S-allylcysteine

S-Allylcysteine (SAC): A Comprehensive Reference

1. Identity and Chemical Characterization

Chemical Names and Formula

S-Allylcysteine (SAC) is an organosulfur compound with the molecular formula HO₂CCH(NH₂)CH₂SCH₂C=CH₂. It is the S-allylated derivative of the amino acid cysteine. Its IUPAC name is (2R)-2-amino-3-prop-2-enylsulfanylpropanoic acid. Only the L-enantiomer is significant biologically. The melting point of SAC is 219–220 °C, its boiling point is 300 °C, its density is 1.191 ± 0.06 g/cm³, and its molecular weight is 161.22 g/mol with a molecular formula of C₆H₁₁NO₂S. SAC belongs to the class of organic compounds known as cysteine derivatives; specifically, it is an S-hydrocarbyl-L-cysteine in which the hydrogen attached to the sulfur is replaced by a prop-2-enyl (allyl) group.

Related Compounds

A number of related compounds are found in garlic, including the disulfide S-allylmercaptocysteine (SAMC) and γ-glutamyl-S-allylcysteine (GSAC). Other phytochemicals including ajoenes, allixin, flavonoids, polyphenols, and thiosulfinates are also present in aged garlic extract, though at lesser concentrations.

Natural Source and Botanical Origin

SAC is a garlic-derived organosulfur compound. Garlic's scientific name is Allium sativum L., belonging to the family Amaryllidaceae (formerly Alliaceae). Allium genus plants include garlic, elephant garlic, scallion, onion, spring onion, green onion, leek, and chives. Studies on the chemical composition of Allium sativum show that the most important constituents of this plant are organosulfur compounds such as allicin, diallyl disulfide, S-allylcysteine, and diallyl trisulfide. In addition to A. sativum, these compounds are also present in A. hirtifolium (shallot).

Biosynthesis

One route of S-allylcysteine biosynthesis in garlic involves serine as a substrate; another pathway proceeds from glutathione (GSH) to S-allylcysteine. During the aging process, γ-glutamyl-S-allylcysteine is converted to S-allylcysteine by a γ-glutamyltransferase.

Distribution: Fresh vs. Aged Garlic

SAC is present in low concentrations in fresh garlic but accumulates significantly during controlled ageing. The conversion of γ-glutamyl-S-allylcysteine (GSAC) to SAC occurs progressively over extended ageing periods, typically ranging from 10 to 20 months under controlled conditions. This accumulation is one reason aged garlic extract preparations have a substantially different compound profile compared to raw garlic, garlic powder, or garlic oil. During the aging process, the odorous, sour, and irritating compounds in fresh raw garlic, such as allicin, are naturally converted into stable and safe compounds with significantly greater therapeutic effects. In aged garlic extract (AGE), SAC and S-allylmercaptocysteine (SAMC) are the major water-soluble organosulfur compounds.

Common Forms and Preparations

Aged garlic extract (AGE) is produced by aging and extracting organic fresh garlic in water at room temperature for approximately 20 months. The resulting extract is high in water-soluble phytochemicals, including cysteine derivatives such as SAC, S-allyl mercaptocysteine (SAMC), S-methyl cysteine, and gamma-glutamyl cysteine derivatives. Many types of commercial garlic products are available, including aged garlic extract, garlic essential oil, and garlic powder. AGE includes water-soluble sulfur compounds such as SAMC and SAC and small amounts of oil-soluble allyl sulfides. Garlic essential oil includes only oil-soluble sulfur components such as diallyl trisulfide and diallyl sulfide. Garlic powder contains alliin and a small amount of oil-soluble sulfur compounds.

SAC is water-soluble and demonstrates substantially higher bioavailability compared to allicin and allicin-derived compounds, which are chemically unstable and degrade rapidly following formation. SAC has been detected in human plasma following oral administration of aged garlic extract preparations, suggesting meaningful systemic absorption. Because SAC concentration in aged garlic extract is predictably related to ageing duration and conditions, it is commonly used as a quality and standardisation marker in both research preparations and commercial products.

