Antioxidant and Prooxidant Actions of Prenylated and Nonprenylated Chalcones and Flavanones in Vitro

Prenylated flavonoids found in hops and beer, i.e., prenylchalcones and prenylflavanones, were examined for their ability to inhibit in vitro oxidation of human low-density lipoprotein (LDL). The oxidation of LDL was assessed by the formation of conjugated dienes and thiobarbituric acid-reactive substances (TBARS)and the loss of tryptophan fluorescence. At concentrations of 5 and 25 íM, all of the prenylchalcones tested inhibited the oxidation of LDL (50 íg protein/ml)induced by 2 íM copper sulfate. The prenylflavanones showed less antioxidant activity than the prenylchalcones, both at 5 and 25 íM. At 25 íM, the nonprenylated chalcone, chalconaringenin (CN), and the nonprenylated flavanone, naringenin (NG), exerted prooxidant effects on LDL oxidation, based on TBARS formation. Xanthohumol (XN), the major prenylchalcone in hops and beer, showed high antioxidant activity in inhibiting LDL oxidation, higher than R-tocopherol and the isoflavone genistein but lower than the flavonol quercetin. When combined, XN and R-tocopherol completely inhibited copper-mediated LDL oxidation. These findings suggest that prenylchalcones and prenylflavanones found in hops and beer protect human LDL from oxidation and that prenylation antagonizes the prooxidant effects of the chalcone, CN, and the flavanone, NG.