Oxidation of low-density lipoprotein (LDL) and activation of the pleiotropic transcription factor nuclear factor kappa B (NF-kappaB), are often the chemical and molecular alterations associated with the development of the atherosclerotic lesion. We have reported previously on the antioxidant properties of a garlic compound, S-allyl cysteine (SAC), and its ability to inhibit damage caused by oxidative stress in bovine endothelial cells. In this study, the antioxidant effects of SAC were further determined, using several in vitro assay systems. First, we determined the effect of SAC on Cu2+-induced oxidation of LDL. Varying concentrations of SAC were co-incubated with a standardized Cu2+/LDL solution, and LDL-oxidation was then ascertained by determining the formation of thiobarbituric acid reactive substances (TBARS). SAC inhibited LDL-oxidation at an optimum concentration of 1 mM. In another experiment, we determined the effects of SAC on oxidized-LDL (ox-LDL) activation of J774 murine macrophages and human umbilical vein endothelial cells (HUVEC). Cells were grown on 96-well plates, preincubated with SAC at 37 degrees C and 5% CO2 for 24 h, washed, and exposed to ox-LDL for 24 h. Levels of hydrogen peroxide (H2O2) were determined by a fluorometric assay. In both cell lines, SAC exhibited dose-dependent inhibition of H2O2 formation. We also studied the effects of SAC on NF-kappaB activation in HUVEC using tumor necrosis factor-a (TNF-alpha) or H2O2 as stimulators. Cells were grown in 75 cm2 flasks at 37 degrees C and 5% CO2 and were preincubated with SAC 24 h before stimulation with TNF-alpha or H2O2. Nuclear extracts were then prepared and NF-kappaB activation was determined using an electrophoretic mobility shift assay with a 32P-labeled probe. SAC exhibited dose-dependent inhibition of NF-kappaB activation. Our data suggest that SAC may act via antioxidant mechanisms to inhibit the atherogenic process.