2. Traditional and Historical Use

Throughout history, garlic has served vital nutritional and therapeutic functions. Some of the oldest references to this therapeutic herb appear in the Avesta, a collection of Zoroastrian holy literature effectively carried together in the sixth century BC. The ancient Sumerians and Egyptians both utilized garlic in medicine. There is some evidence that athletes were given garlic to enhance strength during the first Olympics in Greece. Garlic was suggested for use in traditional Chinese and Indian medicine to treat leprosy, aid digestion, and prevent parasite infestation.

Garlic has been used for traditional medicine in diverse cultures such as in Korea, Egypt, Japan, China, Rome, and Greece. In his Natural History, Pliny gave a list of conditions in which garlic was considered beneficial. Galen, writing in the second century, eulogized garlic as the "rustic's theriac," meaning a cure-all. In the 17th century, Thomas Sydenham valued it as an application in confluent smallpox, and William Cullen's Materia Medica of 1789 found some dropsies cured by it alone.

Garlic is a perennial plant grown in China, Korea, Japan, and many other countries. It is generally known as a tonic or restorative drug and has long been used as a stomachic, diuretic, expectorant, intestinal-controlling, bactericidal, and anthelmintic agent. In Japan, garlic-containing miso soup is used as a remedy for the common cold with headaches, fever, and sore throat.

Although SAC itself was not isolated and identified as a discrete compound until modern analytical chemistry methods became available, aged garlic, which lacks allicin but may have activity due to the presence of SAC, represents a distinct traditional preparation in several Asian cultures. Aged garlic extract is sliced, dried garlic that has been preserved in 15–20% ethanol for more than 1.5 years. More than 3,000 publications in this century have provided evidence of garlic's effectiveness in preventing and treating a variety of diseases, validating its traditional uses.

3. Key Constituents and Active Compounds

SAC as the Principal Bioactive Compound of Aged Garlic Extract

SAC is the main organosulfur compound in aged garlic extract and is recognized for its antioxidant and neuroprotective properties. SAC has been extensively studied, demonstrating remarkable antioxidant, anti-inflammatory, and immunomodulatory capacities.

Relationship to Allicin

The non-proteinogenic amino acid alliin (S-allylcysteine sulfoxide) is converted to allicin in an enzyme-mediated process catalyzed by alliinase when garlic tissue is damaged. SAC and alliin are distinct precursors in the garlic organosulfur metabolic pathway; thiosulfinates such as allicin decompose and disappear during any processing, while SAC accumulates and persists through aging and aqueous extraction, conferring distinctive chemical stability.

4. Mechanisms of Action

Antioxidant Activity: Direct Free-Radical Scavenging

SAC can scavenge superoxide anion, hydrogen peroxide, hydroxyl radical, peroxynitrite radical, and peroxyl radical produced in neuronal cells, as well as hypochlorous acid and singlet oxygen produced in microglial cells. Moreover, SAC exhibits chelating properties on Fe²⁺ and Cu²⁺ ions, thereby avoiding Fenton reaction-mediated oxidative damage.

Nrf2/ARE Pathway Activation

The cellular defense mechanism against oxidative damage involves the activation of transcription factor Nrf2, which enhances the transcription of antioxidant and phase II enzyme genes. The ability of SAC to activate the Nrf2 factor has been previously reported in hepatic cells, and research has extended this finding to normal brain tissue. A study in male Wistar rats administered SAC (25, 50, 100, and 200 mg/kg body weight, intragastric) for 90 days found dose-dependent activation of Nrf2 and enhancement of antioxidant enzyme activities in the striatum, frontal cortex, and hippocampus, alongside alterations in markers of oxidative damage.

