Physochlaina praealta (Don.) Miers – high physochlaina – grows in Southeast Asia, the Himalayas; cultivated in India.
Ph. physaloides (L.) G. Don. – bleached henbane – grows in East Asia, Mongolia, China, Japan, and the Far East.
Both species are perennial herbaceous plants used for the industrial production of atropine. Henbane is popular in Tibetan medicine.
The plant contains alkaloids with fused pyrrolidine and piperidine rings (derivatives of tropine, scopine and ecgonine).
PLANTS CONTAINING ALKALOIDS WITH CONDENSED PYRROLIDINE AND PIPERIDINE RINGS (TROPINE, SCOPINE AND ECGONINE DERIVATIVES)
These alkaloids are based on a system known as tropane, built from piperidine and pyrrolidine rings. The diversity of these alkaloids depends on which amino alcohol was formed in the process of biosynthesis from tropane (tropine, scopine or ecgonine) and with what organic acids these amino alcohols subsequently formed esters.
Alkaloids, which are derivatives of the amino alcohols tropine or scopine, accumulate in plants of the genera of the nightshade family – belladonna, henbane, dope, scopolia, duboisia, mandrake, physochlein.
Tropine in nightshades is esterified with tropic acid to form the alkaloid hyoscyamine. Hyoscyamine is found in plants in the form of a left-handed isomer and in this state passes into galenic products; in the factory isolation of the alkaloid from plants, it is traditionally obtained its inactive racemate – atropine in the form of a salt – Atropini sulfas.
C-copyne also forms an ester with tropic acid. This alkaloid is called scopolamine or hyoscine. Natural scopolamine is also a levorotatory compound. Its racemate, scopolamine, is called atroscin.
Other tropane derivatives, found in the same plants in small quantities, as well as liquid and volatile pyridine derivatives, often associated with tropane alkaloids, are of no practical importance.
The total content of alkaloids in different Solanaceae is very different and varies within 0.04-3%. In the same species, the amount of alkaloids is different in different organs of the plant and, moreover, varies depending on the phase of vegetation and environmental conditions. Usually, the sum of alkaloids in all Solanaceae species includes both main alkaloids, but in all possible proportions, one of them prevails. This circumstance determines the different use of certain types of nightshade.
The action of atropine and thioscyamine is antispasmodic, dilating the pupil, relaxing smooth muscles, analgesic, limiting the secretion of sweat glands, relieving the symptoms of motion sickness, and stimulating the central nervous system. Scopolamine calms the central nervous system; otherwise, its action is similar to that of atropine.
PLANTS CONTAINING ALKALOIDS
Alkaloids are called natural nitrogen-containing compounds of the main nature, formed in plants. Groups of proteinogenic amines (for example, tyramine) and betaines (stakhidrin, trigonelline, etc.) adjoin the alkaloids, which are considered as transitional compounds from the simplest nitrogen-containing compounds (methylamine, trimethylamines, etc.) to the alkaloids proper.
Of natural pharmacologically active substances, alkaloids are the main group from which modern medicine draws the largest number of highly effective drugs.
According to world literature, by the end of the past decade, the number of alkaloids isolated from the higher plants of the Earth’s flora exceeded 5000. According to modern concepts, alkaloid-bearing plants make up 10% of the entire world flora. The families Equisetaceae, Lycopodiaceae, Ephedraceae, Liliaceae, Amaryllidaceae, Dioscoreaceae, Chenopodiaceae, Nymphaeaceae, Ranunculaceae, Berberidaceae, Menispermaceae, Papaveraceae, Fabaceae, Rutaceae, Cactaceae, Punicaceae contain the largest number of alkaloid-bearing genera and species. Loganiaceae, Apocynaceae, Borraginaceae, Solanaceae, Rubiaceae.
