garlic extracts. Allicin can be produced by treating synthetic alliin with purified alliinase. In order to get reproducible amounts of allicin, this process can be done immediately before administration (Rabinkov et al., 1994).
The beneficial effects of garlic consumption are well documented in the traditional medicine of many cultures in the world, and especially in the Chinese system of medicine, where garlic is associated with longevity. Indeed, garlic extracts contain several biologically active compounds; and allicin and its analogues have been the most thoroughly investigated. Allicin shows in vitro and in vivo antimicrobial and anticancer activity, and has beneficial cardiovascular effects, lowering blood lipids and interfering with thrombogenesis. It also shows antibacterial activity, and freshly obtained aqueous garlic extracts as well as crude allicin (15 mg/kg) could counteract a lethal dose of intestinal Shigella in rabbits (Chowdhury et al., 1991).
Both animal studies and epidemiological investigations have shown an inverse association between the consumption of garlic and stomach and colon cancers, presumably due to the inhibition of gastric nitrosation in the acidic stomach environment. Gastric nitrosation is responsible for the formation of carcinogenic N-nitroso compounds, and its inhibition might be related to the antibiotic activity of allicin on nitrate-reducing bacteria (Mei et al., 1989). Allicin, along with other constituents, has also been related to the antitumour effects of garlic. Synergistic interaction with diallyl sulphide, which strengthens the detoxification activity of gluthathione; spirostane saponins (common to other Allium species), which show powerful cytotoxicity in vitro; as well as with the phytoalexin allixin have been proposed. Allicin might also be responsible for most of the serum lipid-lowering effects of garlic. Six major clinical trials with fresh garlic cloves (3—10 g) and almost thirty with garlic powder tablets (0.6—0.9 g) support a beneficial activity on serum triglycerides (approx. 12 per cent reduction) and cholesterol (approx. 11 per cent reduction). Amounts of allicin as low as 0.07 mg/kg can still have effects on serum lipids. The hypolipaemic activity of garlic supplements is apparently stronger in women than in men.
Allicin and allicin-derived garlic compounds (ajoene, diallyl trisulphide, vinyldithi-ins) are strong inhibitors of human platelet aggregation. In vivo clinical studies with fresh garlic (100 mg/kg, corresponding to c.0.4 mg allicin/kg) have shown a 50 per cent decrease of the blood clotting power, presumably mediated by the inhibition of cAMP phosphodiesterase (Kiesewetter et al., 1993).
Despite the excellent record of in vitro and in vivo activity, allicin is difficult to formulate for its instability, and it is not clear if it is superior to garlic extracts or fresh garlic in any of its potential pharmacological applications. Furthermore, nothing is known on the molecular mechanism of its various activities.
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