Nitric oxide initiates iron binding to neocuproine.

It was demonstrated that two species of paramagnetic dinitrosyl iron complex (DNIC) with neocuproine form under the following conditions: in addition of neocuproine to a solution of DNIC with phosphate; in gaseous NO treatment of a mixture of Fe(2+) + neocuproine aqueous solutions at pH 6.5-8; and in addition of Fe(2+)--citrate complex + neocuproine to a S-nitrosocysteine (cys-NO) solution. The first form of DNIC with neocuproine is characterized by an EPR signal with g-factor values of 2.087, 2.055, and 2.025, when it is recorded at 77K. At room temperature, the complex displays a symmetric singlet at g = 2.05. The second form of DNIC with neocuproine gives an EPR signal with g-factor values of 2.042, 2.02, and 2.003, which can be recorded at a low temperature only.The revealed complexes are close to DNIC with cysteine in their stability. The ability of neocuproine to bind Fe(2+) in the presence of NO with formation of paramagnetic DNICs warrants critical reevaluation of the statement that neocuproine is only able to bind Cu(+) ions. It was suggested that the observed affinity of neocuproine to iron was due to transition of Fe(2+) in DNIC with neocuproine to Fe(+). In experiments on cys-NO, it was shown that the stabilizing effect of neocuproine on this compound could be due to neocuproine binding to the iron catalyzing decomposition of cys-NO.     

Nitric oxide mediates iron release from ferritin

Nitric oxide (NO) synthesis by cytotoxic activated macrophages has been postulated to result in a progressive loss of iron from tumor target cells as well as inhibition of mitochondrial respiration and DNA synthesis. In the present study, the addition of an NO-generating agent, sodium nitroprusside, to the iron storage protein ferritin resulted in the release of iron from ferritin and the released iron-catalyzed lipid peroxidation. Hemoglobin, which binds NO, and superoxide anion, which reacts with NO, inhibited nitroprusside-dependent iron release from ferritin, thereby providing evidence that NO can mobilize iron from ferritin. These results suggest that NO generation in vivo could lead to the mobilization of iron from ferritin disrupting intracellular iron homeostasis and increasing the level of reactive oxygen species.     

Nitric oxide and iron: effect of iron overload on nitric oxide production in endotoxemia

The amount of iron within the cell is carefully regulated in order to provide an adequate level of micronutrient while preventing its accumulation and toxicity. Iron excess is believed to generate oxidative stress, understood as an increase in the steady-state concentration of oxygen radical intermediates. Nitric oxide (NO) is an inorganic free-radical gaseous molecule which has been shown over the last decade to play an unprecedented variety of roles in biological systems. The effect of nitrogen reactive species may explain the iron sequestration pattern that characterizes macrophages under inflammatory conditions. From a patho-physiological viewpoint, further studies are required to assess the usefulness of this mechanism to minimize formation and release of free radicals in diseased tissues. However, contrary to the deleterious effects of the reactive nitrogen oxide species formed from either NO/O(2) and NO/O(2)(-), it has been pointed out that NO shows antioxidant properties. A number of studies have described the complex relationships between iron and NO, but controversy remains as to the influence and significance of iron on inflammatory NO production. To explore the initial steps of the effects triggered by LPS administration in the presence of excess iron, male Wistar rats were treated with: lipopolysaccharide from Escherichia coli (serotype 0127:B8) (LPS); iron-dextran; or iron-dextran plus LPS and liver samples were taken after 6 h. EPR spectra of NO-Hb in the venous blood were determined at 77 K. Iron-dextran administered to rats intraperitoneally resulted predominantly in iron uptake by the liver Kupffer cells and led to an increased NO level in blood in the presence of LPS. Further studies will be required to assess the complex role of the Kupffer cells on iNOS induction and NO production.     

Nitric oxide can modify amino acid residues in proteins.

