Garlic Chemistry

The Basis for Antimicrobial Activity of Allicin

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Basis for Antimicrobial Activity of Allicin

After it was clearly shown by that allicin is almost exclusively responsible for the antibiotic properties of garlic, the question of the mechanism of allicin’s antibiotic activity arose.


The fact that a compound possesses antimicrobial activity is based on two principal features. Firstly, the compound must be able to reach potential targets and, if these are intracellular, that means it must be able to get inside the microbial cell. In the case of bacteria, an antibiotic has to penetrate the bacterial cell wall and the cell membrane. In addition to these two boundaries, the slime layers or capsules of certain bacteria can constitute an extra layer of resistance. Following the arrival of the antibiotic in the cell, it must have a target that, if attacked, leads to cell inactivity or cell death. Miron et al. investigated the permeability of artificial and natural phospholipid membranes to allicin and demonstrated that allicin readily diffuses across these membranes.


When allicin is inside the cell, the antibiotic efficiency depends upon reaching and reacting with its targets and on the importance of those targets to the cell. In a seminal study, using logic that has stood the test of time, Cavallito and coworkers investigated the chemistry of several plant-derived antimicrobial compounds. It was found that the active principles of Allium sativum, Erythronium americanum, Asarum reflexum, Arctium minus, Ranunculus acris, Ranunculus bulbosus and of Brassica species as well as the non-plant-derived antibiotics penicillin, citrinin, gliotoxin, clavacin and pyocyanines react with cysteine. Pretreatment of these antibiotics with cysteine led to a total loss of activity against bacteria. The different antibiotics were categorized into groups depending on their reactivity with cysteine residues. The antibiotics were tested with cysteine residues that differed in their chemical microenvironment. In summary, certain antibiotics (group I, e.g., allicin) reacted with every cysteine residue independently of the neighbouring groups, as long as the –SH-group was freely available. Another group of antibiotics (group III, e.g., penicillin) showed increased reaction kinetics with cysteine if other amino-groups were adjacent to the cysteine residue, but much slower reaction kinetics if not. Pyocyanines (group II antibiotics) turned out to be in between, since they showed intermediate reaction kinetics with the cysteines tested. These experiments were prophetically significant considering the present state of knowledge on the effects of surrounding amino acids on the micro pKa values (and therefore reactivity) of cysteine residues in proteins to oxidation. Furthermore, Cavallito’s group speculated that the lower efficiency of allicin compared to penicillin and other antibiotics could be explained depending on their reactivity with cysteine residues. Since allicin reacts readily with all free cysteine residues available, it is buffered by proteins and low molecular weight thiols, independently of their importance for cellular viability. On the other hand, it was speculated that the specificity of penicillin for cysteine residues in the vicinity of amino-groups would lead to less waste of penicillin on unimportant targets, therefore making it more efficient than allicin. Several case studies have subsequently reported the reactivity of allicin with thiol-containing enzymes. For example, Wills used allicin to inhibit the activity of several enzymes in vitro. Among them were important enzymes for primary metabolism like succinic dehydrogenase, hexokinase, triosephosphate dehydrogenase or alcohol dehydrogenase.


But it has to be stated that some enzymes which did not contain a thiol group were also inhibited by allicin, whereas a few enzymes containing a thiol group could not. These findings suggest that not only cysteine is a potential target for allicin. The fact that some enzymes, although containing thiol groups, are not inactivated by allicin could be explained by an adverse pKa of these cysteines which depends on the microenvironment within the protein, so that the reactivity of these cysteines with allicin is very weak. Rabinkov et al. investigated the reaction of cysteine with allicin using RP-HPLC and NMR-analysis of the resulting products. It was shown that allicin reacted with the sulfhydryl-group of cysteine via a disulfide exchange-like reaction (Figure 2B, unstressed situation shown in Figure 2A). By demonstrating that enzymes that are irreversibly inhibited by allicin can be rescued by strong reductants, the idea of the disulfide exchange-like reaction was transferred to a working model of enzyme inhibition by allicin via the same mechanism.

A Closer Look at the Basis for Antimicrobial Activity of Allicin

Possible influence of allicin on proteins and protein synthesis. (A) protein synthesis in unstressed conditions. After translation the protein is folded into its final structure. (B) The cysteine residue that is accessible for attack (indicated in red) reacts with allicin via a disulfide exchange-reaction. The cysteine residue that is sterically blocked (indicated in blue) does not react with allicin. (C) According to Cavallito’s hypothesis, allicin may attack cysteine residues on elongating amino acid chains while the protein is still being synthesized and is not fully developed [48]. In this early stage, cysteine residues that are normally blocked for reactions with allicin (compare B) are now potential targets. Possible results may be an abortion of translation or misfolded proteins with reduced or no function.

Allicin against Methicillin-resistant

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Allicin against Methicillin-resistant


Antibacterial activity of a new, stable, aqueous extract of allicin against methicillin-resistant Staphylococcus aureus

The increasing prevalence of methicillin-resistant Staphylococcus aureus (MRSA) in hospitals and the community has led to a demand for new agents that could be used to decrease the spread of these bacteria. Topical agents such as mupirocin have been used to reduce nasal carriage and spread and to treat skin infections; however, resistance to mupirocin in MRSAs is increasing. Allicin is the main antibacterial agent isolated from garlic, but natural extracts can be unstable. In this study, a new, stable, aqueous extract of allicin (extracted from garlic) is tested on 30 clinical isolates of MRSA that show a range of susceptibilities to mupirocin. Strains were tested using agar diffusion tests, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). Diffusion tests showed that allicin liquids produced zone diameters > 33 mm when the proposed therapeutic concentration of 500 μg/mL (0.0005% w/v) was used.

