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The constituents of medicinal plants - Pengelly A.

Pengelly A. The constituents of medicinal plants - London, 2001. - 109 p.
Download (direct link): theconstituents2001.djvu
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GLUCOSINOLATES (Mustard oil glycosides)
These are pungent tasting compounds found mainly in the Brassicaceae family, though similar compounds are also present in the Allium genus (Liliaceae). While the Allium compounds occur less frequently as glycosides, they are usually classified along with the glucosinolates. These glycosides are formed by decarboxylation of amino acids such as tyrosine, phenylalanine and tryptophan. Sinigrin (potassium isothiocyanate), the glycoside from seeds of the black mustard seed (Brassica nigra), is hydrolised by the enzyme myrosin to the aglycone allyl isothiocyanate. Depending on conditions other thiocyanates and highly toxic nitriles may be formed, the latter when plants are subjected to very hot water (>45°C) [Mills 1994]. More than 70 individual glucosinolate compounds are known, varying only in the character of their side chain.
—giu At least 300 species of Brassicas have been
CH^ssCH—сн2—c studied for their glucosinolate content. The
_о_so3_к compounds are mainly concentrated in the
sinigrin seeds, although they can be found anywhere
in the plants. They can always be identified by their spicy, pungent taste - responsible for the flavours of mustard seeds, horseradish root, cress and rocket leaves etc. They also occur in the garden nasturtium -Tropaeolum majus (Tropaeolaceae), in the form of glucotropaeolinoside, which is hydrolized to the antibiotic compound benzyl isothiocynate.
Actions of glucosinolates.
Mustard oils act as rubefacients or irritants when applied topically, causing local vasodilation. Mustard poultices have been used historically to break up congestion in the lungs and bronchioles, though care must be taken not to induce skin lesions. Taken internally the compounds are effective decongestants for sinus conditions (eg. horseradish & garlic tabs), while also acting to stimulate
digestion. Large doses may induce emesis. As with all sulphur compounds they exhibit some antibiotic effects.
Glucosinolates also depress thyroid function, therefore all Brassicas are potentially goitregenic. Problems mainly arise when cattle or other livestock graze areas infested by Brassica weeds such as mustard weed or shepherds purse, and the milk or meat produced is consumed by local people who may also suffer from deficient iodine intake. Such occurrences have been recorded in some inland regions of Australia though there is no proof of a causal relationship [Bruneton 19951. This goitregenic effect may also be harnessed therapeutically in cases of hyperthyroidism. The compounds are also responsible for some of the well documented antitumor properties of Brassicas [Brinker 1991]. Benzyl isothiocyanate, obtained by hydrolysis of glucosinolates in Tropaeolum majus, is cytotoxic and active against several human tumor cell lines (Pintao et al.1995].
Iridoids are synthesized through the mevalonic acid pathway and are technically known as cyclopentan-(c]-pyran monoterpenoids. They occur mainly as glycosides though non-glycosidic iridoids also occur - these are covered in the monoterpene section of the book. The name iridoid is derived from the common Australian meat ant Iridomyrex detectus, from which it was first detected in 1956 [Sticher 19771. They are derived from plants belonging to many families, most notably the Rubiaceae, Lamiaceae, Scrophulariaceae and Gentianaceae.
Herbs containing iridoid compounds
The first of these compounds to be identified was asperuloside from the woodruff - Asperula odorata (Rubiaceae). Other iridoids with therapeutic properties include:
aucubin from plantain - Plantago spp.
(Plantaginaceae) harpagoside, procumbin from devil’s claw-Harpagophytum procumbens (Pedaliaceae) harpagoside occurs also in figwort Scrophularia nodosa (Scrophuiariaceae) loganin from bogbean Menyanthesspp. (Menyanthaceae)
These glycosides are formed by opening of the 5 carbon ring of the iridoid loganin. They include:
amarogentin, gentiopicroside from gentian - Gentiana spp. (Gentianaceae) picroliv from Picrorrhiza kurroa (Scrophuiariaceae) oleuropein from olive leaves - Olea europea (Oleaceae)
Therapeutics of iridoid glycosides
Iridoids are the most bitter of all plant compounds, often responsible for the so called "bitter principle”. On a scale for bitter value devised by Wagner & Vaserian [described in Sticher 1977], amarogentin and related secoiridoids were the most bitter compounds tested. Its taste is perceptible at a dilution of 1 part/ 50,000. Bitters are known to stimulate release of gastrin in the GIT, leading to increase in digestive secretions including bile flow.
Bitters improve appetite and assist pancreatic function. They are regarded as cooling remedies, useful for fevers and inflammations. Actions include:
Antiinflammatory - aucubin, loganin [Recio et al 1994]. Hepatoprotective - picroliv [ Visen et al 1993], gentiopicroside, aucubin.
Hypotensive - oleuropein (also coronary dilating, anti arrhythmic)
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