DNIC is a Structure Providing Specificity of Physiological Effects of NO.

Metabolism of nitric oxide (NO) donors: dinitrosyl iron complexes (DNIC), nitrosothiols (RSNO), and nitroprusside was studied on a chick embryo model. The obtained results give reason to assume that DNIC constituting the main pool of nitroso compounds in the vast majority of tissues are NO donors immediately interacting with the physiological target of NO, and other NO donors can perform this function after their transformation into DNIC. NO is released from DNIC not spontaneously, but under a joint influence of a factor destroying the complex and a target having chemical affinity for NO. A similar mechanism is apparently implicated in NO passage through the cell membrane.

Putative Role of Ligands of DNIC in the Physiological Action of the Complex.

In a relatively isolated system of avian embryo, the metabolism of NO, a component of the dinitrosyl iron complexes (DNIC), the main NO donor in most tissues, depends on the ligands that make up the complex. This fact corroborates the earlier hypothesis that these ligands perform a regulatory function in NO metabolism. It is also shown that nitrite injected into the embryo is not oxidized to nitrate like NO in DNIC, but is accumulated outside the amniotic sac. Normally, nitrite is present in an embryo in trace amounts. These facts suggest that NO in the embryo is transferred from the donor molecule to a target in the embryo tissues further transformed with minimum oxidation to nitrite.

Synthesis and Metabolism of Nitric Oxide (NO) in Chicken Embryos and in the Blood of Adult Chicken.

In chicken embryos, nitric oxide (NO) is accumulated in the pool of NO donors: S-nitrosothiols, nitrosyl-iron complexes, high-molecular-weight nitro-compounds. Oxidation of NO to nitrate occurs with different intensity in the embryos of different chicken breeds. In some embryos, NO donors accumulate almost without oxidation. Stable concentration of NO donors and nitrate in the blood of adult chicken is a result of dynamic equilibrium between NO synthesis and elimination (oxidation, consumption by other tissues, and excretion). As NO oxidation occurs mainly not in the blood, but in other tissues, decomposition of NO donors and NO oxidation are determined the properties of these tissues, in particular, the presence of physiological targets of NO, rather than spontaneous processes. Hence, evaluation of the intensity of NO metabolism is important for prediction of the efficiency of preparations containing NO donors and stimulators of its synthesis.

Modification of Biochemical Properties of Nitrosothiol by Fe3+ Cation: A Presumable Physiological Role.

In the presence of Fe3+ cation, S-nitrosoglutathione (GSNO) loses the potency to inhibit catalase in the system containing hemoglobin (an NO trap) with iron chelator or -SH inhibitor (a "sulfhydric poison" Hg2+). In the absence of hemoglobin, the inhibitory potency is retained in both cases. These properties are characteristic of dinitrosyl-iron complexes containing ferrous iron and thiols (DNIC/RSH). Since the potency to inhibit catalase results from the presence of -NO group, its loss in the presence of hemoglobin relates probably to transfer of this group to hemoglobin. The nitrosothiols are relatively stable compounds, so their ability to release NO under the action of iron chelators, which is characteristic of DNIC/RSH, can have important physiological implications, because the role of such chelators can be played by some endogenous agents as well. Thus, release of NO from the donor compounds can be controlled and regulated. Probably, the agents such as nitrosothiol+Fe3+ are the major constituents in the pool of nitroso compounds.

[Possibilities for the diagnosis of inflammatory reaction in ischemic stroke]. / Vozmozhnost' diagnostiki vospalitel'noi reaktsii pri ishemicheskom insul'te.

AIM: To study diagnostic possibilities for determining the content of nitrite and N-nitroso compounds (NO2-+RNNO) in blood plasma and cerebrospinal fluid (CSF) in patients with acute ischemic stroke (IS). MATERIAL AND METHODS: Twenty-four patients with IS were examined. The content of NO oxidation products was determined in venous blood and CSF by using an enzyme sensor based on the unique property of nitrite (NO2-), N-nitroso compounds (RNNO), S-nitrosothiols and dinitrosyl iron complexes to inhibit the enzyme catalase in the presence of halide ions. The study was conducted on the 1st day of IS. RESULTS AND CONCLUSION: CSF in patients with IS contained nitrite and N-nitroso compounds (NO2-+RNNO) in concentrations ranging from 0.4 to 2.0 µm. The relationship between the size of IS and the concentration of NO2-+RNNO in CSF was shown. It was 1.01±0.13 µm in patients with medium IS and 0.71±0.07 µm in patients with small IS (U-criterion 16.5; p<0.05). There was no correlation between the severity of neurological deficit at the time of hospitalization and discharge from the hospital and the content of NO2-+RNNO in CSF (r=0.134; p>0.5; r=0.155; p>0.5, respectively). Plasma NO2-+RNNO levels were not associated with the presence and size of IS though they were elevated in patients with inflammatory complications. In conclusion, NO2-+RNNO can be considered as a marker of inflammation in patients with IS. Their presence in CSF reflects the extent of brain damage, but not the presence of concomitant inflammatory diseases.

