Antioxidant and prooxidant actions of prenylated and nonprenylated chalcones and flavanones in vitro

Prenylated flavonoids found in hops and beer, i.e., prenylchalcones and pre nylflavanones, were examined for their ability to inhibit in vitro oxidatio n of human low-density lipoprotein (LDL). The oxidation of LDL was assessed by the formation of conjugated dienes and thiobarbituric acid-reactive sub stances (TBARS) and the loss of tryptophan fluorescence. At concentrations of 5 and 25 mu M, all of the prenylchalcones tested inhibited the oxidation of LDL (50 mu g protein/ml) induced by 2 mu M copper sulfate. The prenylfl avanones showed less antioxidant activity than the prenylchalcones, both at 5 and 25 mu M. At 25 mu M, the nonprenylated chalcone, chalconaringenin (C N), and the nonprenylated flavanone, naringenin (NG), exerted prooxidant ef fects on LDL oxidation, based on TEARS formation. Xanthohumol (XN), the maj or prenylchalcone in hops and beer, showed high antioxidant activity in inh ibiting LDL oxidation, higher than alpha-tocopherol and the isoflavone geni stein but lower than the flavonol quercetin. When combined, XN and alpha-to copherol completely inhibited copper-mediated LDL oxidation. These findings suggest that prenylchalcones and prenylflavanones found in hops and beer p rotect human LDL from oxidation and that prenylation antagonizes the prooxi dant effects of the chalcone, CN, and the flavanone, NG.