Cardiovascular and cerebrovascular diseases now have become the principal diseases that threaten human health and atherosclerosis is their principal pathogenic mechanism. Homocysteine (Hcy) has received much attention due to its role in the development of atherosclerosis. Numerous clinical studies and animal experiments have shown that hyperhomocysteinemia (HHcy) is an independent risk factor for vascular diseases, including ischemic heart disease, stroke and peripheral vascular disease (1,2). HHcy is a pathological condition characterized by an increase (>15 µmol/l) in the plasma concentration of total Hcy. Numerous studies have proposed that the atherogenic effects of HHcy are based on the induction of oxidative stress, endothelial dysfunction, thrombosis and activation of proinflammatory factors (3–8). Yet, the mechanisms by which Hcy promotes cardiovascular disease have remained to be fully elucidated. Carotid artery wall thickness is evaluated by measuring its intima-media thickness (IMT). Coronary artery IMT is an easily, reproducibly and non-invasively measured parameter for estimating the severity of carotid atherosclerosis and has been reported to be an independent predictor of coronary heart disease. Studies have demonstrated that allicin prevents atherosclerosis by exerting anti-aggregatory, anti-migratory (9) and anti-oxidant effects (10) as well as lowing blood pressure (11) and blood lipids (12). A previous study by our group reported that allicin obviously decreases the serum Hcy levels in rats with HHcy and also alleviates the injury of Hcy to endothelial cells (13). These results suggested that allicin may be useful as a drug for atherosclerosis. The present study hypothesized that allicin decreases plasma Hcy levels and exerts anti-atherosclerotic effects in coronary heart disease. The effect of allicin treatment on carotid artery IMT and plasma Hcy in coronary heart disease was assessed.

Atherosclerosis, initiated at sites of endothelial cell injury, is the pathological basis of cardiovascular and cerebrovascular diseases, such as coronary heart disease, cerebral apoplexy and post-angioplasty restenosis. Characteristic features of atherosclerotic lesions include infiltration of monocytic cells, cholesterol deposition, elaboration of extracellular matrix as well as proliferation and migration of vascular smooth muscle cells. HHcy is an independent risk factor of arteriosclerosis (16,17) and has been associated with an increased relative risk of cardiovascular events (18). In 587 coronary disease patients followed up for four years, a mortality rate of 3.8% was observed among those with homocysteinemia (Hcy, <9 µmol/l), and a mortality rate of 24.7% among those with Hcy levels of >15 µmol/l (19). The rate of HHcy among patients with atherosclerotic diseases is up to 40%. In the clinic, HHcy is classified into three categories according to the plasma Hcy concentration: Mild (15–30 µmol/l), moderate (30–100 µmol/l) and severe (>100 µmol/l). The present study identified that coronary heart diseases were closely associated with mild HHcy. Few of the patients with coronary heart diseases had moderate HHcy, but none of them had severe HHcy. In the allicin group, a significant decrease of plasma Hcy levels was observed after 12 weeks of treatment, which was significantly larger than the decrease in the control group, suggesting that the use of allicin may decrease Hcy levels in patients with coronary heart disease. Hcy, a sulfur-containing amino acid, is an intermediate product of methionine metabolism and causes multifold effects through the reactivity of its sulfhydryl group. Laboratory studies have suggested a role of Hcy in activating pathways attributed to atherogenesis and thrombosis (20), including endothelial dysfunction, inflammation, oxidative stress, lipid peroxidation (21), endoplasmic reticulum stress (22), stimulation of vascular smooth muscle cell proliferation and apoptosis (23). Garlic consumption has been inversely correlated with multiple risk factors associated with cardiovascular diseases, including increased platelet aggregation (9,24), reactive oxygen species (25), high blood pressure (11), high cholesterol (12,26), proliferation and migration of vascular smooth muscle cells (27,28), and apoptosis of heart cell (29). Allicin, diallyl trisulfide (CH₂=CH-CH₂-S-S-S-CH₂-CH=CH₂), is an organosulfur compound considered responsible for most of the pharmacological activities of garlic. Therefore, it is hypothesized that allicin has certain lowering effects on Hcy. A previous study by our group reported that allicin obviously decreases the serum Hcy levels in rats with HHcy and also alleviates Hcy-induced injury to endothelial cells (30). Another study suggested that garlic and garlic-derived organic polysulfides induce H₂S production in a thiol-dependent manner (31). H₂S is an endogenous cardioprotective vascular cell-signaling molecule. It can not only activate K_ATP channels, but also react with S-nitrosothiol species to release NO as a reducing agent and nucleophile, causing vascular smooth muscle cell relaxation. Whether H₂S generation from allicin is able to decrease Hcy in HHcy patients requires further investigation. The decrease of Hcy levels in coronary heart disease patients with HHcy after treatment with allicin was confirmed in the present study.

