Involvement of superoxide anion in the reaction mechanism of haemoglobin oxidation by nitrite

The Role of the Natural Antioxidants in the Oxihaemoglobin Oxidation and the Diminution of Nitrite Concentration

The paper includes the study of the inhibition of the process of methemoglobinization at oxidation with nitrites in the presence of sodium dihydroxyfumarate (DFH3Na) and resveratrol (3,4’,5-trihydroxystilben). The experimental study was carried out by treatment of the erythrocyte mass by hemolysis and exposure to nitrite. The kinetic investigations were carried out in following conditions: [Resv] = (5.10-5 – 1.10-3) mol/l, [DFH3Na] = 1.10-6 – 5.10-6 mol/l; [HbO2]=1.10-3 mol/l; pH 7,1; t = 370C. The rate of transformation of HbO2 in the presence of resveratrol and DFH3Na was calculated from kinetic curves of consumption of the substrate and formation of MetHb obtained pectrophotometrically (λmax= 540 nm for HbO2 and λmax=630 for MetHb). It has been found out that the introduction of resveratrol and DFH3Na in the system HbO2 – NO2- causes the decrease of the autooxidation factor φ DFH3Na approximately by 1.1 – 2.5 times and φresveratrol by 1.1 – 1.7 times. The time of achievement of the maximum rate of oxidation of HbO2 dζ/dτ (where ζ is the rate of transformation of HbO2 in MetHb) increases while the phase of fast oxidation of HbO2 decreases with increase of content of inhibitors. The process of interaction of nitrites with reducers (such as DFH4, DFH3Na, resveratrol and (+)-catechine) was carried out as well. It has been established that degree of diminishing of the concentration of nitrites in the system RedH2-NO2- decreases as follows: DFH<4DFH3Na<Resv<(+)Catechol.

Chemical physiology of blood flow regulation by red blood cells: the role of nitric oxide and S-nitrosohemoglobin

Blood flow in the microcirculation is regulated by physiological oxygen (O2) gradients that are coupled to vasoconstriction or vasodilation, the domain of nitric oxide (NO) bioactivity. The mechanism by which the O2 content of blood elicits NO signaling to regulate blood flow, however, is a major unanswered question in vascular biology. While the hemoglobin in red blood cells (RBCs) would appear to be an ideal sensor, conventional wisdom about its chemistry with NO poses a problem for understanding how it could elicit vasodilation. Experiments from several laboratories have, nevertheless, very recently established that RBCs provide a novel NO vasodilator activity in which hemoglobin acts as an O2 sensor and O2-responsive NO signal transducer, thereby regulating both peripheral and pulmonary vascular tone. This article reviews these studies, together with biochemical studies, that illuminate the complexity and adaptive responsiveness of NO reactions with hemoglobin. Evidence for the pivotal role of S-nitroso (SNO) hemoglobin in mediating this response is discussed. Collectively, the reviewed work sets the stage for a new understanding of RBC-derived relaxing activity in auto-regulation of blood flow and O2 delivery and of RBC dysfunction in disorders characterized by tissue O2 deficits, such as sickle cell disease, sepsis, diabetes, and heart failure.

Routes to S-nitroso-hemoglobin formation with heme redox and preferential reactivity in the beta subunits

Previous studies of the interactions of NO with human hemoglobin have implied the predominance of reaction channels that alternatively eliminate NO by converting it to nitrate, or tightly complex it on the alpha subunit ferrous hemes. Both channels could effectively quench NO bioactivity. More recent work has raised the idea that NO groups can efficiently transfer from the hemes to cysteine thiols within the beta subunit (cysbeta-93) to form bioactive nitrosothiols. The regulation of NO function, through its chemical position in the hemoglobin, is supported by response to oxygen and to redox agents that modulate the molecular and electronic structure of the protein. In this article, we focus on reactions in which Fe(III) hemes could provide the oxidative requirements of this NO-group transfer chemistry. We report a detailed investigation of the reductive nitrosylation of human met-Hb, in which we demonstrate the production of S-nitroso (SNO)-Hb through a heme-Fe(III)NO intermediate. The production of SNO-Hb is strongly favored (over nitrite) when NO is gradually introduced in limited total quantities; in this situation, moreover, heme nitrosylation occurs primarily within the beta subunits of the hemoglobin tetramer. SNO-Hb can similarly be produced when Fe(II)NO hemes are subjected to mild oxidation. The reaction of deoxygenated hemoglobin with limited quantities of nitrite leads to the production of beta subunit Fe(II)NO hemes, with SNO-Hb produced on subsequent oxygenation. The common theme of these reactions is the effective coupling of heme-iron and NO redox chemistries. Collectively, they establish a connectivity between hemes and thiols in Hb, through which NO is readily dislodged from storage on the heme to form bioactive SNO-Hb.