The selection of this concentration was based on evidence from the MIC, MBC and agar diffusion tests in this study. Of the strains tested, 88% had MICs for allicin liquids of 16 μg/mL, and al strains were inhibited at 32 μg/mL. Furthermore, 88% of clinical isolates had MBCs of 128 μg/mL, and all were killed at 256 μg/mL. Of these strains, 82% showed intermediate or full resistance to mupirocin; however, this study showed that a concentration of 500 μg/mL in an aqueous cream base was required to produce an activity equivalent to 256 μg/mL allicin liquid.

Number of strains and the size of zones of inhibition with Allicin (500 µg/mL) and mupirocin against Staphylococcus aureus

Number of strains and the size of zones of inhibition with Allicin (500 µg/mL) and mupirocin against Staphylococcus aureus



Antibacterial activity of a new, stable, aqueous extract of allicin against methicillin-resistant Staphylococcus aureus.- By R R. CUTLER* and P. WILSON† Accepted: 19 March 2004

Allicin against methicillin-resistant

Bioactive Compounds of Garlic

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Bioactive Compounds of Garlic


Garlic is considered as a functional spice because of its diverse array of nutritional constituents, phytochemicals, and fiber.

It contains high levels of potassium, phosphorus zinc, and sulfur, moderate levels of selenium, calcium, magnesium, manganese, iron, and low levels of sodium, vitamin A and C and B-complex, 17 amino acids with eight main amino acids, also polyphenols, flavonoids, flavanols, tannins, saponins, polysaccharides, sulfur-containing compounds, and enzymes.

Generally, bioactive compounds are present in intact garlic, but, after chopping or crushing, a higher number of compounds, such as allicin, DAS, DADS, dithiins, and ajoene have been found after different types of chemical reactions.

This article mainly discusses the Bioactive Compounds of Garlic.

Organosulfur compounds from garlic

Non-sulfur garlic phytochemicals


Organosulfur compounds from garlic

Two classes of organosulfur compounds are found in whole garlic cloves: L-cysteine sulfoxides and γ-glutamyl-L-cysteine peptides.

L-Cysteine sulfoxides

S-allyl-L-cysteine sulfoxide (alliin) accounts for approximately 80% of cysteine sulfoxides in garlic. When raw garlic cloves are crushed, chopped, or chewed, an enzyme known as alliinase is released. Alliinase catalyzes the formation of sulfenic acids from L-cysteine sulfoxides. Sulfenic acids spontaneously react with each other to form unstable compounds called thiosulfinates. In the case of alliin, the resulting sulfenic acids react with each other to form a thiosulfinate known as allicin (half-life in crushed garlic at 23°C is 2.5 days). The formation of thiosulfinates is very rapid and has been found to be complete within 10 to 60 seconds of crushing garlic. Allicin breaks down in vitro to form a variety of fat-soluble organosulfur compounds, including diallyl trisulfide (DATS), diallyl disulfide (DADS), and diallyl sulfide (DAS), or in the presence of oil or organic solvents, ajoene and vinyldithiins. In vivo, allicin can react with glutathione and L-cysteine to produce S-allylmercaptoglutathione (SAMG) and S-allylmercaptocysteine (SAMC), respectively.

γ-Glutamyl-L-cysteine peptides                                

Crushing garlic does not change its γ-glutamyl-L-cysteine peptide content. γ-Glutamyl-L-cysteine peptides include an array of water-soluble dipeptides, including γ-glutamyl-S-allyl-L-cysteine, γ-glutamylmethylcysteine, and γ-glutamylpropylcysteine. Water-soluble organosulfur compounds, such as S-allylcysteine and SAMC, are formed from γ-glutamyl-S-allyl-L-cysteine during long-term incubation of crushed garlic in aqueous solutions, as in the manufacture of aged garlic extracts.


Non-sulfur garlic phytochemicals

Although little is known about their bioavailability and biological activities, non-sulfur garlic phytochemicals, including flavonoids, steroid saponins, organoselenium compounds, and allixin, likely work in synergy with organosulfur compounds.


Organosulfur alliin in garlic-allicin



Garlife™ ingredients.

Garlife™ GE-Garlic Extract, standardized by allicin, alliin

Garlife™ BG-Black Garlic Extract, standardized by SAC (S-Allyl-L-Cysteine)

Garlife™ AG-Aged Garlic Extract, standardized by SAC (S-Allyl-L-Cysteine)

Garlife™ GO-Garlic Oil, standardized by allicin.

Garlife™ Alipure- Garlic Extract 98% Alliin

Garlife™ AlIinase- Alliinase 1000U/g; 5000U/g.

Garlife™ Caps- OEM bulk capsules/tablets of garlic extract.

Redlife™ MK-Red yeast rice extract, standardized by monacolin-K