STRUCTURAL ASPECTS OF THE INHIBITORY EFFECT OF GLABRIDIN ON LDL OXIDATION

The inhibitory effects of glabridin, an isoflavan isolated from licori ce (Glycyrrhiza glabra) root, and its derivatives on the oxidation of LDL induced by copper ions or mediated by macrophages were studied, in order to evaluate the contribution of the different parts of the isof lavan molecule to its antioxidant activity. The peak potential (E-1/2) of the isoflavan derivatives, their radical scavenging capacity towar d 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical and their ability to c helate heavy metals were also analyzed and compared to their inhibitor y activity on LDL oxidation. In copper ion-induced LDL oxidation, glab ridin (1), 4'-O-methylglabridin (2), hispaglabridin A (3), and hispagl abridin B (4), which have two hydroxyl groups at positions 2' and 4' o r one hydroxyl at position 2' on ring B, successfully inhibited the fo rmation of conjugated dienes, thiobarbituric acid reactive substances (TBARS) and lipid peroxides, and inhibited the electrophoretic mobilit y of LDL under oxidation. Compounds 1-3 exhibited similar activities, whereas compound 4 was less active. In macrophage-mediated LDL oxidati on, the TEARS formation was also inhibited by these isoflavans (1-4) a t a similar order of activity to that obtained in copper ion-induced L DL oxidation. On the other hand, 2'-O-methylglabridin (5), a synthesiz ed compound, whose hydroxyl at 2'-position is protected and the hydrox yl at 4'-position is free, showed only minor inhibitory activity in bo th LDL oxidation systems. 2',4'-O-Dimethylglabridin (6), whose hydroxy ls at 2'- and 4'-positions are both protected was inactive. Resorcinol (7), which is identical to the phenolic B ring in glabridin, presente d low activity in these oxidation systems. The isoflavene glabrene (8) , which contains an additional double bond in the heterocyclic C ring, was the most active compound of the flavonoid derivatives tested in b oth oxidation systems. The peak potential of compounds 1-5 (300 mu M), tested at pH 7.4, was similar (425-530 mV), and that for compound 6 a nd 8 was 1078 and 80 mV, respectively. Within 30 min of incubation, co mpounds 1, 2, 3, 4, 8 scavenged 31%, 16%, 74%, 51%, 86%, respectively, of DPPH radical, whereas compounds 5 and 6, which almost did not inhi bit LDL oxidation, also failed to scavenge DPPH. None of the isoflavan derivatives nor the isoflavene compound were able to chelate iron, or copper ions. These results suggest that the antioxidant effect of gla bridin on LDL oxidation appears to reside mainly in the 2' hydroxyl, a nd that the hydrophobic moiety of the isoflavan is essential to obtain this effect. It was also shown that the position of the hydroxyl grou p at B ring significantly affected the inhibitory efficiency of the is oflavan derivatives on LDL oxidation, but did not influence their abil ity to donate an electron to DPPH or their peak potential values. (C) 1998 Elsevier Science Inc.