Mechanisms of Specific Embryonic Effects of Nitrogen Oxide.

The study of NO metabolism in chicken embryos showed that the intensity of oxidation of both endogenous and exogenous for the embryo NO donors to nitrate is determined by the presence or state of NO targets, rather than donor concentration. The mechanism of this oxidation and its physiological role are discussed. It was also shown that oxidation product nitrate is actively eliminated from the amnionic sac.

Selectivity in Physiological Action of Nitric Oxide: A Hypothetical Mechanism.

The study showed that dinitrosyl iron complex (NO)2Fe(RS)2 containing the thiolate ligands, which is the basic physiological donor of NO, can transfer NO to other molecule only at the moment of rearrangement. This rearrangement can occur during interaction of the complex with more effective iron chelators than the thiolate ligands. In the absence of NO trap, a new complex is formed with a new ligand. NO transfer to a trap can also occur under the action of the agents such as mercury salts or ROS, which interact with the thiolate ligands. Probably, the ligands in the dinitrosyl iron complexes are the structures responsible for interaction of these complexes with physiological targets and for specificity and effectiveness of this interaction.

Enzymatic Sensor Detects Some Forms of Nitric Oxide Donors Undetectable by Other Methods in Living Tissues.

Studies with the use of highly sensitive enzymatic sensor have shown the presence of various forms of nitrosyl iron complexes, including those undetectable by other methods, in living tissues. All these complexes are long-living compounds and constitute the major part of nitroso compounds in the blood, muscles, liquor, and amniotic fluid.

Effect of Green Light on Nitric Oxide Metabolism in Chick Embryos. A Possible Physiological Role.

The exposure to green light, which serves as a well-known activating factor for myogenesis during incubation of chicken eggs, contributes to intensification of embryonic metabolism of NO. A metabolic product, nitrate, is mainly accumulated in the muscles. These data suggest that light induces a NO-dependent activation of the factor, which intensifies muscle tissue development.

Specific role of nitric oxide (NO) in avian embryonic myogenesis.

NO plays a specific role in avian embryogenesis stimulating the development of muscle tissue. The main consumer of NO synthesized at the initial stage of avian embryogenesis is presumably a factor stimulating myogenesis.

Possible mechanism of generation of nitrite and non-thiolate nitroso compounds in blood plasma during inflammatory processes.

Generation of nitrite (NO2¯) and non-thiolate nitroso compounds in human blood during leukocyte activation mainly occurred due to destruction of NO donors in the plasma, but not due to intensification of NO synthesis. We proposed a mechanism of production of nitrite and non-thiolate nitroso compounds in the blood during inflammation.

Can summary nitrite+nitrate content serve as an indicator of NO synthesis intensity in body tissues?

Studies with the use of a highly specific enzymatic sensor demonstrated that, contrary to the common opinion, normally nitrate is in fact not present in the most important physiological fluids. NO metabolites in the amniotic fluid and semen are mainly presented by NO donor compounds. Therefore, the intensity of NO synthesis can be evaluated by the total content of all its metabolites, but not by the widely used summary nitrite+nitrate content.

Oxidation of thiamine on reaction with nitrogen dioxide generated by ferric myoglobin and hemoglobin in the presence of nitrite and hydrogen peroxide.

It is shown that nitrogen dioxide oxidizes thiamine to thiamine disulfide, thiochrome, and oxodihydrothiochrome (ODTch). The latter is formed during oxidation of thiochrome by nitrogen dioxide. Nitrogen dioxide was produced by incubation of nitrite with horse ferric myoglobin and human hemoglobin in the presence of hydrogen peroxide. After addition of tyrosine or phenol to aqueous solutions containing oxoferryl forms of the hemoproteins, thiamine, and nitrite, the yield of thiochrome greatly increased, whereas the yield of ODTch decreased. In the presence of high concentrations of tyrosine or phenol compounds ODTch was not formed at all. The neutral form of thiamine with the closed thiazole cycle and minor tricyclic form of thiamine do not enter the heme pocket of the protein and do not interact with the oxoferryl heme complex Fe(IV=O) or porphyrin radical. The tricyclic form of thiamine is oxidized to thiochrome by tyrosyl radicals located on the surface of the hemoprotein. The thiol form of thiamine is oxidized to thiamine disulfide by both hemoprotein tyrosyl radicals and oxoferryl heme complexes. Nitrite and also tyrosine, tyramine, and phenol readily penetrate into the heme pocket of the protein and reduce the oxyferryl complex to ferric cation. These reactions yield nitrogen dioxide as well as tyrosyl and phenoxyl radicals of tyrosine molecules and phenol compounds, respectively. Tyrosyl and phenoxyl radicals of low molecular weight compounds oxidize thiamine only to thiochrome and thiamine disulfide. The effect of oxoferryl forms of myoglobin and hemoglobin, nitrogen dioxide, and phenol on thiamine oxidative transformation as well as antioxidant properties of the hydrophobic thiamine metabolites thiochrome and ODTch are discussed.

