Characteristics of the Whole Plants and Their Secondary Metabolites
The genus Nigella belongs to the family Ranunculaceae and includes about 20 species. Assignation to the genus Nigella changed repeatedly during the last century; some selected species are listed in Table 1. Nigella species are located mainly in the Middle East and the Mediterranean region.
The plants are annuals and often have spirally arranged bi- or tripimatisect leaves with linear or capillary lobes and solitary, terminal, hermaphrodite, actinomorphic, bluish flowers. Some subspecies or varieties also have white flowers or flowers of other pigmentation (mostly breeding forms).
Only a small number of Nigella species are of practical interest as ornamental plants or as plants used in traditional folk medicine. Most of them are only known as weeds or wild plants. The habitats of the Nigella species are, for instance, south + east Europe, south + western and central Asia and Asia Minor.
In Nigella species many biologically active compounds, mostly secondary plant products, are found, for example, enzymes, other proteins and peptides, fatty acids, fats, oils, essential oils, and alkaloids, as well as saponins. Detailed tests have shown a clear pharmacological action for only a small part of these compounds or mixtures.
Analyses for a chemical characterization of Nigella sativa seeds were done by Babayan et al. (1978) and Bose et al. (1981). Esters of unsaturated fatty acids (e.g., dehydrostearinic or linolenic acid) with C15 or higher terpenoid or aliphatic alcohols, and α,β-unsaturated hydroxyketone (C18H28O4), as well as a trace of an alkaloid of the pyridine group, were detected. Babayan et al. (1978) found the following chemical composition: protein 21%, fat 5.5%, ash 3.7%, the remaining percent being carbohydrates. The fatty acids are a mixture of linoleic acid (56%), oleic acid (24.6%), palmitic acid (12%), stearic acid (3%), eicosadienoic acid (2.5%), linolenic acid (0.7%), and myristic acid (0.16%). By hydrolysis of the proteins, 15 amino acids, nine of them essential, could be detected.
Two different kinds of proteins of Nigella have been studied in detail. The homologies of seed storage proteins within the genus Nigella compared with proteins obtained from the agriculturally important families Fabaceae and Poaceae, as well as the properties of coding nucleic acids, were studied; they can be useful markers for taxonomic studies.
Rudyuk’s group, Higginson (1981) Gmoshinskii et al. (1982) characterized Upases from Nigella damascena seeds as surfactants, possibly therapeutics, or for use in technical systems. Main lines for isolation, purification, stabilization, and some forms for application were elaborated.
Datta et al. (1987) analyzed proteins extracted from normal and mutant lines of Nigella sativa by use of PA A electrophoresis. The different normal and mutant lines studied showed great heterogenities.
Some workers have analyzed Nigella species for their alkaloid and terpenoid content. Protoanemonine-forming compounds and berberine could not be detected. Only magnoflorine (8) was detectable in these experiments. The alkaloid-like substance damascenine formed by N. damascena was a model for the study of alkaloid formation in the whole plant because it has a simple structure. Nevertheless, the biosynthetic pathway for the formation of damascenine is not as yet completely clear. The formation of this compound is bound to a differentiation process in the development of the plants. Only seeds contain this compound. The biosynthesis probably starts from anthranilic acid, an intermediate of the tryptophan branch in the biosynthesis of aromatic amino acids. Some methylations and a hydroxylation step convert anthranilic acid into damascenine and related compounds.
Much experience in the use of Nigella species, plant extracts or isolated compounds or their mixtures, can be found in the area of traditional medicine, for example in Sri Lanka, India, Arabia, or Saudi Arabia. Namba et al. (1985) analyzed the use of these drugs in Sri Lanka. Among other things, extracts of N sativa seeds (in 50% methanol) also have the effect of inhibiting viable cells of Streptococcus mutans (IC 10-30 μg/ml). Besides other drugs, Al-Yahya (1986) also qualitatively analyzed the Nigella species used in traditional medicine. Sayed (1980) discussed some plant uses in folk medicine. In this report, N. sativa is termed a drug plant which is already industrialized and known in Arabia as Habbett et Barakh. In Egypt this drug is used as a diuretic and carminative. The oil shows activity in treating asthma, respiratory congestion, and coughs. Sayed named nigellone as an active principle. The nigellone-containing fraction of the volatile oil of N. sativa is described as a useful agent in the treatment of bronchial asthma. Canonica et al. (1963) showed that nigellone is probably thymoquinone or a polymer of it. Going further, El-Dakhakhny (1965) isolated thymoquinone as a yellow compound from seeds of Nigella sativa. He also termed this compound the main pharmacologically active principle used in folk medicine in the orient. Hampel (1957) made reference to the use of N. arvensis seeds as a spice and in folk medicine as a diuretic. The unripe seeds are described as a source for homeopathic mixtures.
Vohora et al. (1973) studied the influence of different plant extracts on the fertility of animals. In their experiments N. sativa seeds did not show any anti-ovular activity in rabbits. Deshpande et al. (1974) described pest control through N. sativa extracts. Oleic acid- and linoleic acid-containing extracts have shown insecticidal activity against insects which attack stored grains.
In a German patent a pharmaceutical preparation containing five components, e.g. fluid extracts, are given:
1. extracts of Carum carvi (caraway);
2. extracts of Foeniculum vulgare;
3. plant extracts containing terpenoid ketones, and/or terpenoid ethers, and/or terpenoid alcohols, and/or phenolic derivatives of terpenes;
For preparation of the third component, N. sativa was used among other plants. The agents for extraction and the parts of the plants used for this patent cover a wide range. The described mixtures were tested for an indirect parasympathomimetic action and can be applied in the therapy of dystonic (atonic and/or spastic) and dysbacterial illness as well as flatulence in stomach or intestine.
Data for the cultivation of Nigella species in the field are rare.
Nigella spp.: Concluding Remarks
Some species of the genus Nigella are of interest for use in traditional medicine, for cultivation as ornamentals, and for study in physiology and genetics because the simple sets of chromosomes allow closer studies in this range. In vitro cultivation of some species is possible mainly for the production of regenerated plants. Suspension cultures have been established, but primarily only with N. damascena and N. sativa. It was also shown that these cultures are suitable for cultivation in fermentors which work with low shear forces. Some of the influences of light, phytohormones, and selected antimetabolites were shown. It was, in fact, not possible to induce a specific synthesis of secondary products in the suspension cultures which are of practical interest. In the future some experiments will be carried out for the isolation and/or selection of lines with special product formation. A practical use of Nigella suspension cultures for commercial production of such compounds is, however, unlikely. A direct practical application is within sight for the in vitro methods for plant propagation and breeding.
Selections from the book: “Medicinal and Aromatic Plants III”, 1991.