Alliin (/ˈæli.ɪn/) is a sulfoxide that is a natural constituent of fresh garlic. It is a derivative of the amino acid cysteine. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic.
Garlic has been used since antiquity as a therapeutic remedy for certain conditions now associated with oxygen toxicity, and, when this was investigated, garlic did indeed show strong antioxidant and hydroxyl radical-scavenging properties, it is presumed owing to the alliin contained within.
Alliin has been found to affect immune responses in blood, and was the first natural product found to have both carbon- and sulfur-centered stereochemistry.
Allicin is an organosulfur compound obtained from Wild garlic, a species in the family Alliaceae. It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944. This colorless liquid has a distinctively pungent smell. This compound exhibits antibacterial and anti-fungal properties. Allicin is garlic’s defense mechanism against attacks by pests.
Several animal studies published between 1995 and 2005 indicate that allicin may: reduce atherosclerosis and fat deposition,normalize the lipoprotein balance, decrease blood pressure, have anti-thrombotic and anti-inflammatory activities, and function as an antioxidant to some extent.
Other studies have shown a strong oxidative effect in the gut that can damage intestinal cells, although these results were in laboratory animals, not humans.
Furthermore, many of these results were obtained by excessive amounts of allicin, which has been clearly shown to have some toxicity at high amounts, or by physically injecting the lumen itself with allicin, which may not be indicative of what would happen via oral ingestion of allicin or garlic supplements.
A randomized clinical trial funded by the National Institutes of Health (NIH) in the United States and published in the Archives of Internal Medicine in 2007 found that the consumption of garlic in any form did not reduce blood cholesterol levels in patients with moderately high baseline cholesterol levels.
The fresh garlic used in this study contained substantial levels of allicin so this study casts doubt on the ability of allicin when taken orally to reduce blood cholesterol levels in human subjects.
In 2009, Vaidya, Ingold and Pratt clarified the mechanism of the antioxidant activity of garlic, such as trapping damaging free radicals. When allicin decomposes, it forms 2-propenesulfenic acid, and this compound is what binds to the free-radicals. The 2-propenesulfenic formed when garlic is cut or crushed has a lifetime of less than one second.
Allicin has been found to have numerous antimicrobial properties, and has been studied in relation to both its effects and its biochemical interactions.
One potential application is in the treatment of methicillin-resistant Staphylococcus aureus (MRSA), an increasingly prevalent concern in hospitals. A screening of allicin against 30 strains of MRSA found high level of antimicrobial activitity, including against strains that are resistant to other chemical agents.
Of the strains tested, 88% had minimum inhibitory concentrations for allicin liquids of 16 mg/L, and all strains were inhibited at 32 mg/L. Furthermore, 88% of clinical isolates had minimum bactericidal concentrations of 128 mg/L, and all were killed at 256 mg/L. Of these strains, 82% showed intermediate or full resistance to mupirocin. This same study examined use of an aqueous cream of allicin, and found it somewhat less effective than allicin liquid. At 500 mg/L, however, the cream was still active against all the organisms tested—which compares well with the 20 g/L mupirocin currently used for topical application.
A water-based formulation of purified allicin was found to be more chemically stable than other preparations of garlic extracts.They proposed that the stability may be due to the hydrogen bonding of water to the reactive oxygen atom in allicin and also to the absence of other components in crushed garlic that destabilize the molecule.
A small (146 healthy adults) double-blind, placebo-controlled study found that a daily supplement containing purified allicin, had dramatic results by reducing the risk of catching a cold by 64%, the symptom duration was reduced by 70% and those in the treatment group were much less likely to develop more than 1 cold.
- Iberl, B et al. (1990). “Quantitative Determination of Allicin and Alliin from Garlic by HPLC”. Planta Med 56 (3): 320–326. doi:10.1055/s-2006-960969. PMID 17221429.
- Kourounakis, PN; Rekka, EA (November 1991). “Effect on active oxygen species of alliin and Allium sativum (garlic) powder”. Res Commun Chem Pathol Pharmacol. 74 (2): 249–252. PMID 1667340.
- Salman, H et al. (September 1999). “Effect of a garlic derivative (alliin) on peripheral blood cell immune responses”. Int J Immunopharmacol. 21 (9): 589–597. doi:10.1016/S0192-0561(99)00038-7. PMID 10501628.
- Eric Block (2009). Garlic and other alliums: the lore and the science. Royal Society of Chemistry. pp. 100–106.