The nitrite reductase activity of xanthine oxidoreductase sustains cardiovascular health

BackgroundXanthine oxidoreductase (XOR) is a multi-functional enzyme that metabolises purines generating uric acid and is a generator of reactive oxygen species. Both functions have been implicated in the pathogenesis of cardiovascular disease. More recently, a third function of XOR as a nitrite reductase has been identified and been shown to play a key role in the benefits of targeting the non-canonical pathway for nitric oxide (NO) generation in the cardiovascular disease setting. This effect has been specifically attributed to XOR dependent recovery of NO levels. However, whether XOR derived NO plays any role in maintaining cardiovascular homeostasis in health is unknown. To explore this, we used global and hepatocyte-specific Xdh-deleted mice to assess cardiovascular homeostasis. MethodsXdh+/+ and Xdh+/-, Xdhfl/fl and Xdhfl/flAlbCre+/-(HXOR KO) mice littermates, matched for sex and age, were used for in vivo cardiovascular phenotyping; blood pressure, cardiac function, endothelial reactivity, and leukocyte trafficking. Tissues from these mice were used for biochemical measurements of nitrate, nitrite, and markers of NO downstream signalling. ResultsXdh+/- and HXOR KO mice expressed significantly attenuated liver and plasma nitrite reductase activity and platelet cGMP levels versus littermate controls. These effects were associated with increased systolic blood pressure, left ventricular remodelling, and increased leukocyte activation. These effects were associated with and likely driven by endothelial dysfunction evident in both mouse models. This dysfunction was reflected by increased endothelial adhesion molecule expression (P-selectin), increased ischaemia-induced vasoconstriction, during vessel occlusion, and an impaired flow-mediated dilation response of the iliac artery in vivo. ConclusionsIn summary, XOR derived NO is critical for maintaining vascular homeostasis under physiological conditions and is key in mediating the benefits of dietary nitrate regimes in cardiovascular pathology