Alkaloids are “bitter” to many plant lovers, including a number of Botanists. But almost all of us encounter them at least once in a day. We use coffee, cocoa, black pepper, vasaa (Ardusi), Amruta , tobacco, betelnut , pomegranate, etc in our daily life. Among medicines quinine for malaria, reserpine for blood pressure, codeine as pain reliever, ephedrine for fever etc are familiar to most of us.
Alkaloids are basic (alkali-like, thus the name), plant products having a heterocyclic nitrogen (nitrogen in a ring) and marked pharmacological activity. There are more than 12,000 Alkaloids in plant world, and this group was considered the largest group of Secondary metabolites. They are almost unheard in lower plants (except for ergot and Amanita ), rare in Gymnosperms (present only in Taxaceae and Gnetaceae) and seen mostly in Angiosperms. But only about 15% of Angiosperm families contain them. The alkaloid rich families in Dicots are the Fabaceae, Papaveraceae, Ranunculaceae, Rubiaceae, Solanaceae, Berberidaceae, Apocynaceae, Asclepiadaceae, Asteraceae, Menispermaceae and Loganiaceae and in Monocots, the Amaryllidaceae and Liliaceae, Stemonaceae, Dioscoreaceae, Arecaceae, Poaceae and Orchidaceae.
There are about 12 major groups of alkaloids, classified based on the basic chemical strucures. Thus the complex isoquinoline alakaloids are located in early families of Ranales like Ranunculaceae (Aconitum- Ativish), Berberidaceae (Berberis- daruhaldi), Menispermaceae (Tinospora- Amruta), Papaveraceae(Poppy, Argemone) and indole alkaloids in somewhat advanced families like Apocynaceae (Rauwolfia- Sarpagandha, Alstonia-Sapthaparni, Holarrhena-Kurchi), Asclepiadaceae (Calotropis-Akhro, Hemidesmus-Ananthamul, Gymnema- Madhunashi) and simple tropane alkaloids in advanced families like Solanaceae`(Atropa, Datura). Normally one type of alkaloid will be distributed in members of same family or related families so that alkaloids can be used as Chemotaxonomic markers or as leads to Bioprospecting.
Alkaloids are biosynthesised from amino acids like ornithine (tropane and pyrrolidine alkaloids), lysine and cadavarine(piperidine and lupine alkaloids), phenylalanine and tyrosine(isoquinoline alkaloids and protoalkaloids) and tryptophan (indole and quinoline alkaloids). They do not occur singly, but in groups.
Alkaloids occur as salts with organic acids of the plant and these salts are soluble in water and insoluble in non-polar solvents. But it is always advisable to use slightly acidic (1% acetic acid/HCl) water to get better extractions, when done for pharmaceutical preparations. But free alkaloids are usually insoluble in water, or only sparingly so, but are freely soluble in ether, CHCl3 and other relatively non-polar solvents. This property is made use of in the extraction, purification and estimation of alkaloids. Alkaloid salts are crystalline, often with characteristic crystal forms and habit. Alkaloids can be precipitated out of their aqueous or acid solutions by a number of substances like picric acid and tannic acid.
Apart from the medicinal values some of the alkaloids are used as Biopesticides (Aconitum, pepper, Datura, tobacco etc- about 50 plants) and Natural dyes (Berberis).
Alkaloids were once considered toxic excretory products, but now are understood as survival mechanisms of plants warding off herbivores, insects and microbes.
References:
1. M. Daniel (2006) Medicinal Plants: Chemistry and Properties. Oxford & IBH Publishers, Delhi.
Mammen Daniel