V. Yu. Titov

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.

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.

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.

Detection of nitrite and nitrosocompounds in chemical systems and biological liquids by the calorimetric method

The capacity of nitrite, S-nitrosothiols (RS-NO), dinitrosyl iron complexes (DNICs) with thiolcontaining ligands, and nitrosoamines to inhibit catalase has been used for the selective determination of these compounds in purely chemical systems and biological liquids: cow milk and colostrum. The limiting sensitivity of the method is 50 nM. A comparison of the results of the determinations of RS-NO, DNIC, and nitrite by the catalase method and the Griess method conventionally used for nitrite detection showed that, firstly, Griess reagents decompose DNIC and RS-NO to form nitrite. Therefore, the Griess method cannot be used for nitrite determination in solutions of these substances. Secondly, Griess reagents interact with complexes of mercury ions with RS-NO, inducing the release of nitrosonium ions from the complex followed by the hydrolysis of nitrosonium to nitrite. Thus, the proposition about the spontaneous decay of the complexes of mercury ions with RS-NO is incorrect. Keeping in mind a high sensitivity of the method, the use of catalase as an enzyme detector of nitrosocompounds allows one to detect these compounds in neutral medium without prior purification of the object, thereby preventing artificial effects due to noncontrolled modifications of the compounds under study.

Features of the metabolism of nitric oxide in normal state and inflammation

Brief analysis of the metabolism of nitric oxide in living cells in normal state and pathology and also the analysis of the causes that hampered the progress of these studies were carried out. It was established that most of physiological fluids, including blood, normally contain nitrite and non-thiolate nitroso compounds in concentration less than 100 nM. Literature data from different researchers on the normal range of nitrite concentration in plasma of healthy people from several hundreds of nM to several μM is probably the result of low selectivity of the methods used. But nitrite and non-thiolate nitroso compounds concentration in blood is dramatically increased in case of inflammatory diseases. It is proposed that the main mechanism for the production of these substances in blood is the nitrosyl iron complexes transformation by active oxygen species but not the activation of NO production as it was considered previously.

Mechanism of hydrogen peroxide induced oxidation of oxyhemoglobin

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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.

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.

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 intensifi cation of NO synthesis. We proposed a mechanism of production of nitrite and non-thiolate nitroso compounds in the blood during infl ammation.