The present study analyzed the antioxidative properties of natural compounds from the root of the plant Glycyrrhiza glabra (licorice) toward LDL oxidation. Seven constituents, with antioxidant capacity were isolated from Glycyrrhiza glabra. The isolated compounds were identified as the isoflavans Hispaglabridin A (1), Hispaglabridin B (4), Glabridin (3), and 4'-O-Methylglabridin (2), the two chalcones, isoprenylchalcone derivative (5) and Isoliquiritigenin (6), and the isoflavone, Formononetin (7). Among these compounds, Glabridin constituted the major amount in the crude extract (11.6%, w/w) as detected by high-performance liquid chromatography (HPLC) analysis. The antioxidative capacities of the isolated compounds (1-7) were tested against beta-carotene destruction and LDL oxidation. The isoflavans (1-4) at a concentration of 50 microM inhibited beta-carotene consumption, following 90 min of incubation at 50 degrees C, similar to the inhibitory effect of the whole licorice crude extract (at 16 mg/1). The chalcones (5 and 6) exhibited moderate inhibition and the isoflavone 7 was almost inactive, whereas vitamin E (50 microM) completely inhibited beta-carotene consumption. The inhibitory effect of the constituents 1-7, at a concentration of 30 microM on 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH)-induced LDL oxidation was determined by measuring the amount of the thiobarbituric acid reactive substances (TBARS) and the amount of lipid peroxides. While compounds 1-6 exhibited high inhibitory activity, compound 7 and vitamin E were not active. A dose-dependent inhibitory effect of Glabridin, on the formation of cholesteryl linoleate hydroperoxide (CLOOH), in an AAPH-induced LDL oxidation system was also shown. Glabridin, at 5 or 40-60 microM concentration, inhibited the CLOOH formation by 62% and 90%, respectively. These results suggest that constituents 1-6 are very potent antioxidants toward LDL oxidation with Glabridin being the most abundant and potent antioxidant. As LDL oxidation is a key event in the formation of the early atherosclerotic lesion, the use of these natural antioxidants may be proven beneficial to attenuate atherosclerosis.