over the last two decades , the biological preeminence of cellular nitric oxide ( no ) signaling pathways has been intimately linked to many processes and to its regulated enzymatic formation from l - arginine via the actions of no synthases ( nos ) and to secondary activation of soluble guanylate cyclase as a major physiological target / effector system [ 120 ] . evolutionary pressure has established a functional diversity in cellular expression of nos isoenzymes derived from three distinct genes and designated as endothelial ( e ) , neuronal ( n ) and inducible ( i ) nos .
e- and n - nos are constitutively expressed , display ca dependent activation , and rapidly produce and release no within spatially defined cellular domains .
in contrast , inos expression is intimately linked to proinflammatory processes , displays a significant latency period due to transcriptional and translational processing , and effects unregulated ca independent release of no for extended periods of time [ 7,2128 ] .
interestingly , a significant body of literature supports the contention that constitutively released no can attenuate the expression of inos in vascular smooth muscle , neutrophils , microglia , astrocytes and hepatocytes [ 2935 ] .
work from our laboratory has demonstrated significant feedback inhibition of no on constitutively derived no release [ 12,1416,3641 ] as well as inos derived no release . within the past decade ,
an important body of work has challenged the primacy of nos / l - arginine derived no in cellular signaling processes and involves the existence of chemically stable nitrite and nitrite reductase activities in these same cell / tissue types [ 4256 ] .
nitrite is a major metabolic product of no and is found in all cell and tissue types that utilize no signaling processes [ 4246,52,53,5567 ] .
accordingly , the establishment of a parallel and complementary no signaling pathway utilizing recycled nitrite chemical equivalents , independently expressed from well established nos / l - arginine signaling pathway , requires the identification and biochemical characterization of key candidate enzymes displaying significant nitrite reductase activities within meaningful biological contexts . until now , accumulated no / nitrite reductase literature has focused on xanthine oxidoreductase ( xor ) as the major candidate nitrite reductase enzyme linked to cellular no signaling events [ 49,51,52,5458,6064,6876 ] .
other candidate nitrite reductases displaying potentially important biological roles as accessory players in no signaling events include the mitochondrial enzymes aldehyde dehydrogenase , cytochrome c / cytochrome c oxidase , deoxymyoglobin   and deoxyhemoglobin .
xanthine oxidoreductase has been previously characterized as a housekeeping enzyme responsible for cellular uric acid formation via enzymatic conversion of hypoxanthine and xanthine . based on its intrinsic state - dependent biochemical properties to exist as both a dehydrogenase and an oxidase
, it became apparent to several investigators that xor possessed multi - functional enzymatic activities outside the realm of xanthine metabolism [ 5456,70,79 ] .
hallmark positive vascular effects were well established to be mediated by cellular nos / l - arginine no signaling pathways [ 711 ] .
a small but significant literature has also established a compelling functional association between administered sodium nitrite , xor activation , and pharmacologically characterized no transductive effects in positive cardiovascular function [ 62,63,75,8082 ] , enhanced pulmonary perfusion , and protection against ischemia / reperfusion injury [ 64,7275 ] and hypoxic damage [ 56,58,8385 ] and oxidative stress .
similar positive vascular and cellular effects were observed to be functionally associated with mitochondrial aldehyde dehydrogenase , cytochrome c / cytochrome c oxidase . nitric oxide derived from nos / l - arginine systems functions not only as a vasodilator but as a general antibacterial and antiviral agent and , counter - intuitively , it can down - regulate proinflammatory events [ 27,8692 ] .
accordingly , significant anti - inflammatory properties of administered sodium nitrite have been attributed to xor activation via pharmacologically characterized no transductive effects .
work from our laboratory supports the contention that constitutively derived no provides a basal or 
tonal level of chemical mediator keeps particular types of cells in a state of inhibition .
on , i.e. , respond to environmental changes , and that this low basal level of no   provides an organism with a major pathway that functions to dampen microenvironmental 
this kind of activation really represents a disinhibition process , i.e. , an overcoming of the inhibitory influence of no by changing the balance between basal no and the levels of excitatory signals . in support of the hypothesis stated above
, there is considerable evidence that constitutively derived no down - regulates the immunocyte - endothelial interaction .
the adherence of monocytes and granulocytes is reduced following the stimulation of cnos   and , in the presence of no , monocytes , granulocytes and endothelial cells become round and inactive .