Contribution of nitric oxide to potassium bromate-induced elevation of methaemoglobin concentration in mouse blood

Bromate, an inorganic oxyhalide disinfection by-product, is known to cause kidney damage, haemolysis and methaemoglobinemia. In potassium bromate (KBrO3)-treated mice (1.2 mmol/kg), elevation of methaemoglobin (MetHb) concentration in blood was observed simultaneously with an elevation of the NO concentration and attenuation of glutathione peroxidase (GPx) activity. Renal oxidative stress and kidney damage were also confirmed in the KBrO3-treated mice. A pre-administered GPx-mimic ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) dose-dependently diminished the KBrO3-induced changes in MetHb concentration and GPx activity. Renal oxidative stress and kidney damage caused by the KBrO3 administration were also dose-dependently suppressed by ebselen. On the other hand, ebselen did not suppress the KBrO3-induced elevation of the NO concentration. KBrO3-induced methaemoglobinemia, renal oxidative stress and kidney damage, consequently, seemed to result from the attenuation of GPx activity. Besides, the enhancement of NO production was not likely to be a result but a cause for the KBrO3-induced attenuation of GPx activity. In in vitro experiments, oxidation of human oxyhaemoglobin (HbO2) to MetHb was observed in a reaction mixture containing HbO2 and an NO donor, NOC-7 (1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene) or SIN-1 (3-(4-morpholinyl)sydnonimine), and this oxidation was inhibited by the NO scavenger carboxy-PTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide). However, no MetHb formation was observed in a reaction mixture containing HbO2 and KBrO3. These results suggest that KBrO3-induced methaemoglobinemia results from the reduction of GPx activity in blood by the KBrO3-induced increases in superoxide, NO and ONOO-.

Copper and carcinogenesis

Metal ions play an important role in biological systems, and without their catalytic presence in trace or ultratrace amounts many essential co-factors for many biochemical reactions would not take place. However, they become toxic to cells when their concentrations surpass certain optimal (natural) levels. Copper is an essential metal. Catalytic copper, because of its mobilization and redox activity, is believed to play a central role in the formation of reactive oxygen species (ROS), such as O2-* and *OH radicals, that bind very fast to DNA, and produce damage by breaking the DNA strands or modifying the bases and/or deoxyribose leading to carcinogenesis. The chemistry and biochemistry of copper is briefly accounted together with its involvement in cancer and other diseases.

Oscillatory oxido-reductive reaction of intracellular hemoglobin in human erythrocyte incubated with o-aminophenol

When human erythrocytes were incubated with o-aminophenol at pH 7.0 at 37 degrees C for 46 hours, intracellular oxyhemoglobin was completely oxidized to methemoglobin during the initial 6 hours, and methemoglobin formed was then reduced to oxyhemoglobin during the following 20 hours. This was demonstrated by the changes in absorption spectra of intracellular hemoglobin. Such oscillatory behavior of intracellular hemoglobin during reaction with o-aminophenol was explained by the fact that o-aminophenol has the ability to both oxidize oxyhemoglobin and reduce methemoglobin. In order to study the mechanism of oxido-reductive reactions of hemoglobin with aromatic reductants including o-aminophenol, the oxidation of ferrous hemoglobin and reduction of methemoglobin with various aromatic reductants such as o-aminophenol, 2-amino-4-methyl-phenol, 2-amino-5-methylphenol, and homogentisic acid were investigated under various conditions. It was found that oxyhemoglobin was oxidized by these aromatic compounds, and the oxidation rate was accelerated in the presence of inositol hexaphosphate, but was not affected in the presence of catalase and superoxide dismutase, except for the case with homogentisic acid. The oxidation of ferrous hemoglobin by these compounds did not proceed under anaerobic conditions. Methemoglobin was reduced by these aromatic compounds, and the reduction rate was much accelerated in the presence of inositol hexaphosphate, but was not affected in the presence of catalase and superoxide dismutase, except for the case with homogentisic acid. The reduction of methemoglobin by these compounds proceeded under anaerobic conditions, suggesting that ferric heme of hemoglobin reacts directly with aromatic reductants. On the basis of these results, the mechanism of oxido-reductive reaction of ferrous and ferric hemoglobin with aromatic reductants was proposed.

