Allicin is an organosulfur compound obtained from garlic.[1] When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic.[2] Allicin is unstable and quickly changes into a series of other sulfur-containing compounds such as diallyl disulfide.[3] Allicin is an antifeedant, i.e. the defense mechanism against attacks by pests on the garlic plant.[4]
Allicin is an oily, slightly yellow liquid that gives garlic its distinctive odor. It is a thioesterofsulfenic acid. It is also known as allyl thiosulfinate.[5] Its biological activity can be attributed to both its antioxidant activity and its reaction with thiol-containing proteins.[6]
Allicin features the thiosulfinate functional group, R-S-(O)-S-R. The compound is not present in garlic unless tissue damage occurs,[1] and is formed by the action of the enzyme alliinaseonalliin.[1] Allicin is chiral but occurs naturally only as a racemate.[7] The racemic form can also be generated by oxidation of diallyl disulfide:[8][9]
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.[10][11] Furthermore, allicin can be unstable, breaking down within 16 hours at 23 °C.[12]
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.[6] At room temperature, two molecules of allylsulfenic acid condense to form allicin.[5][9]
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.[13]
In a small clinical trial, a daily high dose of extracted allicin (20 times the amount in a garlic clove) showed effectiveness to prevent the common cold.[14] A Cochrane review found this to be insufficient to draw conclusions.[15]
A study from 2021 has shown "a combination of the short half-life, high reactivity and non-specificity to particular proteins are reasons most bacteria 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".[16]
It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944.[17][7]
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.[18]: 302
^Kourounakis PN, Rekka EA (November 1991). "Effect on active oxygen species of alliin and Allium sativum (garlic) powder". Research Communications in Chemical Pathology and Pharmacology. 74 (2): 249–52. PMID1667340.
^ abNikolic V, Stankovic M, Nikolic L, Cvetkovic D (January 2004). "Mechanism and kinetics of synthesis of allicin". Die Pharmazie. 59 (1): 10–4. PMID14964414.
^ abRabinkov A, Miron T, Konstantinovski L, Wilchek M, Mirelman D, Weiner L (February 1998). "The mode of action of allicin: trapping of radicals and interaction with thiol containing proteins". Biochimica et Biophysica Acta (BBA) - General Subjects. 1379 (2): 233–44. doi:10.1016/s0304-4165(97)00104-9. PMID9528659.
^ abBlock E (2010). Garlic and Other Alliums: The Lore and the Science. Cambridge: Royal Society of Chemistry. ISBN978-0854041909.
^Cremlyn RJ (1996). An introduction to organosulfur chemistry. Wiley. ISBN0-471-95512-4.
^Hahn G (1996). Koch HP, Lawson LD (eds.). Garlic: the science and therapeutic application of Allium sativum L and related species (2nd ed.). Baltimore: Williams and Wilkins. pp. 1–24. ISBN978-0-683-18147-0.
^Marchese A, Barbieri R, Sanches-Silva A, Daglia M, Nabavi SF, Jafari NJ, Izadi M, Ajami M, Nabavi SM (2016). "Antifungal and antibacterial activities of allicin: A review". Trends in Food Science and Technology. 52: 49–56. doi:10.1016/j.tifs.2016.03.010.
^Cavallito CJ, Bailey JH (1944). "Allicin, the Antibacterial Principle of Allium sativum. I. Isolation, Physical Properties and Antibacterial Action". Journal of the American Chemical Society. 66 (11): 1950. doi:10.1021/ja01239a048.
^Bettendorff L (2014). "Chapter 7 - Thiamine". In Zempleni J, Suttie JW, Gregory JF, Stover PJ (eds.). Handbook of vitamins (Fifth ed.). Hoboken: CRC Press. pp. 267–324. ISBN978-1-4665-1557-4.
