Allicin is an organosulfur compound obtained from garlic, a species in the family

Alliaceae Allioideae is a subfamily of monocot flowering plants in the family Amaryllidaceae, order Asparagales. It was formerly treated as a separate family, Alliaceae. The subfamily name is derived from the generic name of the type genus, '' Allium'' ...

. 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 In enzymology, an alliin lyase () is an enzyme that catalyzes the chemical reaction :an ''S''-alkyl-L-cysteine ''S''-oxide \rightleftharpoons an alkyl sulfenate + 2-aminoacrylate Hence, this enzyme has one substrate, ''S''-alkyl-L-cysteine ...

converts alliin into allicin, which is responsible for the aroma of fresh garlic. The allicin generated is unstable and quickly changes into a series of other sulfur-containing compounds such as

diallyl disulfide Diallyl disulfide (DADS or 4,5-dithia-1,7-octadiene) is an organosulfur compound derived from garlic and a few other genus ''Allium'' plants. Along with diallyl trisulfide and diallyl tetrasulfide, it is one of the principal components of the dist ...

. Allicin is part of a defense mechanism against attacks by pests on the garlic plant. Allicin is an oily, slightly yellow liquid that gives garlic its distinctive odor. It is a

thioester In organic chemistry, thioesters are organosulfur compounds with the functional group . They are analogous to carboxylate esters () with the sulfur in the thioester playing the role of the linking oxygen in the carboxylate ester, as implied by t ...

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. Produced in garlic cells, allicin is released upon disruption, producing a potent aroma when garlic is cut or cooked, and is among the chemicals responsible for both the smell and flavour of garlic.

Structure and occurrence

Allicin features the

thiosulfinate In organosulfur chemistry, thiosulfinate is a functional group consisting of the linkage R-S(O)-S-R (R are organic substituents). Thiolsulfinates are also named as alkanethiosulfinic (or arenethiosulfinic) acid esters. They are the first member o ...

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

on alliin. Allicin is

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

but occurs naturally only as a

racemate In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...

. The

racemic In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...

form can also be generated by oxidation of

: :(SCH₂CH=CH₂)₂ + 2 RCO₃H + H₂O → 2 CH₂=CHCH₂SOH + 2 RCO₂H :2 CH₂=CHCH₂SOH → CH₂=CHCH₂S(O)SCH₂CH=CH₂ + H₂O 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 hours at 23 °C.

Biosynthesis

The biosynthesis of allicin commences with the conversion of cysteine into S-allyl-L-cysteine. Oxidation of this thioether gives the sulfoxide ( alliin). The enzyme

, which contains

pyridoxal phosphate Pyridoxal phosphate (PLP, pyridoxal 5'-phosphate, P5P), the active form of vitamin B6, is a coenzyme in a variety of enzymatic reactions. The International Union of Biochemistry and Molecular Biology has catalogued more than 140 PLP-dependent a ...

(PLP), cleaves alliin, generating allylsulfenic acid (CH₂=CHCH₂SOH), pyruvate, and ammonium ions. At room temperature, two molecules of allylsulfenic acid condense to form allicin.

Research

Allicin has been studied for its potential to treat various kinds of

multiple drug resistance Multiple drug resistance (MDR), multidrug resistance or multiresistance is antimicrobial resistance shown by a species of microorganism to at least one antimicrobial drug in three or more antimicrobial categories. Antimicrobial categories are c ...

bacterial infections, as well as viral and fungal infections

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...

, but as of 2016, the safety and efficacy of allicin to treat infections in people was unclear. In a small

clinical trial Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, diet ...

, a daily high dose of extracted allicin (20 times the amount in a garlic clove) showed effectiveness to prevent the common cold. A Cochrane review found this to be insufficient to draw conclusions. A study from 2021 has shown "a combination of the short half-life, high reactivity and non-specificity to particular

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s are reasons most

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

cannot deal with allicin’s mode of action and develop effective defence mechanism" and argue "that could be the key to sustainable drug design addressing serious problems with escalating emergence of multidrug-resistant bacterial strains".

History

Allicin was discovered as part of efforts to create

thiamine Thiamine, also known as thiamin and vitamin B1, is a vitamin, an essential micronutrient, that cannot be made in the body. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosphorylated forms of thi ...

derivatives in the 1940s, mainly in Japan. Allicin became a model for

medicinal chemistry Medicinal or pharmaceutical chemistry is a scientific discipline at the intersection of chemistry and pharmacy involved with designing and developing pharmaceutical drugs. Medicinal chemistry involves the identification, synthesis and developme ...

efforts to create other thiamine disulfides. The results included sulbutiamine,

fursultiamine Fursultiamine ( INN; chemical name thiamine tetrahydrofurfuryl disulfide or TTFD; brand names Adventan, Alinamin-F, Benlipoid, Bevitol Lipophil, Judolor, Lipothiamine) is a medication and vitamin used to treat thiamine deficiency. Chemically, it ...

(thiamine tetrahydrofurfuryl disulfide) and benfothiamine. These compounds are hydrophobic, easily pass from the intestines to the bloodstream, and are reduced to thiamine by cysteine or

glutathione Glutathione (GSH, ) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources such as reactive oxygen species, free radicals, pe ...

References

{{Authority control Thiosulfinates Anti-inflammatory agents Antibiotics Dietary antioxidants Pungent flavors Allium Garlic Antifungals Allyl compounds

Structure and occurrence

Biosynthesis

Research

History

See also

References