Nitric oxide derived from sodium nitroprusside binds to the heme moiety of hemoglobin and also modifies some functional groups in the protein. As hemoglobin concentration is increased, globin modification is decreased presumably due to formation of the NO complex with heme. The SH groups of hemoglobin are probably not involved in the formation of the stable product formed by NO. In the presence of inositol hexaphosphate, which binds preferentially in the cleft between the two beta-chains of hemoglobin, formation of one modified derivative was selectively reduced. With hemoglobin specifically blocked on its N-terminal residues, globin modification was also significantly reduced. Carbonic anhydrase, which is blocked at its N-terminus, was also refractory to modification. The results suggest that the N-terminal groups of some proteins can be modified by nitric oxide, perhaps by deamination.     

Nitric oxide and changes of iron metabolism in exercise

Accumulated data imply that exercise itself might not lead to a true iron deficiency or 'sport anaemia' in a healthy athlete who has adequate iron intake. The higher prevalence of iron deficiency anaemia in younger female athletes might be not due to exercise itself, but probably results from dietary choices, inadequate iron intake and menstruation. These factors can also induce iron deficiency or anaemia in the general population. However, exercise does affect iron metabolism, leading to low or sub-optimal iron status. The underlying mechanism is unknown. In this review, recent advances in the study of the effect of exercise on iron metabolism and nitric oxide, and the relationship between nitric oxide and iron status in exercise are discussed. A hypothesis that increased production of nitric oxide might contribute to sub-optimal iron status in exercise is proposed.     

Nitric oxide improves internal iron availability in plants

Iron deficiency impairs chlorophyll biosynthesis and chloroplast development. In leaves, most of the iron must cross several biological membranes to reach the chloroplast. The components involved in the complex internal iron transport are largely unknown. Nitric oxide (NO), a bioactive free radical, can react with transition metals to form metal-nitrosyl complexes. Sodium nitroprusside, an NO donor, completely prevented leaf interveinal chlorosis in maize (Zea mays) plants growing with an iron concentration as low as 10 microM Fe-EDTA in the nutrient solution. S-Nitroso-N-acetylpenicillamine, another NO donor, as well as gaseous NO supply in a translucent chamber were also able to revert the iron deficiency symptoms. A specific NO scavenger, 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, blocked the effect of the NO donors. The effect of NO treatment on the photosynthetic apparatus of iron-deficient plants was also studied. Electron micrographs of mesophyll cells from iron-deficient maize plants revealed plastids with few photosynthetic lamellae and rudimentary grana. In contrast, in NO-treated maize plants, mesophyll chloroplast appeared completely developed. NO treatment did not increase iron content in plant organs, when expressed in a fresh matter basis, suggesting that root iron uptake was not enhanced. NO scavengers 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and methylene blue promoted interveinal chlorosis in iron-replete maize plants (growing in 250 microM Fe-EDTA). Even though results support a role for endogenous NO in iron nutrition, experiments did not establish an essential role. NO was also able to revert the chlorotic phenotype of the iron-inefficient maize mutants yellow stripe1 and yellow stripe3, both impaired in the iron uptake mechanisms. All together, these results support a biological action of NO on the availability and/or delivery of metabolically active iron within the plant.     

Nitric oxide and atherosclerosis.

Endothelial dysfunction has been shown in a wide range of vascular disorders including atherosclerosis and related diseases. Here, we examine and address the complex relationship among nitric oxide (NO)-mediated pathways and atherogenesis. In view of the numerous pathophysiological actions of NO, abnormalities could potentially occur at many sites: (a) impairment of membrane receptors in the arterial wall that interact with agonists or physiological stimuli capable of generating NO; (b) reduced concentrations or impaired utilization of l-arginine; (c) reduction in concentration or activity both of inducible and endothelial NO synthase; (d) impaired release of NO from the atherosclerotic damaged endothelium; (e) impaired NO diffusion from endothelium to vascular smooth muscle cells followed by decreased sensitivity to its vasodilator action; (f) local enhanced degradation of NO by increased generation of free radicals and/or oxidation-sensitive mechanisms; and (g) impaired interaction of NO with guanylate cyclase and consequent limitation of cyclic GMP production. Therefore, one target for new drugs should be the preservation or restoration of NO-mediated signaling pathways in arteries. Such novel therapeutic strategies may include administration of l-arginine/antioxidants and gene-transfer approaches.     

