Interference of the polyphenol epicatechin with the biological chemistry of nitric oxide- and peroxynitrite-mediated reactions

Abstract

The formation of reactive nitrogen species in mammalians has both beneficial and undesirable effects. Nitric oxide (NO) production in endothelial cells leads to vascular smooth muscle relaxation, but if reactive nitrogen species are generated in high amounts by cells under inflammatory conditions they are toxic. Flavonoids like (−)-epicatechin show an inverse association of their intake with diseases thought to be associated with overproduction of reactive nitrogen species. We found that the formation of cyclic GMP in cultured porcine aortic endothelial cells was not affected by up to 1 mM (−)-epicatechin. Half maximal inhibition of interferon-γ/lipopolysaccharide induced nitrite accumulation in murine macrophages required about 0.5 mM of the flavonoid. In contrast, nitration of free tyrosine triggered by 0.1 and 1 mM authentic peroxynitrite was inhibited by (−)-epicatechin with ic 50 values of 6.6 and 28.0 μM, respectively. The presence of 15 mM sodium bicarbonate had no significant effect. Nitration of protein-bound tyrosine in phorbol 12-myristate 13-acetate treated HL-60 cells in the presence of nitrite was inhibited by (−)-epicatechin at a similar concentration range ( ic 50=10 –100 μM). Myeloperoxidase activity of phorbol 12-myristate 13-acetate stimulated HL-60 cells was inhibited by (−)-epicatechin with an ic 50 value of 77.4 μM. Epicatechin inhibited dihydrorhodamine oxidation by 50 μM authentic peroxynitrite and 1 mM 3-morpholino-sydnonimine with ic 50 values of 11.8 and 0.63 μM, respectively. Our data suggest that at up to 0.1 mM (−)-epicatechin preferentially inhibits NO-related nitration and oxidation reactions without affecting NO synthesis and cyclic GMP signaling.