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 alliinase on 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 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 of sulfenic 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, cysteine
is first converted into alliin. The enzyme alliinase, which contains
pyridoxal phosphate, 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 an antioxidant 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 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.
= Antibacterial activity= 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, 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.
= Antiviral activity= 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 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. See also
Allyl isothiocyanate, the active piquant chemical in mustard, radishes,
horseradish and wasabi Capsaicin, the active piquant chemical
in chili peppers Piperine, the active piquant chemical in
black pepper syn-Propanethial-S-oxide, the chemical
found in onions List of phytochemicals in food
References External links
Johnston, Nicole. "News in Brief: Garlic: A natural antibiotic". Mdd
modern drug discovery 5. Allicin, the major component of garlic, is one such
agent, and it was recently shown to be potent against VRE and MRSA in two
studies presented at the 41st Interscience Conference on Antimicrobial
Agents and Chemotherapy in Chicago in December.