Nitric oxide and neurons.

In the past 2 years powerful evidence has emerged to suggest that nitric oxide functions as a neurotransmitter in both the central and peripheral nervous systems. Recent evidence suggests that it may play a role in mediating forms of synaptic plasticity such as long-term potentiation in the CA1 region of the hippocampus, and long-term depression in the cerebellum. Abnormal secretion of nitric oxide may be responsible for the neurotoxicity mediated by NMDA receptors that results in the pathophysiology of strokes and neurodegenerative diseases.     

Nitric oxide does not promote iron release from ferritin

It has been previously reported that iron release from ferritin could be promoted by nitric oxide (NO) generated from sodium nitroprusside. It was thus proposed that some of the toxic effects of NO could be related to its ability to increase intracellular free iron concentrations and generate an oxidative stress. On the contrary, the iron exchange experiments reported here show that NO from S-nitrosothiols is unable to promote iron release from ferritin. The discrepancy may be explained by the disregarded ability of ferrozine, the ferrous trap used in the previous report, to mobilize iron both from ferritin and from sodium nitroprusside spontaneously.     

Nitric oxide and cell death.

Nitric oxide (NO) has several essential roles in mammals, but unregulated NO production can cause cell death through oxidative stress, disrupted energy metabolism, DNA damage, activation of poly(ADP-ribose) polymerase, or dysregulation of cytosolic calcium. Such disturbances can lead to either apoptotic or necrotic cell death, depending on the severity and context of the damage. Here I review the mechanisms by which NO kills cells and discuss how NO thereby contributes to ischaemia-reperfusion injury and neurodegeneration.     

Nitric oxide metabolism and breakdown.

The steady-state concentration and thus the biological effects of NO are critically determined not only by its rate of formation, but also by its rate of decomposition. Bioreactivity of NO at physiological concentrations may differ substantially from that suggested by in vitro experiments. The charge neutrality and its high diffusion capacity are hallmarks that characterize NO bioactivity. Reactive oxygen derived species are major determinants of NO breakdown. Biotransformation of NO and its related N-oxides occurs via different metabolic routes within the body. S-Nitrosothiols formed upon reaction of NO with redox-activated thiols represent an active storage pool for NO. The major oxidative metabolites represent nitrite and nitrate, the ratio of both is determined by the microenvironmental redox conditions. In humans, circulating nitrite represents an attractive estimate of regional endothelial NO formation, whereas nitrate, with some caution, appears useful in estimating overall nitrogen/NO turnover. Within the near future, more specific biochemical tools for diagnosis of reduced NO bioactivity will become available. Increasing knowledge on the complex metabolism of NO in vivo will lead to the development of new therapeutic strategies to enhance bioactivity of NO via modulation of its metabolism.     

Nitric oxide and thiol groups.

S-Nitroso(sy)lation reactions have recently been appreciated to regulate protein function and mediate 'nitrosative' stress. S-Nitrosothiols (SNOs) have been identified in a variety of tissues, and represent a novel class of signaling molecules which may act independently of homolytic cleavage to NO - and, indeed, in a stereoselective fashion - or be metabolized to other bioactive nitrogen oxides. It is now appreciated that sulfur-NO interactions have critical physiological relevance to mammalian neurotransmission, ion channel function, intracellular signaling and antimicrobial defense. These reactions are promising targets for the development of new medical therapies.     

Nitric oxide and lipid peroxidation.

Nitric oxide can both promote and inhibit lipid peroxidation. By itself, nitric oxide acts as a potent inhibitor of the lipid peroxidation chain reaction by scavenging propagatory lipid peroxyl radicals. In addition, nitric oxide can also inhibit many potential initiators of lipid peroxidation, such as peroxidase enzymes. However, in the presence of superoxide, nitric oxide forms peroxynitrite, a powerful oxidant capable of initiating lipid peroxidation and oxidizing lipid soluble antioxidants. The role of nitric oxide in vascular pathology is discussed.     

