonds or, more commonly, as di-, tri- or tetrasaccharides. This is done by one sugar attaching to the aglycone and the others linking onto that sugar.

Figure 24.1 The structure of glucose, a common component of glycosides.

The linkage between the sugar and the aglycone (glycosidic linkage) is difficult for human enzymes to break down. This is where having a healthy gut is important, as this job is done by the various microbes in the gut. When the two parts have been separated, the aglycone (the non-sugar part) is small enough to be transported through the gut wall into the bloodstream, where it is sent around the body.

The linkage between the two groups can vary. It can be:

• Phenolic hydroxyl group O-glycoside.

• Carbon group C-glycoside (Figure 24.2), e.g. aloin (which was one of the first glycosides to be isolated).

• Nitrogen-glycoside, N-glycoside: this term is falling out of used and is being replaced by glycosylamine.

• Sulphur-glycoside, S-glycoside: thioglycoside (glucosinolates; see p. 184).

Figure 24.2 The C-glycoside linkage. The sugars attached vary in composition and can consist of one or more sugars.

The well-established naming of glycosides using the termination ‘in’ (e.g. salicin, aloin) has persisted and results in some confusion, as some substances (e.g. pectin) are not glycosides. Similar examples of this, involving isoflavones, are discussed in Chapter 21 ‘Phenols’ (p. 161). The more modern termination ‘-oside’ can be used (e.g. sennoside) to prevent confusion.

Glycosides are generally inactive and are activated by a process of hydrolysis into:

• a sugar moiety (glycone)

• a non-sugar moiety (aglycone).

This is usually done with the help of specialized bacteria in the colon. Classification of a glycoside is based on the type of aglycone. For example, phenolic glycosides have a phenol attached.

Anthraquinones

It has been shown experimentally that the sugar moiety certainly of sennosides A and B (see Figure 21.9, p. 155) remains unaltered in both the stomach and gut, but is cleaved off in the caecum by the activity of microorganisms, which convert them to dianthrones. The dianthrones that remain in the gut are cleaved again to form an anthrone and anthraquinone (see Figure 21.8, p. 154), which produce the laxative effects.

The activity of the glycosides of anthrones is very high, which is why herbs containing them have to be stored for a while or subjected to heat treatment so that the majority are converted to anthraquinone glycosides.

Saponins

• The glycosides of triterpenes, steroids or steroidal alkaloids found in plants.

• So called because these compounds froth when shaken with water.

• They also cause haemolysis of red bloods cells on direct contact.

Glycyrrhizin (see Figure 24.4) is a saponin.

Cardiac Glycosides

• Asclepiadaceae: Cynanchum spp. (Bai Wei, Bai Qian).

• Lilaceae: Convallaria majalis (lily of the valley).

• Ranunculaceae: Helleborus niger (black hellebore).

• Scrophulariaceae: Digitalis purpurea (foxglove), Scrophularia spp. (figwort).

The Heart

Cardiac glycosides exert a positive inotropic effect on the heart in cardiac failure (see Chapter 26 ‘Cardiovascular disorders’, p. 191). Cardiac failure occurs when the heart is unable to pump blood effectively at a rate that meets the needs of the body. The heart muscle can perform only weakly, particularly on ventricular contraction. This reduced pumping capacity results in a reduced heart output. However, as new blood continues to enter the heart, the volume of blood in the heart increases. As the heart is unable to pump this blood out, it becomes congested, hence ‘congestive heart failure’. Cardiac glycosides (or the aglycones) increase the capacity of the heart muscle to pump.

The mechanism of action of the cardiac glycosides is still not clear, but the most widely accepted idea is that the cardiac glycosides inhibit the membrane-bound sodium and potassium pumps responsible for the sodium and potassium exchange (see Figure 31.1, p. 235).

There are two types of cardiac glycoside (Figure 24.3):

• bufadienolides: C24 (24 carbon atoms)

• cardenolides: C23 (23 carbon atoms).

Figure 24.3 Comparison of structures of aglycones of cardiac glycosides. Boxes highlight the difference in structure.

Cyanogenic Glycosides

Cyanogenic glycosides (Figure 24.4) yield hydrocyanic acid (HCN) as one of the products of hydrolysis, hence the name ‘cyanogenic’. They are capable of producing cyanide when broken down.