Inhibition of peroxynitrite-mediated LDL oxidation by prenylated flavonoids: the alpha,beta-unsaturated keto functionality of 2'-hydroxychalcones as a novel antioxidant pharmacophore.

Prenylated 2'-hydroxychalcones and flavanones from the inflorescences of the female hop plant (Humulus lupulus) were shown to inhibit peroxynitrite-mediated oxidation of low-density lipoproteins (LDL) at low micromolar concentrations. LDL oxidation was induced by the peroxynitrite generator, 3-morpholinosydnonimine (SIN-1), and measured by the formation of conjugated dienes and thiobarbituric reactive substances. Human intake of prenylated chalcones and flavanones is mainly through beer, which contains up to 4 mg/L of these polyphenols. The two main oxidation products obtained by SIN-1 and peroxynitrite treatment of xanthohumol (XN), the principal prenylflavonoid of hops, were the aurone, auroxanthohumol (AUXN), and an endoperoxy derivative of XN, named endoperoxyxanthohumol (EPOX). In addition, the reaction produced smaller amounts of the nitro and nitroso derivatives of XN and EPOX. The formation of these nitrated products was enhanced in the presence of sodium bicarbonate (25 mM). SIN-1-induced formation of AUXN is considered to be a superoxide-mediated reaction, while the structure of EPOX points to a two electron oxidation reaction involving a Michael type addition with peroxynitrite as the nucleophile, followed by cyclization yielding a (1,2)-dioxepin-5-one ring structure. The flavanone isomer of XN, isoxanthohumol (IsoXN), unexpectedly showed a slight prooxidant effect instead of an inhibitory effect on LDL oxidation. Except for the formation of minor nitrated products, IsoXN remained largely unmodified upon treatment with SIN-1/peroxynitrite. Taken together, our results suggest that the alpha,beta-unsaturated keto functionality of chalcones is most reactive toward superoxide and peroxynitrite anions.