NF-κB Inhibition and Anti-Inflammatory Effects

SAC inhibits NF-κB translocation into the nucleus, thus preventing apoptotic signaling. This mechanism underlies much of SAC's anti-inflammatory activity, since NF-κB governs the transcription of numerous pro-inflammatory cytokines. SAC has been shown to demonstrate immunomodulatory capacities alongside its antioxidant and anti-inflammatory properties.

Neuroprotective Mechanisms: ER Stress and Calpain Inhibition

SAC is a sulfur-containing amino acid exhibiting a wide range of biological activities including antioxidant, anti-inflammatory, and anticancer properties. An earlier study demonstrated that SAC ameliorates oxidative damage in a model of experimental stroke; however, the antioxidant property alone does not fully explain its beneficial effects. Endoplasmic reticulum (ER) stress and ER stress-induced cell death have been implicated in neurological diseases such as brain ischemia, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. SAC appears to interact with the Ca²⁺-binding site of calpain. This finding of a novel target for SAC strongly suggests that the concept of SAC as a mere antioxidant must be expanded to include the property of inhibiting ER stress. The calpain inhibitory potential of SAC may prove useful in deriving therapeutic agents for neurological disorders associated with ER stress or overactivation of calpain.

Nitric Oxide and Hydrogen Sulfide Pathways (Vascular)

The AGE hypotensive mechanism involves the participation of S-allylcysteine in vasodilator pathways, such as nitric oxide (NO) and hydrogen sulfide (H₂S) synthesis. Garlic-derived polysulfides stimulate the production of the vascular gasotransmitter hydrogen sulfide and enhance the regulation of endothelial nitric oxide, which induce smooth muscle cell relaxation, vasodilation, and blood pressure reduction.

Glutathione System

Another biosynthetic pathway for SAC in garlic proceeds from glutathione (GSH), suggesting a close metabolic relationship between SAC and the glutathione antioxidant system. Research has further indicated that SAC can upregulate glutathione-dependent antioxidant enzymes (glutathione peroxidase, glutathione reductase) through Nrf2 activation, thereby reinforcing the cellular redox buffer.

Neurotrophic Activity

Research has evaluated whether SAC supports neurotrophin signaling, including tropomyosin receptor kinase B (TrkB), protein kinase B (AKT), and extracellular signal-regulated kinase (ERK) pathways following ischemia/reperfusion injury. SAC must be classified not only as an antioxidant, but also as an anti-inflammatory and neurotrophic molecule.

5. Pharmacokinetics

S-allylcysteine was rapidly and easily absorbed following oral administration, distributed mainly in plasma, liver, and kidney. Bioavailability was 98.2% in rats, 103% in mice, and 87.2% in dogs; the compound was mainly excreted into urine unchanged and/or N-acetylated, depending on the species. In human volunteers who received oral garlic supplement containing SAC, SAC was rapidly absorbed from the gastrointestinal tract; however, the half-life was more than 10 hours and the excretion time was more than 30 hours.

6. Scientific Evidence by Area of Use

6.1 Cardiovascular Health: Hypertension

Evidence Level: Moderate (multiple RCTs; systematic reviews and meta-analyses available).

A double-blind randomized placebo-controlled trial of 12 weeks enrolled 88 patients with uncontrolled hypertension, investigating the effect of daily intake of aged garlic extract (1.2 g containing 1.2 mg S-allylcysteine) or placebo on blood pressure, central hemodynamics, and other cardiovascular markers. Mean blood pressure was significantly reduced by 5.0 ± 2.1 mmHg systolic, and in responders by 11.5 ± 1.9 mmHg systolic and 6.3 ± 1.1 mmHg diastolic compared to placebo. Central hemodynamic measures tended to improve in the garlic group more than in the placebo group, including central blood pressure, central pulse pressure, mean arterial pressure, augmentation pressure, pulse-wave velocity, and arterial stiffness. Trends in beneficial effects on inflammatory markers TNFα, total cholesterol, LDL cholesterol, and apolipoproteins were observed, though these did not reach significance due to small subgroup numbers.

