Allicin is an organosulfur compound obtained from 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. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic. The allicin generated is very unstable and quickly changes into a series of other sulfur containing compounds such as diallyl disulfide.
It exhibits antibacterial, antifungal, antiviral, and antiprotozoal activity. Allicin is garlic's defense mechanism against attacks by pests.
Structure and occurrence
Allicin features the thiosulfinate functional group, R-S(O)-S-R. The compound is not present in garlic unless tissue damage occurs, and is formed by the action of the enzyme alliinaseon alliin. Allicin is chiral but occurs naturally only as a racemate. The racemic form can also be generated by oxidation of diallyl disulfide:
(SCH2CH=CH2)2 + RCO3H → CH2=CHCH2S(O)SCH2CH=CH2 + RCO2H
(SCH2CH=CH2)2 + RCO3H → CH2=CHCH2S(O)SCH2CH=CH2 + RCO2H
Alliinase is irreversibly deactivated below pH 3; as such, allicin is generally not produced in the body from the consumption of fresh or powdered garlic. Furthermore, allicin can be unstable, breaking down within 16 h at 23 °C.
Biosynthesis of Allicin
Allicin is an oily, slightly yellow liquid that gives garlic its unique odor. It is a thioester ofsulfenic acid and is also known as allyl thiosulfinate. Its biological activity can be attributed to both its antioxidant activity and its reaction with thiol containing proteins.
In the biosynthesis of allicin (thio-2-propene-1-sulfinic acid S-allyl ester), cysteine is first converted into alliin (+ S-allyl-L-cysteine sulfoxide). The enzyme alliinase, which containspyridoxal phosphate (PLP), cleaves alliin, generating allysulfenic acid, pyruvate, and ammonium. At room temperature allysulfenic acid is unstable and highly reactive, which cause two molecules of it to spontaneously combine in a dehydration reaction to form allicin.
Allicin is produced in garlic cells when they are damaged, which is why garlic’s scent is most potent once it is being cut or cooked. It is believed that alliin and alliinase are kept in separate compartments of the cells and can only combine once these compartments have been ruptured.
Potential health benefits
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 anantioxidant to some extent. Other animal studies have shown a strong oxidative effect in the gut that can damage intestinal cells, though 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 the 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 half-life 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 activity, 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
Allicin has antiviral activity both in vitro and in vivo. Among the viruses susceptible to allicin are Herpes simplex type 1 and 2, Parainfluenza virus type 3, human Cytomegalovirus, Influenza B, Vaccinia virus, Vesicular stomatitis virus and Human rhinovirus type 2.
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 one cold.