Liquorice (licorice; glycyrrhiza) is the dried unpeeled rhizome and root of the perennial herb Glycyrrhiza glabra (Leguminosae/Fabaceae). A number of different varieties are cultivated commercially, including G. glabra var. typica (Spanish liquorice) in Spain, Italy, and France, and G. glabra var. glandulifera (Russian liquorice) in Russia. Russian liquorice is usually peeled
before drying. Glycyrrhiza uralensis (Manchurian liquorice) from China is also commercially important. Much of the liquorice is imported in the form of an extract, prepared by extraction with water, then evaporation to give a dark black solid. Most of the liquorice produced is used in confectionery and for flavouring, including tobacco, beers, and stouts. Its pleasant sweet taste and foaming properties are due to saponins. Liquorice root contains about 20% of water soluble extractives, and much of this (typically 3-5% of the root, but up to 12% in some varieties) is comprised of glycyrrhizin, a mixture of the potassium and calcium salts of glycyrrhizic (glycyrrhizinic) acid (Figure 5.63). Glycyrrhizic acid is a diglucuronide of the aglycone glycyrrhetic (glycyrrhetinic) acid. The bright yellow colour of liquorice root is provided by flavonoids (1-1.5%) including liquiritin and isoliquiritin (Figure 5.63), and their corresponding aglycones (see page 150). Considerable amounts (5-15%) of sugars (glucose and sucrose) are also present.
Glycyrrhizin is reported to be 50-150 times as sweet as sucrose, and liquorice has thus long been used in pharmacy to mask the taste of bitter drugs. Its surfactant properties have also been exploited in various formulations, as have its demulcent and mild expectorant properties. More recently, some corticosteroid-like activity has been recognized, liquorice extracts displaying mild anti-inflammatory and mineralocorticoid activities. These have been exploited in the treatment of rheumatoid arthritis, Addison's disease (chronic adrenocortical insufficiency), and various inflammatory conditions. Glycyrrhetic acid has been implicated in these activities, and has been found to inhibit enzymes that catalyse the conversion of prostaglandins and glucocorticoids into inactive metabolites. This results in increased levels of prostaglandins, e.g. PGE2 and PGF2a (see page 54), and of hydrocortisone (see page 268). Perhaps the most important current application is to give systematic relief from peptic ulcers by promoting healing through increased prostaglandin activity. A semi-synthetic derivative of glycyrrhetic acid, the hemisuccinate carbenoxolone sodium (Figure 5.63), is widely prescribed for the treatment of gastric ulcers, and also duodenal ulcers. The mineralocorticoid effects (sodium and water retention) may exacerbate hypertension and cardiac problems. Surprisingly, a deglycyrrhizinized liquorice preparation is also marketed for treatment of peptic ulcers, but its efficiency has been questioned.
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