Allicin
Chemical compound
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Through Ammonium and Common cold
Allicin is an oily, slightly yellow liquid that gives garlic its distinctive odor. It is a thioester of a sulfinic acid. It is also known as allyl thiosulfinate. Its biological activity can be attributed to both its antioxidant activity and its reaction with thiol-containing proteins.
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 + 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 alliinase, which contains pyridoxal phosphate (PLP), cleaves alliin, generating allylsulfenic acid (CH2=CHCH2SOH), pyruvate, and ammonium ions. At room temperature, two molecules of allylsulfenic acid condense to form allicin.
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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.
A Cochrane review found there to be insufficient clinical evidence regarding the effects of allicin in preventing or treating common cold.
History
It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944. 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.