Alkaloids

3.1.1 Alkaloids

Definition and Distribution in Plants

Alkaloids are a large class of nitrogen-containing secondary metabolites of plants, microbes, or mammals. Alkaloids are famous for their manifold pharmacological activities. Since the discovery of morphine from the opium poppy, Papaver som- niferum, in 1806, more than 10,000 alkaloids have been purified and identified from natural resources. Many modern drugs are produced from naturally occurring alkaloids or their synthetic analogs.

Alkaloids are mainly distributed in higher plants such as the Taxus genus, the Pinus genus, the Picea genus, the Ketelearia genus, the Ephedra genus, the Cephalotaxus genus, the Ranunculaceae family, the Berberidaceae family, the Menispermaceae family, the Magnoliaceae family, and the Liliaceae family, but rarely found in lower plants.

Biological Activities of Alkaloids

Alkaloids are a group of substances possessing various remarkable biological activi- ties, such as antibacterial (e.g., berberine), anti-malaria (e.g., quinine), analgesia (e.g., morphine), anesthesia (e.g., cocaine), anticancer (e.g., vincrinstine), cardiant (e.g., dl-demethylcoclaurine), antihypertention (e.g., resepine), cholinomimeric action (e.g., galatamine), relieving cough (e.g., codeine), spasmolysis (e.g., atro- pine), vasodilatation (e.g., vincamine), anti-arhythmia (e.g., quinidine), and anti- asthma (e.g., ephedrine). For example, the morphine alkaloids are powerful pain relievers and narcotics, and vincristine, isolated from Vinca rosea (now Catharanthus roseus), is one of the most potent anti-leukemic drugs in use today.

Properties of Alkaloids

Alkaloids in plants exist as free states, as salts, or as N-oxides. The nitrogen com- pounds may exist as a primary amine, as a secondary amine, as a tertiary amine, or as a quaternary amine. They are generally white or yellowish solids with a few exceptions (i.e., nicotine is a brown liquid) and have bitter taste. They can produce a precipitate when reacted with heavy metal iodides. Most alkaloids are able to form cream-colored precipitate with Mayer’s reagent (potassiomercuric iodide solution). The reaction of Dragendorff’s reagent (solution of potassium bismuth iodide) with alkaloids gives an orange-colored precipitate, which is sensitive and can be used for the detection of alkaloids on thin layer chromatography (TLC).

Structures and Classification

Alkaloids are commonly subclassified according to their biosynthetic origin. Most alkaloids can be covered in the following subgroups.

1. Alkaloids derived from the nonprotein amino acid L-ornithine including pyrrolidine/tropane and pyrrolizidine alkaloids (see Fig. 3.1A).

2. Alkaloids derived from the amino acid L-lysine including piperidine, quino- lizidine, and indolizidine alkaloids (see Fig. 3.1B)

3. Alkaloids derived from nicotinic acid including pyridine alkaloids (see Fig. 3.1C).

4. Alkaloids derived from the amino acid L-phenylalanine and L-tyrosine including phenyethylamines, tetrahydroisoquinoline, benzyltetrahydroiso- quinoline, phenethylisoquinoline, and terpenoid tetrahydroisoquinoline alkaloids (see Fig. 3.2 A).

5. Alkaloids derived from the amino acid L-tryptophan including indole, ter- penoid indole, quinoline, pyrroloindole, and ergot alkaloids (see Fig. 3.2B).

6. Alkaloids derived from the origin of terpenoids (see Fig. 3.3A).

7. Alkaloids derived from the origin of steroids (see Fig. 3.3B).

8. Alkaloids derived from anthranilic acid (see Fig. 3.4A).

9. Alkaloids derived from histidine (see Fig. 3.4B).

10. Alkaloids derived from purine derivatives (see Fig. 3.4C).

 Soure: Traditional Herbal Medicine Research Methods; Edited by Willow J.H. Liu; A John Wiley & Sons, Inc., Publication