Alkaloids

Alkaloids constitute the single largest class of plant secondary metabolites. Around 10,000 of them are reported and many more newer ones are being identified. They are generally referred to as organic nitrogenous bases of plant origin that are pharmacologically active. All alkaloids cannot be described by a clear-cut definition because not all are basic, and though most commonly encountered in the plant kingdom, to a lesser extent they are also found in bacteria, fungi and animals.

Structurally alkaloids are extremely diverse ranging in complexity from relatively simple molecules to very complex structural forms. Typically alkaloids contain one or more nitrogen atoms present as primary, secondary, or tertiary amines usually as part of a cyclic system. They are extremely variable with respect to their botanical, biochemical origin, chemical structure, and pharmacological action. Though several forms of classification is possible, alkaloids may be conveniently classified based on their biogenetic origin or on the nature of the nitrogen-containing ring system. They usually occur either free or as salts bound to a typical organic acid or occur as N-oxides. Protoalkaloids are those that do not have nitrogen in a ring system, and steroidal terpene alkaloids and purines not derived from amino acids are referred to as pseudo alkaloids. The degree of basicity of alkaloids is variable, depending on the structure and presence and location of other functional groups within the molecule.

Distribution of alkaloids in the plant kingdom is restricted to specific groups of families and genera. In angiosperms they are well represented in many dicotyledonous families. To a very limited extent, they are also distributed in monocots as also in gymnosperms and pteridophytes. Toxic alkaloids of more than 24 classes are reported from skins of amphibians. Many alkaloid-containing plants have been used since early times as drugs, poisons, stimulants, and as psychedelics. Many of our present drugs include alkaloids or their synthetic substitutes and they are of great medicinal significance on account of their powerful pharmacological/toxicological properties.