The antioxidant properties of three garlic preparations and organosulfur compounds in garlic have been determined. Aged garlic extract inhibited the emission of low level chemiluminescence and the early formation of thiobarbituric acid-reactive substances (TBA-RS) in liver microsomal fraction initiated by t-butyl hydroperoxide. However, the water extracts of raw and heat-treated garlic enhanced the emission of low level chemiluminescence. Among the variety of organosulfur compounds, S-allylcysteine (SAC) and S-allylmercaptocysteine (SAMC), the major organosulfur compounds found in aged garlic extract, showed radical scavenging activity in both chemiluminescence and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assays, indicating that these compounds may play an important role in the antioxidative activity of aged garlic extract.

Oxidative modification of LDL has been recognized as playing an important role in the initiation and progression of atherosclerosis. In this study, we determined the effects of aged garlic extract (AGE) and its major compound, S-allylcysteine (SAC), on oxidized LDL (Ox-LDL)-induced injury in endothelial cells (EC). Lactate dehydrogenase (LDH) release as an index of membrane damage, methylthiazol tetrazoium (MTT) assay for cell viability and thiobarbituric acid reactive substances (TBARS) indicating lipid peroxidation were measured. Ox-LDL caused an increase of LDH release, loss of cell viability and TBARS formation. Both AGE and SAC prevented all of these changes. To elucidate the mechanism, effects of AGE or SAC on intracellular glutathione (GSH) level in EC, and release of peroxide from EC and macrophages (M Phi) were determined. Ox-LDL depleted intracellular GSH and increased release of peroxides. Both AGE and SAC inhibited these changes. Effects of SAC on hydrogen peroxide (H(2)O(2)) or tumor necrosis factor (TNF)-alpha-induced nuclear factor (NF)-kappa B activation were determined. Pretreatment of EC with SAC inhibited NF-kappa B activation. We demonstrated that both AGE and SAC can protect EC from Ox-LDL-induced injury by preventing intracellular GSH depletion in EC and by minimizing release of peroxides from EC and M Phi. SAC also inhibited H(2)O(2)- or TNF-alpha-induced NF-kappa B activation. Our data suggest that AGE and its main compound, SAC, may be useful for prevention of atherosclerosis.

Aged garlic extract (AGE) has been shown to have antioxidant activity. The organosulfur compounds, S-allyl-L-cysteine and S-allylmercapto-L-cysteine, are responsible, at least in part, for the antioxidant activity of AGE. To identify major active components, we fractionated AGE, using hydrogen peroxide scavenging activity as an antioxidative index. Strong activity in the amino acid fraction was found and the major active compound was identified as N alpha-(1-deoxy-D-fructos-1-yl)-L-arginine (Fru-Arg). Antioxidant activity of Fru-Arg was comparable to that of ascorbic acid, scavenging hydrogen peroxide completely at 50 micromol/L and 37% at 10 micromol/L. Quantitative analysis using the established HPLC system revealed that AGE contained 2.1-2.4 mmol/L of Fru-Arg, but none was detected in either raw or heated garlic juice. Furthermore, it was shown that a minimum of 4 mo aging incubation was required for Fru-Arg to be generated. These findings indicate that the aging process is critical for the production of the antioxidant compound, Fru-Arg. These results may explain some of the variation in benefits among different commercially available garlic preparations.

We determined the changes in the contents of three gamma-glutamyl peptides and four sulfoxides in garlic cloves during storage at -3, 4, and 23 degrees C for 150 days using a validated high-performance liquid chromatography method that we reported recently. When garlic was stored at 4 degrees C for 150 days, marked conversion of the gamma-glutamyl peptides, gamma-L-glutamyl-S-allyl-L-cysteine and gamma-L-glutamyl-S-(trans-1-propenyl)-L-cysteine (GSPC), to sulfoxides, alliin and isoalliin, was observed. Interestingly, however, when garlic was stored at 23 degrees C, a decrease in GSPC and a marked increase in cycloalliin, rather than isoalliin, occurred. To elucidate in detail the mechanism involved, the conversion of isoalliin to cycloalliin in both buffer solutions (pH 4.6, 5.5, and 6.5) and garlic cloves at 25 and 35 degrees C was examined. Decreases in the concentration of isoalliin in both the solutions and the garlic cloves during storage followed first-order kinetics and coincided with the conversion of cycloalliin. Our data indicated that isoalliin produced enzymatically from GSPC is chemically converted to cycloalliin and that the cycloalliin content of garlic cloves increases during storage at higher temperature. These data may be useful for controlling the quality and biological activities of garlic and its preparations.