A second randomized, double-blind, placebo-controlled clinical trial included 19 hypertensive volunteers treated daily with 1.2 g of AGE (containing 1.2 mg of S-allylcysteine) or placebo for 12 weeks. Only the AGE group showed significant reductions in systolic blood pressure at weeks 4 and 12.

Numerous clinical trials have examined the effects of aged garlic on metabolic factors with inconsistent results; a systematic review and meta-analysis that evaluated 19 trials found that aged garlic consumption significantly reduced systolic blood pressure (weighted mean difference: −2.49 mmHg; 95% CI: −4.02 to −0.95) and LDL cholesterol (WMD: −4.41 mg/dL; 95% CI: −8.28 to −0.54).

An open-label clinical trial investigated a SAC-rich garlic extract formula in hypertensive subjects. Twelve participants completed the clinical trial. The supplement decreased systolic blood pressure by approximately 11 mmHg at two weeks, persisting beyond four weeks with daily supplementation, and decreased diastolic blood pressure in hypertensive subjects. This study is limited by its open-label design and small sample size.

A separate meta-analysis on the safety and efficacy of long-term garlic consumption as an adjunctive treatment for hypertension reported that S-allylcysteine content was negatively correlated with blood pressure outcome, with an optimal dosage range identified at 0.5–1.5 mg, and that garlic intervention had a significant effect on reducing blood pressure in populations including obese individuals, those aged 50–60, or those with higher diastolic blood pressure.

6.2 Cardiovascular Health: Atherosclerosis and Lipids

Evidence Level: Preliminary-to-moderate (preclinical studies, some RCT data with inconsistent lipid results).

Among garlic preparations, aged garlic extract has been shown to improve atherosclerosis in clinical trials and animal studies. AGE contains various compounds with potential anti-atherosclerotic properties, such as S-1-propenylcysteine, S-allylcysteine, and other sulfur-containing constituents. AGE has also been shown to inhibit cholesterol synthesis in rat hepatocytes; the cooperative action of several AGE components, including SAC, may contribute to this effect.

6.3 Neuroprotection and Neurodegenerative Diseases

Evidence Level: Preclinical (animal models and cell lines); no completed large human clinical trials specifically for SAC as a standalone intervention in neurodegeneration.

ER stress and ER stress-induced cell death have been shown to be involved in neurological diseases such as brain ischemia, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. In a rat ischemia/reperfusion model, the neuroprotective effect of SAC, a water-soluble compound from garlic, was investigated against cerebral ischemia/reperfusion-induced mitochondrial dysfunction in the hippocampus using a transient rat middle cerebral artery occlusion (MCAO) model. SAC (300 mg/kg) was given twice intraperitoneally. SAC significantly restored ATP content and the activity of mitochondrial respiratory complexes, which were severely altered in the MCAO group.

While SAC has shown neuroprotective properties, its subacute long-term effects remain underexplored, particularly in females. A recent study evaluated whether SAC supports functional recovery after ischemia/reperfusion, focusing on neurotrophin signaling including TrkB, AKT, and ERK pathways. Adult female Wistar rats underwent 1 hour of ischemia and 15 days of reperfusion; SAC (100 mg/kg, intraperitoneally) was administered at the onset of reperfusion and daily for 15 days, with motor and cognitive deficit tests performed.

The dietary intake of SAC has been shown to decrease the production of amyloid-beta (Aβ) in the brains of mice with D-galactose-induced aging. In a study examining Alzheimer's disease-related pathology, SAC demonstrated anti-amyloidogenic activity and the ability to destabilize Alzheimer's beta-amyloid fibrils in vitro (Pubmed: 18023978). All neurodegenerative disease findings for SAC remain preclinical; human trial data specific to SAC in Alzheimer's or Parkinson's disease are not available.

6.4 Hepatoprotection

Evidence Level: Preclinical (animal and cell models); limited direct human data.

