Alkaloids

Abstract

Alkaloids are a subgroup of natural nitrogenous compounds. The term “alkaloid” have many definitions because, it refers to compounds with very heterogeneous structures, however, alkaloid is a basic substance containing one or more nitrogen atoms in its molecule, as part of a heterocyclic ring. Chemical classification of alkaloids is based on their carbon-nitrogen skeletons. Alkaloids is the largest class of secondary plant substances (more than 15,000 organic compounds). Although these substances are mainly found in higher plants, but they also occur in lower plants, animals, microorganisms and marine organisms. Alkaloids are found in about 15% of vascular plants from more than 150 families. Some particular alkaloids in plants are often restricted to seeds, leaves, bark or roots. Plant alkaloids occur largely as salts of organic acids (acetic, citric, malic, tannic, aconitic and quinic acids), however, some alkaloids (nicotine, myosmine and anabasine) occur in free state. Classification: Alkaloids may be classified based on their chemical structures: (i). True alkaloids: These are derived from amino acids and with a heterocyclic ring, (ii). Proto-alkaloids: These are derived from amino acids but the nitrogen atom is not contained in a heterocyclic ring, (iii). Pseudo-alkaloids: These are not derived from amino acids but from terpenoids (solanine) or purines (caffeine), containing a heterocyclic ring with a nitrogen atom in their chemical structures. Alkaloids play major role in plant defence and they protect plants against pathogens, insects and herbivorous animals. Some alkaloids are also mentioned as putative allelochemicals. Their allelopatic potential has been identified in natural agroecosystems i.e., grasses, particularly hordenine and gramine from barley (Hordeum vulgare L.). These compounds affects the nutrient uptake, membrane permeability, protein synthesis, photosynthesis, respiration, enzyme activity, hormone balance and plant water potential. Alkaloids could be reserve substances rich in nitrogen or other elements needed for plant growth and development. They may also act as regulatory growth factors similar to hormones or appear as byproducts of plant metabolism. Some alkaloids are very toxic to humans, while, others are used in medicine.