A class of nitrogen-containing usually basic plant products, which are often poisonous. Many alkaloids, e.g. morphine, codeine, nicotine, and cocaine, have been utilized in medicine and other fields. Their natural functions are not well understood. Some, through their bitterness and toxicity, may serve to protect the plants from herbivorous animals. Others are thought to be involved in nitrogen metabolism (e.g. nicotine is involved in the absorption of nitrate through plant roots). Many are thought to be simply end products of nitrogen metabolism, which are stored in leaves, fruits, and flowers before being discarded. Certain families, e.g. the Solanaceae, Leguminosae, and Papavera-ceae, contain many alkaloidal species while monocotyledons rarely produce alkaloids. Most groups of lower plants contain alkaloidal species though alkaloids have not been found in the algae. Three classes of alkaloids have been recognized. The true alkaloids have a nitrogen-containing heterocyclic nucleus, examples being the * isoquinoline alkaloids . The protoalkaloids lack a heterocyclic ring and are usually simple amines. Examples are mescaline and ephedrine. Some of the protoalkaloids may be precursors of true alkaloids. Both the true alkaloids and the protoalkaloids are derived from amino acids. The pseudoalka-loids are not derived from amino acids but from such compounds as terpenes, purines, and sterols. Pseudoalkaloids include theobromine and caffeine, which are both methylated purines. See also indole alkaloids , pyridine alkaloids , piperidine alkaloids , tropane alkaloids. |