Aged garlic extract possesses hepatoprotective and antioxidative activities. SAC has historically been reported in the scientific literature as effective in controlling hepatopathy. In a study of doxorubicin-induced toxicity, severe doxorubicin toxicity was induced in mice by a single intraperitoneal injection (15 mg/kg body weight), and SAC (30 mg/kg) was injected intraperitoneally daily for 5 days starting two days before doxorubicin administration. Histopathological analysis of the heart and liver and serum creatine phosphokinase (CPK) measurements were performed 6 days after doxorubicin administration. These findings suggest a potential cytoprotective role against chemotherapy-related organ toxicity in animal models; translation to human outcomes requires further study.

6.5 Anticancer Effects

Evidence Level: Preclinical (cell-line and animal models); no clinical trials specifically for SAC as a standalone anticancer intervention.

Hepatocellular carcinoma (HCC) is highly malignant and metastatic, and the garlic derivative SAC has been investigated for its effect on the proliferation and metastasis of HCC. The proliferation rate and colony-forming abilities of MHCC97L cells were suppressed by SAC, together with significant suppression of proliferation markers Ki-67 and PCNA. Moreover, SAC hindered the migration and invasion of MHCC97L cells corresponding with upregulation of E-cadherin and downregulation of VEGF. Furthermore, SAC significantly induced apoptosis and necrosis of MHCC97L cells through suppressing Bcl-xL. These findings are from preclinical in vitro and xenograft models and do not establish clinical efficacy in humans.

6.6 Metabolic Health: Diabetes and Obesity

Evidence Level: Preclinical; limited indirect human evidence through AGE studies.

A narrative review examining the pathophysiological mechanisms underlying metabolic syndrome summarized current evidence on the protective role of SAC against key pathological features of this condition, including oxidative stress and inflammation. Animal research on SAC in diabetic rat models has shown effects on thyroid hormone and circulatory antioxidant systems, and nuclear factor κB-dependent anti-inflammatory effects have been observed in the kidneys of diabetic mice. These findings are preclinical and do not constitute clinical evidence in humans.

6.7 Immune Function

Recently, AGE has been suggested as a promising candidate for the maintenance of immune system homeostasis through modulation of cytokine secretion, promotion of phagocytosis, and activation of macrophages. Additional clinical studies are necessary to establish the concrete mechanisms and pathways that underpin the therapeutic role of SAC as an immunomodulatory agent.

7. Body Systems and Health Areas Associated with SAC

• Cardiovascular system: A wide range of therapeutic effects of garlic has been observed, including cardioprotective and antihypertensive effects. SAC specifically is implicated in blood pressure regulation via NO and H₂S vasodilator pathways.
• Central nervous system: SAC exhibits a broad spectrum of protective effects characterized by antioxidant, anti-inflammatory, and neuromodulatory actions.
• Hepatic system: Aged garlic extract, whose primary active compound is SAC, possesses hepatoprotective activities.
• Endocrine/metabolic system: Preclinical studies have linked SAC to improvements in insulin sensitivity and markers of glycation-related damage.
• Immune system: SAC has been extensively studied, demonstrating antioxidant, anti-inflammatory, and immunomodulatory capacities.
• Oncology (preclinical): SAC is of interest for its potential medicinal properties and as a chemopreventive agent.

8. Dosage Forms and Dosages Reported in Studies

SAC is not typically marketed as an isolated supplement at present; rather, it is primarily consumed as a constituent of aged garlic extract (AGE) preparations, in which it is used as a standardization marker.

• Human hypertension trials: A 12-week RCT used daily intake of 1.2 g of aged garlic extract containing 1.2 mg S-allylcysteine. A separate trial also administered 1.2 g of AGE (containing 1.2 mg of S-allylcysteine) per day for 12 weeks.
• Optimal SAC dosage range (meta-analysis): S-allylcysteine content was negatively correlated with blood pressure outcome, with an optimal dosage range identified at 0.5–1.5 mg per day in meta-analytic findings.
• Animal neuroprotection studies: In a rat cerebral ischemia model, SAC was administered at 300 mg/kg body weight intraperitoneally. In a female rat ischemia/reperfusion study, SAC was administered at 100 mg/kg intraperitoneally at the onset of reperfusion and daily for 15 days.
• Animal Nrf2 activation study: Male Wistar rats were administered SAC at doses of 25, 50, 100, and 200 mg/kg body weight intragastrically every 24 hours for 90 days to assess brain Nrf2 activation.
• Animal cardiotoxicity protection study: SAC (30 mg/kg) was injected intraperitoneally daily for 5 days in a mouse doxorubicin-toxicity model.

