Alkaloids

Distribution, Identification tests, pharmaceutical applications, isolation, the therapeutic activity of Alkaloids. 

Distribution:

Most of the alkaloids yields from the angiosperms, Leguminosae, Papaveraceae, Ranunculaceae, Solanaceae, Rubiaceae, Berberidaceae, Liliaceae and Amaryllidaceae plant families.
Physical and Chemical Properties:

  1. Alkaloids are colourless, crystalline, non-volatile and bitter solids.
  2. Most of them are optically active and are laevo-rotatory.
  3. They are insoluble or sparingly soluble in water and fully soluble in organic solvents like alcohols, ether, chloroform.

Exceptions are the following:
1. Coniine, nicotine and hygrine are liquids.

2. Berberine salts are coloured.

3. Ephedrine, colchicine and mesocaline are non-heterocyclic alkaloids.

4. Papaverine is optically inactive and ricinine is neutral.

Isolation of Alkaloids: Alkaloids are biosynthesised due to participation of several amino acids and enzymatic systems.
a. Tobacco alkaloids are derived from ornithine. b. In the biogenesis of solanaceous alkaloids, phenylalanine, ornithine and methionine are involved. c. The indole alkaloids of rauwolfia, vinca, nux-vomica etc. are derived from tryptophan. d. Phenylalanine is also the precursor for biogenesis of ephedra, opium and colchicum alkaloids.

Extraction of Alkaloids:
The specificity of the method of extraction depends upon the chemical nature of alkaloid to be extracted. The general method of extraction is as follows:
Step-1 The drug is powdered and extracted with ethanol. Step-2 The solvent is removed, and residue is treated with dilute inorganic acid, whereupon the bases are extracted as their soluble salts. Step-3 The free bases are liberated by addition of sodium carbonate and extracted with organic solvents like ether or chloroform. Step-4 The solvent is removed under vacuum and the crude alkaloidal extract is separated by physical means such as chromatography.

Classification of Alkaloids: Alkaloids are classified in the following four types.

A. Based on chemical nature & Structure:

Non–heterocyclic & Heterocyclic alkaloids are:  

B. Based on pharmacological action:

A. Based on Biosynthesis Pathway:

Identification of Alkaloids by Precipitation Method:

  1. Mayer’s reagent (potassium mereuric iodide solution): Cream or pale-yellow precipitate.
  2. Dragendorff’s reagent (potassium bismuth iodide solution): Brown or reddish-brown colour or precipitate.
  3. Wagner’s reagent (Iodine and potassium iodide solution): Brown or reddish-brown colour or precipitate.
  4. Hager’s reagent (saturated solution of picric acid): Yellow precipitate.

Therapeutic activity of Alkaloids: 

  1. Erythromycin and chloramphenicol are antibiotic alkaloids. Alkaloids are also obtained from synthetic sources, e.g., heroin; homotropine etc.
  2. Alkaloids are highly potent medicaments and possess curative properties, eg. a) Morphine (opium) has a narcotic action. b) Reserpine (rauwolfia) is a tranquiliser. c) Strychnine (nux-vomica) is a nervine stimulant. d) Cocaine (coca leaf) is a local anaesthetic. e) Hyoscine (belladonna) is an antispasmodic. Note: Alkaloids in higher doses are poisonous.

Functions of Alkaloids

  1. Detoxicating agents.
  2. Reservoirs of protein synthesis.
  3. Protective agents and discourage animal or insect attacks.
  4. Plant stimulants or regulators in physiological activities, such as growth, reproduction and metabolism.