these findings strongly indicate that no can diminish the adherence and level of activation of leukocytes and endothelial cells .
it also suggests these are phenomena that occur within a microenvironment given no short - half life and the strength of the effect produced by many of these cells via autocrine and / paracrine signaling .
it is now possible to add a functionally reinforcing mechanism whereby basal levels of cellular nitrite are recycled to active no equivalents via the actions of xor and accessory nitrite reductases upon physiological demand ( figure 1 ) .
it has been well established that mitochondrial respiration linked to homeostasis of intermediary energy metabolism is regulated by no signaling systems [ 12,98104 ] .
for example , pharmacological inhibition of constitutively derived no has been shown to increase oxygen consumption in many animal species [ 105109 ] .
furthermore , a novel nos isoform , mtnos , is present in mitochondria   and appears to modulate local circuit regulatory functions within electron transport complexes .
interestingly , nitrite - derived no has been shown to potently regulate respiration , reactive oxygen species , and energy metabolism in plant mitochondria [ 83,111113 ] .
the apparent redundancy of plant mitochondrial nos / l - arginine- and nitrite - derived no signaling systems [ 83,111113 ] provides a compelling platform for further investigation into reciprocal regulatory effects of mtnos and concerted nitrate reductase actions in mammalian mitochondria ( figure 1 ) [ 4246,53,85,114 ] .
a recent important publication has described local circuit nitrite / no cycling to produce biologically active no within liver mitochondria .
the investigators have demonstrated that nitrite mediates cellular signaling through its reduction to no via reactions with the mitochondrial electron carrier cytochrome c. cytochrome c - mediated nitrite reductase activity is dependent on pentacoordination of the heme iron in the protein and occurs under anoxic and in the presence of nitrite , pentacoordinate cytochrome c generates bioavailable no that is able to inhibit mitochondrial respiration .
an elegant complementary study has demonstrated in yeast that state - dependent hypoxia recruits cytochrome c oxidase as a functionally competent nitrite reductase .
the investigators have also evaluated nitrite - dependent no production by specific isoforms of cytochrome c oxidase in support of a functional role of the enzyme in hypoxic signaling events .
additionally , the study findings suggest a positive feedback mechanism for nitrite - derived mitochondrial no on selective gene expression of a cytochrome c oxidase subunit that is functionally associated with enhanced production of no in hypoxic / anoxic cells .
on a functional basis it has become clear that the basal level of no derived from cnos in concert with cellular nitrite reduction by xor within a diverse class of nitrite reductases may serve as a key regulatory mechanism underlying complex , cascading , physiological processes associated with maintaining cellular and organ viability .
further studies are required to probe selective regulatory effects of nos - derived and nitrite - derived no on gene expression of their cognate synthetic enzymes .
similar compelling studies are needed to elucidate biologically meaningful cellular coupling of cytosolic xor and mitochondrial nitrite reductases in normal and pathophysiological states ( figure 1 ) [ 6870,80,115117 ] .
finally , holistic pre - clinical and studies to evaluate conversion of dietary nitrate to recycling active cellular nitrite pools hold great promise for improving quality of life in human and animal populations .
 summaryour mini - review focuses on dual regulation of cellular nitric oxide ( no ) signaling pathways by traditionally characterized enzymatic formation from l - arginine via the actions of no synthases ( nos ) and by enzymatic reduction of available cellular nitrite pools by a diverse class of cytosolic and mitochondrial nitrite reductases . 
 nitrite is a major metabolic product of no and is found in all cell and tissue types that utilize no signaling processes . 
 xanthine oxidoreductase ( xor ) has been previously characterized as a housekeeping enzyme responsible for cellular uric acid formation via enzymatic conversion of hypoxanthine and xanthine . 
 it has become apparent that xor possesses multi - functional enzymatic activities outside the realm of xanthine metabolism and a small but significant literature also established a compelling functional association between administered sodium nitrite , xor activation , and pharmacologically characterized no transductive effects in positive cardiovascular function enhanced pulmonary perfusion , and protection against ischemia / reperfusion injury and hypoxic damage and oxidative stress . 
 similar positive vascular and cellular effects were observed to be functionally associated with mitochondrial aldehyde dehydrogenase and cytochrome c / cytochrome c oxidase . 
 the profound implications of a reciprocal regulatory mechanism responsible for cytosolic and mitochondrial no production are discussed below . 