Phenolic Antioxidants and the Protection of Low Density Lipoprotein from Peroxynitrite-Mediated Oxidations at Physiologic CO2

Dietary phenolic antioxidants have been shown to prevent LDL modifications mediated by several physiologic oxidants including peroxynitrite. However, more recent data demonstrated that CO2 affected the fate of peroxynitrite in biological fluids and significantly reduced peroxynitrite scavenging by polyphenols, raising doubts concerning their antioxidant activity. We found that the oxidation of LDL lipids mediated by peroxynitrite decreased in the presence of bicarbonate, while Trp oxidation and 3-nitroTyr formation increased, suggesting a redirection of peroxynitrite reactivity toward the protein moiety. We therefore evaluated the protective activity of some phenolic antioxidants (quercetin, oleuropein, resveratrol, (+)-catechin, (−)-epicatechin, tyrosol, α- and γ-tocopherol, ascorbate) on peroxynitrite-mediated oxidation of LDL aromatic residues. Some of these phenols protected LDL Trp from oxidation better than ascorbate or α-tocopherol, although protection at 100 μM did not exceed 30−40%. However, the same phenolic antioxidants were more active in inhibiting 3-nitroTyr formation and those with a catechin structure provided significant protection (IC50% 40−50 μM). Red wine, a polyphenol-rich beverage, showed a protective effect comparable to that of the most active phenolic antioxidants. Direct EPR studies showed that bicarbonate significantly increased the peroxynitrite-dependent formation of O-semiquinone radicals in red wine, supporting the hypothesis that polyphenols are efficient scavengers of radicals formed by peroxynitrite/CO2. Ascorbate was a poor inhibitor of peroxynitrite/CO2-induced LDL tyrosine nitration, but the simultaneous addition to the most active polyphenols halved their IC50%. In conclusion, although cooperation with other antioxidants can further decrease the IC50% of polyphenolics, as demonstrated for ascorbate, their antioxidant activity appears to occur at concentrations at least 1 order of magnitude higher than their bioavailability. Keywords: Lipoprotein; 3-nitrotyrosine; tryptophan oxidation; peroxynitrite; phenolic antioxidants; bicarbonate