Mechanism of inhibition of catalase by nitro and nitroso compounds.

Dinitrosyl iron complexes (DNIC) with thiolate ligands and S-nitrosothiols, which are NO and NO+ donors, share the earlier demonstrated ability of nitrite for inhibition of catalase. The efficiency of inhibition sharply (by several orders in concentration of these agents) increases in the presence of chloride, bromide, and thiocyanate. The nitro compounds tested--nitroarginine, nitroglycerol, nitrophenol, and furazolidone--gained the same inhibition ability after incubation with ferrous ions and thiols. This is probably the result of their transformation into DNIC. None of these substances lost the inhibitory effect in the presence of the well known NO scavenger oxyhemoglobin. This fact suggests that NO+ ions rather than neutral NO molecules are responsible for the enzyme inactivation due to nitrosation of its structures. The enhancement of catalase inhibition in the presence of halide ions and thiocyanate might be caused by nitrosyl halide formation. The latter protected nitrosonium ions against hydrolysis, thereby ensuring their transfer to the targets in enzyme molecules. The addition of oxyhemoglobin plus iron chelator o-phenanthroline destroying DNIC sharply attenuated the inhibitory effect of DNIC on catalase. o-Phenanthroline added alone did not influence this effect. Oxyhemoglobin is suggested to scavenge nitrosonium ions released from decomposing DNIC, thereby preventing catalase nitrosation. The mixture of oxyhemoglobin and o-phenanthroline did not affect the inhibitory action of nitrite or S-nitrosothiols on catalase.

Proposed mechanism of nitrite-induced methemoglobinemia.

A scheme of development of nitrite-induced oxyhemoglobin oxidation in erythrocytes based on the analysis of experimental data is proposed. It was found that, contrary to widespread opinion, direct oxidative-reductive interaction between hemoglobin and nitrite is absent or negligible under physiological conditions. The driving stage of this process is methemoglobin-catalyzed peroxidase oxidation of nitrite. The product of the oxidation (presumably NO2*) directly oxidizes oxyhemoglobin to methemoglobin-peroxide complex without hydrogen peroxide release into the environment. The oxidant itself is reduced to nitrite or oxidized to nitrate as a result of interaction with another NO2* molecule. Thus, the stoichiometry of the process depends on the ratio of rates of these two reactions. Substances that are able to compete with nitrite for peroxidase and therefore to prevent the nitrite oxidation effectively protect hemoglobin from oxidation. Catalase is not able to destroy methemoglobin-peroxide complexes, but it can prevent their production in the course of interaction of methemoglobin and free peroxide by destroying the latter.

Nitrite-catalase interaction as an important element of nitrite toxicity.

It was established that nitrite in the presence of chloride, bromide, and thiocyanate decreases the rate of hydrogen peroxide decomposition by catalase. The decrease was recorded by the permanganatometric method and by a method of dynamic calorimetry. Nitrite was not destroyed in the course of the reaction and the total value of heat produced in the process was not changed by its presence. These facts suggest that nitrite induces inhibition of catalase with no change in the essence of the enzymatic process. Even micromolar nitrite concentrations induced a considerable decrease in catalase activity. However, in the absence of chloride, bromide, and thiocyanate inhibition was not observed. In contrast, fluoride protected catalase from nitrite inhibition in the presence of the above-mentioned halides and pseudohalide. As hydrogen peroxide is a necessary factor for triggering a number of important toxic effects of nitrite, the latter increases its toxicity by inhibiting catalase. This was shown by the example of nitrite-induced hemoglobin oxidation. The naturally existing gradient of chloride and other anion concentrations between intra- and extracellular media appears to be the most important mechanism of cell protection from inhibition of intracellular catalase by nitrite. Possible mechanisms of this inhibition are discussed.