Glycosides In Pharmacognosy

Glycosides are the naturally occurring organic compounds found in plants and some animals, which upon hydrolysis (either acid or enzymatic) give one or more sugar (glycone) moiety and non-sugar (aglycone) moiety. The non-sugar or aglycone moiety is called genin. The pharmacological activity of any glycoside is mainly due to the presence of the genin part whereas the glycone part facilitates the transportation of the genin part to the site of action. The most frequently occurring sugar is β-D-glucose, although rhamnose, digitoxose, cymarose, and other sugars are components of glycosides. When the sugar part is glucose then it is known as a glucoside. Other sugars may be developed during hydrolysis then the term glycoside is applied.

Glycosides are acetals in which a hydroxyl group of sugar is attached with a hydroxyl group of a non-sugar component. The secondary hydroxyl group is condensed within the sugar molecule itself and forms an oxide ring. In another way, glycosides are considered sugar ethers. Both alpha and beta glycosides are possible which depends upon the stereo configuration of the glycosidic linkage. The glycosidic linkage is the link between glycone (sugar) and aglycone (non-sugar) part of a glycoside. Beta glycosides widely occur in plants.

Biologically, glycosides have a role in plant life include regulatory, protective, and sanitary functions. They have many functions like cardioactive (digitalis, stropanthus, squill, convallaria, Apocynum, etc), laxative (senna, aloe, rhubarb, cascara, frangula, etc.), analgesic (methyl salicylate from wintergreen), a local irritant (allyl isothiocyanate from black mustard).

It’s very difficult to classify the glycosides because if the classification is based on the sugar group then several rare sugars are involved and the structures of which are not well known. On the other hand, if the aglycone group is used as a classification base, one encounters groups from probably all classes of plant constituents like tannins, sterols, carotenoids, anthocyanins, and many others whose structures are still unknown. A therapeutic classification is good from a pharmaceutical point of view which omits many glycosides of pharmacognostic interest.

Classification of Glycosides

(a) Based on the Linkage:

  1. O-glycosides: In these glycosides, the sugar part is linked with alcoholic or phenolic hydroxyl or carboxyl group.
  2. S-glycosides: In these glycosides, the sugar is attached to a sulfur atom of aglycone such as in sinigrin.
  3. N-glycosides: In these glycosides, the sugar is linked with the nitrogen atom of (-NH2,-NH-) amino group of aglycone like in nucleosides DNA, RNA.
  4. C-glycosides: In these glycosides, the sugar is linked (condensed) directly to a carbon atom of aglycone like in aloin.

Note: N & C-glycosides are not hydrolyzed by acids or alkalis or by enzymes.

Schematic representation of various types of glycosides
Fig.1: Schematic representation of various types of glycosides

(b) According to aglycone part:

  1. If aglycone part alcohol: This group is called an alcoholic group like Salicin.
  2. If aglycone part aldehyde: This group is called an aldehydic group like glucovanillin.
  3. If aglycone part phenol, it is called phenolic group like arbutin.
  4. If aglycone part cyanone, it is called cyanogenic or cyanophoric or cyanoside like amygdalin.
  5. If aglycone part thio, it is called isothiocyanate glycosides like sinigrin or sinalbin.
  6. If aglycone part anthracene, it is called anthraquinone glycoside. example sennoside.
  7. If aglycone part steroid, it is known as steroidal glycoside (cardiac) e.g. Digoxin.
  8. If flavone, flavonol, flavanone known, it is as flavonoid glycoside.
  9. If triterpenoid it is called saponin glycoside e.g. glycyrrhizin, melanthin, ginsenoside.

(c) According to glycone part:

  1. Glucose: Glucoside group like in Sennoside.
  2. Rhamnose: Rhamnoside like in frangulin.
  3. Digitoxose: Digitoxoside like in digoxin.
  4. Glucose and Rhammnose: Glucorhamnoside – glucofrangulin.
  5. Rhamnose and glucose: Rhamnoglucoside – Rutin

Physico-Chemical Properties of Glycosides

  • They are colorless, amorphous, solid, non-volatile compounds.
  • They give a positive test with Molisch and Fehling’s solution test.
  • They have solubility in water but are insoluble in organic solvents.
  • Most of them possess bitter taste but some exceptions are populin, glycyrrhizin, stevioside.
  • They are odorless compounds except for saponin (glycyrrhizin).
  • Glycosides get hydrolyzed by mineral acids and temperature or by enzymes like

(a) Emolsin: Bitter almond seeds.

(b) Myrosin or Myrosinase: Black mustard seeds.

(c) Rhamnase: Glycosides containing rhamnose as sugar part.

Tests For Identification of Glycosides

General Test:

Test A: In this test, 200 mg drug is extracted with 5 ml sulphuric acid (dilute), warm on water bath and filter. Take the acidic extract and neutralize it by adding sodium hydroxide solution (5 percent). After neutralization Fehling solutions A and B (0.1 ml) are added and heated on a water bath for a few minutes (2-3 minutes). Red precipitate produced, note the quantity of precipitate and compare it with red precipitate formed in test B.

Test B: Here 200 mg drug extracted with water (5ml), boil it then add water in the same volume as used for sodium hydroxide in the previous test. After that add Fehling solution A and B (0.1ml) and heat it in a water bath for 2 to 3 minutes. Red precipitate forms. Note the quantity of precipitate and compare it with precipitate formed in test A.

Note: If test A precipitate ≥ test B precipitate————-Glycoside may be present.

Where test B indicates the amount of free reducing sugar already present in the drug and test A shows the presence of free reducing sugar which forms upon acid hydrolysis of glycoside in crude drug.

Structure of alpha and beta methyl glucoside
Fig.2: Structure of alpha and beta methyl glucoside

Extraction Method of Glycosides- Stas-Otto Method

  • The drug should be coarsely powdered for extraction purposes.
  • The temperature should not exceed 45°C for thermolabile drugs.
Stas - Otto method for extraction of Glycoside
Fig.3: Stas – Otto method for extraction of Glycoside
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