Lipid dysbolism is involved in the development of atherosclerosis. However, evidence linking allicin to blood lipids remains inconclusive. In the present study, significant decreases in TC and TG were observed in the allicin group compared with those in the control group. The possible mechanisms of allicin in reducing blood lipids are as follows: A displacement reaction of allicin with an enzyme containing an SH moiety may take place, which interferes the synthesis of lipids. Allicin may increase lipoidase activity and accelerate the hydrolysis of triglycerides. In the metabolic processes, allicin requires NADPH, which is also the sine qua non substrate in the biosynthesis of cholesterol and triglycerides. Allicin reacts with oxidized glutathione, causing a deficiency of glutathione, which affects the metabolism of insulin in the liver and indirectly affects the metabolism of lipids.

Due to its simplicity, reproducibility, cost-effectiveness and non-invasiveness, IMT measurement of carotid arteries has been widely adopted as a method for assessing systemic atherosclerosis and cardiovascular risk in adults. IMT has been reported to be an independent predictor of coronary heart disease (32–34). In carotid atherosclerosis patients, an increase in the IMT by 0.1 mm is associated with an 11% increase in the risk of acute myocardial infarction (35). The increased thickness of the carotid artery IMT is a signal of the total body vascular intima. Several biological mechanisms of the increased thickness of the vascular intima associated with HHcy include impaired endothelial function, dysregulation of cholesterol and triglyceride biosynthesis and stimulation of vascular smooth muscle cell proliferation. The oxidant stress hypothesis is frequently used to explain the damaging effects of Hcy on vascular cells. Due to the reactive sulfhydryl group (−SH), Hcy, as most thiols (RSH), can undergo oxidation to the disulfide (RSSR) at physiological pH in the presence of O₂: 2RSH + O₂ → RSSR + [O₂⁻] + 2H → H₂O₂. By inducing oxidative stress, the free radicals can injure the vascular endothelium. Allicin is able to quench oxygen and hydroxyl radicals to alleviate oxidative stress in a dose-effect manner. Nutritional deficiencies in B vitamin cofactors, including folic acid, vitamin B₆ (pyridoxal phosphate) and/or B₁₂ (methylcobalamin) are required for Hcy metabolism. Of note, dietary enrichment in B vitamins can decrease the levels of Hcy but does not attenuate these adverse effects of HHcy. Therefore, only decreasing the Hcy levels is insufficient. The present findings indicated that the carotid artery IMT in the allicin group was significantly decreased compared with that in the control group, which showed the beneficial effect of allicin in atherosclerosis. It is therefore indicated that the beneficial effects may be associated with the anti-oxidative effects of allicin.

In conclusion, the present study indicated that allicin may be useful for the prevention of atherosclerosis, at least in part by decreasing Hcy levels, regulating lipid metabolism and decreasing the carotid artery IMT. Medication prescribed for patients with coronary heart disease may also modify Hcy levels. However, certain studies have indicated that vitamin therapy, such as folic acid and B vitamins, lowered Hcy levels but failed to rescue cardiovascular or neurovascular disease outcomes (36–39). Therefore, allicin may be a prospective drug for the prevention of cardiovascular and cerebrovascular diseases.