Allicin

Allicin is an organosulfur compound obtained from garlic and leeks.<ref name="Eric Block 1985 114–119">Template:Cite journal</ref> When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic.<ref>Template:Cite journal</ref> Allicin is unstable and quickly changes into a series of other sulfur-containing compounds such as diallyl disulfide.<ref name="ilic">Template:Cite journal</ref> Allicin is an antifeedant, i.e. the defense mechanism against attacks by pests on the garlic plant.<ref name="mol">Template:Cite journal</ref>

Allicin is an oily, slightly yellow liquid that gives garlic its distinctive odor. It is a thioester of sulfenic acid. It is also known as allyl thiosulfinate.<ref name="PMID 14964414">Template:Cite journal</ref> Its biological activity can be attributed to both its antioxidant activity and its reaction with thiol-containing proteins.<ref name="PMID 9528659">Template:Cite journal</ref>

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,<ref name="Eric Block 1985 114–119"/> and is formed by the action of the enzyme alliinase on alliin.<ref name="Eric Block 1985 114–119"/> Allicin is chiral but occurs naturally only as a racemate.<ref name="eric">Template:Cite book</ref> The racemic form can also be generated by oxidation of diallyl disulfide:<ref>Template:Cite book</ref><ref name=Mol>Template:Cite journal</ref>

(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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Furthermore, allicin can be unstable, breaking down within 16 hours at 23 °C.<ref>Template:Cite book</ref>

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 alliinase, which contains pyridoxal phosphate (PLP), cleaves alliin, generating allylsulfenic acid (CH₂=CHCH₂SOH), pyruvate, and ammonium ions.<ref name="PMID 9528659"/> At room temperature, two molecules of allylsulfenic acid condense to form allicin.<ref name="PMID 14964414"/><ref name=Mol/>

Research

Allicin has been studied for its potential to treat various kinds of multiple drug resistance bacterial infections, as well as viral and fungal infections in vitro, but as of 2016, the safety and efficacy of allicin to treat infections in people was unclear.<ref>Template:Cite journal</ref>

A Cochrane review found there to be insufficient clinical evidence regarding the effects of allicin in preventing or treating common cold.<ref>Template:Cite journal</ref>

History

It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944.<ref name=j1>Template:Cite journal</ref><ref name=eric/> Allicin was discovered as part of efforts to create thiamine derivatives in the 1940s, mainly in Japan. Allicin became a model for medicinal chemistry efforts to create other thiamine disulfides. The results included sulbutiamine, fursultiamine (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.<ref name=BettendorfHandbook>Template:Cite book</ref>Template:Rp

See also

• Allyl isothiocyanate, the active piquant chemical in mustard, radishes, horseradish and wasabi
• syn-Propanethial-S-oxide, the lachrymatory chemical found in onions
• List of phytochemicals in food