Pharmaceutical Value of Onions (Allium L.) and related species of Central Asia
Since ancient times, onions, garlic and some other species of the genus Allium L. (onions) have been used as phyto-pharmaceutics, seasonings, and vegetables. Carvings in pyramid walls of the Old Kingdom of Egypt and written sources of the ancient cultures of the Greece and Romans mentioned the impor...
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|Summary:||Since ancient times, onions, garlic and some other species of the genus Allium L. (onions) have been used as phyto-pharmaceutics, seasonings, and vegetables. Carvings in pyramid walls of the Old Kingdom of Egypt and written sources of the ancient cultures of the Greece and Romans mentioned the importance of common onion (A. cepa L.) and garlic (A. sativum L.) in the daily diet of man. The medicinal benefits of these two species were intensely investigated during the last decades and lipid lowering, antibiotic, anti-atherosclerotic and ant-diabetic effects were described. Also, a canceroprotective effect was proven by a number of ethnic studies. The health benefits of Allium vegetables are mainly related to sulphur containing compounds as well as saponins.
The species-rich genus Allium has a main centre of distribution reaching from Southwest Asia to the high mountains of Middle Asia. In this area, several wild species are used by the local population, as one can be concluded from casual remarks in some floras. The so called cysteine sulphoxides of these plants are believed to be mainly responsible for these health benefits. These compounds are converted to thiosulphinates like alliin, if plant material is disrupted. This reaction is catalysed by the action of the enzyme alliinase.
In this investigation, 98 Allium samples with different Accession No. belonging to 77 wild species were investigated. Most of the investigated species belonged to the subgenera Allium, Melanocrommyum and Reticulatobulbosa were chemically analysed and bioactivity was tested. For many species, a remarkable radical scavenger activity and an antibiotic activity, especially against fungi, were observed. It can be concluded, that traditionally used Allium species do have a real biological effect.
In the first step of investigation, the total amount of cysteine sulphoxides of a variety species was determined by a biosensoric method, which was based on immobilized alliinase. Besides thiosulphinates, ammonia is produced by this reaction, which can be used for quantification of these cysteine sulphoxides. It was found that this method was suitable for a rapid pre-screening on these compounds. As a disadvantage, sample preparation is rather time consuming. In comparison to HPLC, this method gave averagely lower amounts of total cysteine sulphoxides, which can be explained by the substrate specificity of immobilized alliinase. Methiin is converted slower as alliin or isoalliin leading to lower total amounts for cysteine sulphoxides for wild species, which are usually rich in methiin.
Besides these known cysteine sulphoxides, also a new sulphur compound containing two pyrrole ring systems and a disulpho-bridge was isolated. This dithiodipyrrole is typical for a number of members of the subgenus Melanocrommyum like A. macleanii, A. jesdianum, A. rosenorum, A. winklerianum and A. rosenbachianum and has a deep red colour, which occurs after heating or wounding of plant material. It must be assumed that an alliinase-like enzyme is also involved in the formation of this compound. Many of these species leading the dithiodipyrrole are used as traditional medicinal plants. In our investigation, a very high radical scavenger activity could be found for this substance.
In total, 39 species were tested on radical scavenger activity using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. All species showing a significantly higher radical scavenger activity as 100%, related to butylated hydroxytoluene (BHT), belonged to the subgenus Melanocrommyum, mostly leading the dithiodipyrrole.
Also antifungal, antialgal and antibacterial activity of some selected Allium species were tested. It can be assumed, that Allium species are most potent against algae and fungi (e.g., A. longicuspis). However, the found results for activity against human pathogenic bacteria were not in accordance with literature data. 17 Different Allium species were tested, colleted in Tajikistan (7), Uzbekistan (4), Iran (5) and obtained from the IPK collection (4). Extracts showed nearly now activity against Gram- bacteria and only a moderate activity against Gram+ bacteria strains. These differences to literature data can be explained by the fact that most active volatile sulphur compounds got lost during sample preparation.
It could be demonstrated by this investigation that the genus Allium contains a fairly high number of wild species, which must be considered as medicinal plants, mostly belonging to the subgenus Melanocrommyum. This subgenus it only used in the western world for ornamental plants. In contrast, many species of this subgenus are traditionally used in Central Asia. Also the chemistry of Allium seems not to be clarified yet. Biogenesis of dithiodipyrrols is totally unknown. Also some unknown compounds exposing a rather strong radical scavenger activity must be considered. Further studies are necessary to answer these open questions.|