Alkaloids are nitrogenous bases in plants which frequently cause physiological responses when administered to animals.
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The term is applied to a heterogenous collection of bases both purine and pyrimdine bases are usually excluded. They are derived from amino acids, ornithine, lysine, phenylalanine, tyrosine, tryptophan. Presently around 3000 alkaloids are known and are suspected to play role in 1-Storage of waste nitrogen, 2-Cationic balance, 3-Protection against parasites, infections, predators.
They are classified into four main classes:
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A. Derivatives of aliphatic aminoacids : e.g.; Nicotene alkaloids, tropon alkaloids.
B. Derivatives of aromatic aminoacids : e.g.; Colchicine, Betacyanin.
C. Indole alkaloids and their derivatives : e.g.; Reserpine, strychinine.
D. Purine alkaloids : e.g.; Theobromine, Caffein.