Putting xanthine oxidoreductase and aldehyde oxidase on the NO metabolism map: Nitrite reduction by molybdoenzymes

Nitric oxide radical (NO) is a signaling molecule involved in several physiological and pathological processes and a new nitrate-nitrite-NO pathway has emerged as a physiological alternative to the ''classic'' pathway of NO formation from L-arginine. Since the late 1990s, it has become clear that nitrite can be reduced back to NO under hypoxic/anoxic conditions and exert a significant cytoprotective action in vivo under challenging conditions. To reduce nitrite to NO, mammalian cells can use different metalloproteins that are present in cells to perform other functions, including several heme proteins and molybdoenzymes, comprising what we denominated as the ''non-dedicated nitrite reductases''. Herein, we will review the current knowledge on two of those ''non-dedicated nitrite reductases'', the molybdoenzymes xanthine oxidoreductase and aldehyde oxidase, discussing the in vitro and in vivo studies to provide the current picture of the role of these enzymes on the NO metabolism in humans. Keywords: Nitric oxide, Nitrite, Xanthine oxidoreductase, Aldehyde oxidase, Oxygen availability, Molybdenu