Physical, chemical, and biological properties of SAC were investigated, and SAC showed stable properties under tested conditions. Its acute/subacute toxicity was very minor in mice and rats (LD₅₀ value >54.7 mM/kg by oral route; >20 mM/kg intraperitoneally).

9. Safety Considerations and Drug Interactions

General Toxicology and Safety Profile

SAC showed stable properties under tested conditions, and its acute/subacute toxicity was very minor in mice and rats (LD₅₀ value >54.7 mM/kg oral; >20 mM/kg intraperitoneally). SAC has demonstrated overall tolerability in humans when supplemented with aged garlic, and low toxicity in animals after 30 days of consumption.

Tolerability in Clinical Trials

In the 88-patient AGE hypertension trial, aged garlic extract was highly tolerable and acceptable, and did not increase the risk of bleeding in patients on blood-thinning medication.

Anticoagulant Interactions

Because of a lack of major clinical data regarding the safety of concomitant use of garlic supplements and anticoagulants, a double-blind, randomized, placebo-controlled pilot study was conducted in which 52 patients were randomized and 48 completed the study. The study medication (AGE or placebo) was administered at 5 mL twice daily for 12 weeks, and potential bleeding and thromboembolic episodes were monitored. Aged garlic extract, which contains S-allylcysteine as the bioactive sulfur compound, appears standardizable and highly tolerable, with little or no known harmful interaction when taken with other blood-pressure-reducing or blood-thinning medications.

However, garlic has antiplatelet and mild anticoagulant properties; it may potentiate warfarin, aspirin, clopidogrel, and direct oral anticoagulants (DOACs), and can interact with antiretrovirals, antihypertensives, and hypoglycaemic agents. Conflicting data are often obtained in controlled clinical studies on garlic interactions, and these inconclusive results are likely due to differences in the duration of treatment and/or the use of different garlic-derived materials. The distinction between AGE-based products (which are rich in SAC) and allicin-based or oil-based preparations (which have different phytochemical profiles) is important when evaluating interaction risk.

Cytochrome P450 System

One study on Phase I and Phase II biotransformation enzymes reported that rats treated with a single dose of garlic oil showed a significant depression of hepatic cytochrome P-450; by contrast, a daily dose over 5 days induced an increase in hepatic cytochrome P-450. Specific CYP enzyme effects attributable to SAC (as distinct from other garlic constituents) have been evaluated in pharmacological studies, but clinical magnitude of interaction is not firmly established for the isolated compound.

Gastrointestinal Effects

Some people suffer from allergies to garlic and other species of Allium. Symptoms can include irritable bowel, diarrhea, mouth and throat ulcerations, nausea, breathing difficulties, and, in rare cases, anaphylaxis. These effects are attributed to garlic broadly and its reactive sulfur compounds; AGE preparations, having reduced allicin content, are generally considered better tolerated gastrointestinally.

Perioperative Considerations

High-dose garlic supplements should be stopped at least 7–10 days before elective surgery, in line with NHS and Royal College of Anaesthetists guidance.

Overall Evidence Gap

Additional clinical studies are necessary to establish the concrete mechanisms and pathways that underpin the therapeutic role of SAC as an anti-cancer, neuroprotective, hepatoprotective, antidiabetic, cardioprotective, anti-asthmatic, and nephroprotective agent. Elaborative experiments on human cell lines and in vivo human supplementation studies are needed to elucidate these health benefits.

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