Tissue Processing of Nitrite in Hypoxia

Although nitrite ( ${\mathrm{NO}}_2^{-}$) and nitrate ( ${\mathrm{NO}}_3^{-}$) have been considered traditionally inert byproducts of nitric oxide (NO) metabolism, recent studies indicate that ${\mathrm{NO}}_2^{-}$ represents an important source of NO for processes ranging from angiogenesis through hypoxic vasodilation to ischemic organ protection. Despite intense investigation, the mechanisms through which ${\mathrm{NO}}_2^{-}$ exerts its physiological/pharmacological effects remain incompletely understood. We sought to systematically investigate the fate of ${\mathrm{NO}}_2^{-}$ in hypoxia from cellular uptake in vitro to tissue utilization in vivo using the Wistar rat as a mammalian model. We find that most tissues (except erythrocytes) produce free NO at rates that are maximal under hypoxia and that correlate robustly with each tissue's capacity for mitochondrial oxygen consumption. By comparing the kinetics of NO release before and after ferricyanide addition in tissue homogenates to mathematical models of ${\mathrm{NO}}_2^{-}$ reduction/NO scavenging, we show that the amount of nitrosylated products formed greatly exceeds what can be accounted for by NO trapping. This difference suggests that such products are formed directly from ${\mathrm{NO}}_2^{-}$, without passing through the intermediacy of free NO. Inhibitor and subcellular fractionation studies indicate that ${\mathrm{NO}}_2^{-}$ reductase activity involves multiple redundant enzymatic systems (i.e. heme, iron-sulfur cluster, and molybdenum-based reductases) distributed throughout different cellular compartments and acting in concert to elicit NO signaling. These observations hint at conserved roles for the ${\mathrm{NO}}_2^{-}$-NO pool in cellular processes such as oxygen-sensing and oxygen-dependent modulation of intermediary metabolism.