The nitrite reductase activity of xanthine oxidoreductase sustains cardiovascular health as mice age.

BACKGROUND: Xanthine oxidoreductase (XOR) is a multi-functional enzyme that metabolises purines generating uric acid and generates 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. This nitrite reductase activity has been proposed to play a key role in the benefits of targeting the non-canonical pathway for nitric oxide (NO) generation in the cardiovascular disease setting; an effect 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. METHODS: Xdh+/+ and Xdh+/-, Xdhfl/fland 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 were used for biochemical measurements of nitrate, nitrite, and measurement of markers of NO downstream signalling. RESULTS: Xdh+/- and HXOR KO mice expressed significantly attenuated liver and plasma nitrite reductase activity and platelet cGMP levels versus littermate controls. As mice aged Xdh-deficient mice developed increasing systolic blood pressure, left ventricular remodelling, endothelial dysfunction and increased leukocyte activation versus their age and sex matched littermate controls. Endothelial 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. Furthermore, the absence of XOR eliminates the benefits of dietary inorganic nitrate treatment. CONCLUSIONS: In summary, XOR derived NO is critical for maintaining, during ageing, vascular homeostasis under physiological conditions and is key in mediating the benefits of dietary nitrate regimes in cardiovascular pathology.