Nitric oxide and mitochondrial respiration.

Nitric oxide (NO) and its derivative peroxynitrite (ONOO-) inhibit mitochondrial respiration by distinct mechanisms. Low (nanomolar) concentrations of NO specifically inhibit cytochrome oxidase in competition with oxygen, and this inhibition is fully reversible when NO is removed. Higher concentrations of NO can inhibit the other respiratory chain complexes, probably by nitrosylating or oxidising protein thiols and removing iron from the iron-sulphur centres. Peroxynitrite causes irreversible inhibition of mitochondrial respiration and damage to a variety of mitochondrial components via oxidising reactions. Thus peroxynitrite inhibits or damages mitochondrial complexes I, II, IV and V, aconitase, creatine kinase, the mitochondrial membrane, mitochondrial DNA, superoxide dismutase, and induces mitochondrial swelling, depolarisation, calcium release and permeability transition. The NO inhibition of cytochrome oxidase may be involved in the physiological regulation of respiration rate, as indicated by the finding that isolated cells producing NO can regulate cellular respiration by this means, and the finding that inhibition of NO synthase in vivo causes a stimulation of tissue and whole body oxygen consumption. The recent finding that mitochondria may contain a NO synthase and can produce significant amounts of NO to regulate their own respiration also suggests this regulation may be important for physiological regulation of energy metabolism. However, definitive evidence that NO regulation of mitochondrial respiration occurs in vivo is still missing, and interpretation is complicated by the fact that NO appears to affect tissue respiration by cGMP-dependent mechanisms. The NO inhibition of cytochrome oxidase may also be involved in the cytotoxicity of NO, and may cause increased oxygen radical production by mitochondria, which may in turn lead to the generation of peroxynitrite. Mitochondrial damage by peroxynitrite may mediate the cytotoxicity of NO, and may be involved in a variety of pathologies.     

Nitric oxide and iron in plants: an emerging and converging story

Although iron is plentiful, it exists primarily in its insoluble form and is therefore not freely available to plants. Thus, complex strategies involving chelators, production of reductive agents, reductase activities, proton-mediated processes, specialized storage proteins, and others, act in concert to mobilize iron from the environment into the plant and within the plant. Because of its fundamental role in plant productivity and ultimately in human nutrition, several unsolved and central questions concerning sensing, trafficking, homeostasis and delivery of iron in plants are currently a matter of intense debate. Here, we discuss some recent studies focusing on iron nutrition in plants as well as evidence from iron homeostasis in animals and propose a new scenario involving the formation of nitric oxide and iron-nitrosyl complexes as part of the dynamic network that governs plant iron homeostasis.     

Nitric oxide and carotid body chemoreception.

Nitric oxide (NO) has been proposed as an inhibitory modulator of carotid body chemosensory responses to hypoxia. It is believed that NO modulates carotid chemoreception by several mechanisms, which include the control of carotid body vascular tone and oxygen delivery and reduction of the excitability of chemoreceptor cells and petrosal sensory neurons. In addition to the well-known inhibitory effect, we found that NO has a dual (dose-dependent) effect on carotid chemoreception depending on the oxygen pressure level. During hypoxia, NO is primarily an inhibitory modulator of carotid chemoreception, while in normoxia NO increased the chemosensory activity. This excitatory effect produced by NO is likely mediated by an impairment of mitochondrial electron transport and oxidative phosphorylation, which increases the chemosensory activity. The recent findings that mitochondria contain an isoform of NO synthase, which produces significant amounts of NO for regulating their own respiration, suggest that NO may be important for the regulation of mitochondrial energy metabolism and oxygen sensing in the CB.     

Nitric oxide

Nitric oxide (NO)--a 1:1 combination of the two most abundant gaseous elements--is a biological mediator of complexity, subtlety and protean effects. The history of its discovery as a mediator is fascinating, and its role in mammalian biology and medicine is proving to be of fundamental importance.