THE ANTIOXIDATIVE EFFECTS OF THE ISOFLAVAN GLABRIDIN ON ENDOGENOUS CONSTITUENTS OF LDL DURING ITS OXIDATION

The effect of the consumption of glabridin, an isoflavan isolated from Glycyrrhiza glabra (licorice) root, on the susceptibility of low dens ity lipoprotein (LDL) to oxidation was studied in atherosclerotic apol ipoprotein E deficient (E degrees mice) and was compared with that of the known flavonoids, quercetin and catechin. Glabridin inhibitory act ivity on in vitro oxidation of human LDL was also investigated by dete rmining the formation of lipid peroxides and oxysterols and the consum ption of LDL-associated lipophilic antioxidants. Determination of the extent of LDL oxidation by measuring the formation of thiobabituric ac id reactive substances (TBARS) after 2 h of LDL incubation with CuSO4 (10 mu M) or 2,2'-azobis (2-amidino-propane) dihydrochloride (AAPH) (5 mM), revealed that glabridin or quercetin consumption resulted in a 5 3 and 54% reduction in copper ion induced oxidation, respectively, and a 95 and 83% reduction in AAPH induced LDL oxidation, respectively. N o inhibition was obtained with consumption of catechin. About 80% of g labridin was found to bind to the LDL human particle. In the in vitro oxidation of LDL induced by AAPH (5 mM), glabridin inhibited the forma tion of TBARS, lipid peroxides and cholesteryl linoleate hydroperoxide (CLOOH) at all the concentrations tested (5-60 mu M), while in oxidat ion induced by copper ions (10 mu M), glabridin exhibited a pro-oxidan t activity at concentrations lower than 20 mu M, and a clear antioxida nt activity at concentrations greater than 20 mu M. Glabridin (30 mu M ) inhibited the formation of cholest-5-ene-3,7-diol (7-hydroxycholeste rol): cholest-5-ene-3-ol-7-one (7-ketocholesterol) and cholestan-5,6-e poxy-3-ol (5,6-epoxycholesterol) after 6 h of AAPH induced LDL oxidati on, by 55, 80 and 40%, respectively, and after 6 h of copper ion induc ed LDL oxidation, by 73, 94 and 52%, respectively. Glabridin also inhi bited the consumption of beta-carotene and lycopene by 38 and 52%, res pectively, after 0.5 h of LDL oxidation with AAPH, but failed to prote ct vitamin E. The in vivo and in vitro reduction of the susceptibility of LDL to oxidation obtained with glabridin, may be related to the ab sorption or binding of glabridin to the LDL particle and subsequent pr otection of LDL from oxidation by inhibiting the formation of lipid pe roxides and oxysterols, and by protecting LDL associated carotenoids. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.