(-)-Epicatechin elevates nitric oxide in endothelial cells via inhibition of NADPH oxidase

Dietary (-)-epicatechin is known to improve bioactivity of NO in arterial endothelium of humans, but the mode of action is unclear. We used the fluorophore 4,5-diaminofluorescein diacetate to visualize the NO level in living human umbilical vein endothelial cells (HUVEC). Untreated cells showed only a weak signal, whereas pretreatment with (-)-epicatechin (10 μM) or apocynin (100 μM) elevated the NO level. The effects were more pronounced when the cells were treated with angiotensin II with or without preloading of the cells with NO via PAPA-NONOate. While (-)-epicatechin scavenged O2·-, its O-methylated metabolites prevented O2·- generation through inhibition of endothelial NADPH oxidase activity, even more strongly than apocynin. From the effect of 3,5-dinitrocatechol, an inhibitor of catechol-O-methyltransferase (COMT), on HUVEC it is concluded that (-)-epicatechin serves as 'prodrug' for conversion to apocynin-like NADPH oxidase inhibitors. These data indicate an NO-preserving effect of (-)-epicatechin via suppression of O2·--mediated loss of NO.