Usually plants that are phylogenetically close contain alkaloids that are very similar in structure, thus forming a natural group of genera. For example, plants of the genera Atropa, Datura, Hyoscyamys, Scopolia, Physochlaina, Duboisia. Mandragora (all from the same Solananeae family) contain a well-defined group of tropane alkaloids. This far-reaching pattern, however, has exceptions that have not yet been explained. So, for example, caffeine is found in plants that are not systematically related to each other: tea (Theaceae), coffee (Rubiaceae), cocoa (Sterculiaceae), mate (Aquifoliaceae), guarana (Sapindaceae), erodium (Geraniaceae). Along with this, there are cases when their 2 very close systematically species, one is rich in alkaloids, and the other either does not contain them at all, or contains alkaloids of a different structure.
Alkaloids can be found throughout the plant, or they can be formed and accumulated only in one or more specific organs. The plant traditionally contains not one, but several alkaloids. In individual plants, there may be 20 or more of them (cinchona, hypnotic poppy, etc.), and they may be similar in structure or belong to different chemical groups. In the sum of alkaloids, 1–3 traditionally predominate quantitatively (the main alkaloids). In plants, alkaloids are dissolved in the cell sap of the main parenchyma, phloem, and other tissues in the form of salts, mainly organic acids (malic, succinic, citric, oxalic, fumaric, quinic, etc.); of mineral acids, phosphoric acid is more often involved.
The quantitative content of alkaloids is, in principle, a species characteristic, and it varies over a very wide range. For example, in black henbane they are only 0.05-0.1%, and up to 15% accumulate in the cinchona bark. In the process of ontogenetic development of plants, their alkaloid content undergoes quantitative and sometimes qualitative changes, and each species has its own regularities.
The content of alkaloids in plants is influenced by their geographical location and various factors (air and soil temperature, precipitation, duration and intensity of sunlight, shading, height above sea level, etc.), as well as human impact in the case of transferring the plant to cultivation or its acclimatization. The largest number of alkaloid-bearing species, moreover, with a high content of alkaloids, is common in subtropical and tropical states with a humid climate. Alkaloids of different structure are confined to certain latitudes, and in connection with this, their pharmacological activity changes.
There is no consensus on the biological role and causes of the formation of alkaloids in plants. The main hypotheses proposed at different times interpret alkaloids as: 1) waste products of the vital activity of a plant organism; 2) spare substances; 3) protective substances; 4) active substances necessary for biosynthesis. The latter hypothesis is currently considered by most scientists to be the most general one, which, however, does not exclude other biological functions of alkaloids.
The exceptional diversity in the structure of alkaloid molecules does not allow us to imagine a single way of their formation in plants. Their biosynthesis proceeds according to specific schemes with the most complex chemical transformations (ring opening and closing, oxidation, deamination, ring condensation, etc.) through many intermediate products. Some alkaloids begin biogenesis from amino acids, others from acetic acid (in other words, from carbohydrates).
The modern classification of alkaloids is based on the nature of the heterocycles included in their molecules, with the release into a separate group of alkaloids with an aliphatic structure and with nitrogen in the side chain.
1. Alkaloids with an aliphatic structure or with nitrogen in the side chain;
2. Pyrrolizidine alkaloids.
3. Piperidine and pyridine alkaloids.
4. Alkaloids with condensed and pyrrolidone and piperidine rings.
5. Quinoline alkaloids.
6. Quinazoline alkaloids.
7. Isoquinoline alkaloids.
8. Indole alkaloids.
9. Alkaloid of the imidazole group.
10. Purine alkaloids.
11. Diterpene alkaloids.
12. Steroid alkaloids (glycoalkaloids).
13. Alkaloids of unknown structure.
In conclusion of this brief review, it should be pointed out that most alkaloids are highly active substances with selective pharmacological action. The selectivity of the action of alkaloids determines their widespread use for medicinal purposes. The main forms are extraction products (tinctures, extracts, novogalenic preparations, etc.) and pure alkaloids isolated from plants, converted into soluble salts of certain inorganic and organic acids.