241 - Oxidation of Quercetin Can Markedly Enhance Its Antioxidant and Cytoprotective Properties

Abstract

A mixture of quercetin oxidation products (Qox) was generated following the direct dissolution of quercetin (Q) in alkali (pH 12). Upon ascertaining the total disappearance of Q, the fully oxidized preparation was neutralized and assayed immediately after for its reactive oxygen species (ROS)-scavenging (ORAC), Folin-Ciocalteau (FC)- and Ferric-reducing capacity. In addition, Qox was investigated for its antioxidant (DCF oxidation-inhibiting) and cytoprotective (LDH leakage-preventing) properties in HS-68/Caco2 cells exposed to indomethacin. Relative to Q, Qox conserved 93%, 77% and 77% of the ORAC, FC and FRAP, respectively. Experiments conducted in both cell types revealed that the concentration of Qox required to afford full protection against the pro-oxidant and cell-lytic effects of indomethacin was only one-tenth (500 nM) that of Q, and that such effects were equally seen in cells co-incubated with Qox (during 40 min) or only pre-incubated (during 40 min and washed-off after). HPLC-DAD/ESI-MS-MS chromatograms of Qox revealed that concomitant to the time-dependent (0-13 min) disappearance of quercetin, up to 12 major peaks emerged. Among these, only one peak (numbered 8) accounted for the antioxidant/cytoprotective effects of Qox. Peak-8, identified as a Q-o-quinone, afforded full cell-protection at 100 nM, a concentration 5- and 50-fold lower that that needed by Qox and Q, respectively. Exposure of Qox or peak-8 to supra-physiological concentrations of cysteine (500 µM) totally prevented these preparations from protecting against the oxidative and lytic cell damage. To our knowledge, these studies are first in revealing that upon full oxidation of Q, a mixture of its oxidation products largely conserves its original redox, ROS-scavenging and antioxidant properties, and that relative to the intact Q molecule, one of the here-found oxidized metabolites displays by far considerably higher antioxidant and cytoprotective properties. Fondecyt # 1150090/3150359