Use of in vitro methaemoglobin generation to study antioxidant status in the diabetic erythrocyte

Poor glycaemic control in diabetes and a combination of oxidative, metabolic, and carbonyl stresses are thought to lead to widespread non-enzymatic glycation and eventually to diabetic complications. Diabetic tissues can suffer both restriction in their supply of reducing power and excessive demand for reducing power. This contributes to compromised antioxidant status, particularly in the essential glutathione maintenance system. To study and ultimately correct deficiencies in diabetic glutathione maintenance, an experimental model would be desirable, which would provide in vitro a rapid, convenient, and dynamic reflection of the performance of diabetic GSH antioxidant capacity compared with that of non-diabetics. Xenobiotic-mediated in vitro methaemoglobin formation in erythrocytes drawn from diabetic volunteers is significantly lower than that in erythrocytes of non-diabetics. Aromatic hydroxylamine-mediated methaemoglobin formation is GSH-dependent and is indicative of the ability of an erythrocyte to maintain GSH levels during rapid thiol consumption. Although nitrite forms methaemoglobin through a complex GSH-independent pathway, it also reveals deficiencies in diabetic detoxification and antioxidant performance compared with non-diabetics. Together with efficient glycaemic monitoring, future therapy of diabetes may include trials of different antiglycation agents and antioxidant combinations. Equalization in vitro of diabetic methaemoglobin generation with that of age/sex-matched non-diabetic subjects might provide an early indication of diabetic antioxidant status improvement in these studies.

Methaemoglobin formation due to nitrite, disulfiram, 4-aminophenol and monoacetyldapsone hydroxylamine in diabetic and non-diabetic human erythrocytes in vitro

Nitrite, monoacetyl dapsone hydroxylamine, 4-aminophenol and disulfiram-mediated methaemoglobin formation was studied in human diabetic and non-diabetic erythrocytes in vitro. Diabetic intact erythrocytes were significantly less sensitive compared with those of non-diabetics to haemoglobin oxidation caused by the hydroxylamine, nitrite and 4-aminophenol, but not disulfiram. In haemolysates, differential sensitivity did occur with disulfiram and was partially retained with 4-aminophenol and nitrite. The differences were lost with 4-aminophenol, nitrite and disulfiram in the presence of haemoglobin purified from the respective erythrocyte types. Diethyl maleate reduced methaemoglobin formation in non-diabetic intact erythrocytes with 4-aminophenol, the hydroxylamine and disulfiram, but not with nitrite. Overall, the differential sensitivity to methaemoglobin formation seen in diabetic compared with non-diabetic erythrocytes, is probably linked to differences in the respective cells' cytosolic anti-oxidant systems.

Direct measurement of nitrite transport across erythrocyte membrane vesicles using the fluorescent probe, 6-methoxy-N-(3-sulfopropyl) quinolinium

Nitrite was shown to quench the fluorescence of 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ) almost twofold more than chloride. SPQ loaded inside vesicles prepared from asolectin and isolated erythrocyte ghosts allowed for the direct measurement of nitrite movement across these membranes. Movement of nitrite across asolectin occurred by diffusion as HNO2 in a pH-dependent manner. By contrast, erythrocyte ghosts had very low diffusion rates for nitrous acid. Erythrocyte ghosts preloaded with 50 mM nitrite to quench SPQ fluorescence were utilized to study heteroexchange with externally added anions. SPQ fluorescence increases (becomes unquenched) with added bicarbonate and nitrate, indicating that nitrite is moving out of the preloaded vesicles. The pH optimum for this exchange was approximately 7.6 and exchange was inhibited by N-ethylmaleimide (NEM) and dihydro-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). These data indicate that nitrite moves across erythrocyte plasma membranes as NO2- by a heteroexchange mechanism with other monovalent anions.

Production of nitric oxide (NO) during the oxidation of human oxyhemoglobin by nitrite: application of a NO-selective electrode for the measurement of NO

By using a nitric oxide (NO) selective electrode system. NO produced during the oxidation of human hemoglobin by nitrite was monitored. When 160 microM oxyhemoglobin (in heme) was reacted with 500 microM nitrite. NO was generated quickly at the initial lag phase of the oxidation of oxyhemoglobin by nitrite and decreased gradually during the second burst phase of the reaction. While the oxidation of oxyhemoglobin by nitrite proceeded in a sigmoidal manner including the initial lag phase and second burst phase. The maximal amount of NO produced under this condition was estimated to be 48 microM. According to the increase of nitrite concentrations added, the amounts of NO produced at the initial phase increased, being in good accordance with the increased rate of the oxidation of oxyhemoglobin. These results strongly suggest the critical role of NO in the oxidation mechanism of oxyhemoglobin by nitrite.

Changes in nitric oxide generated by the oxidation of oxymyoglobin by nitrite

We found that nitric oxide (NO) was produced during the oxidation of oxymyoglobin by nitrite, by using a NO-selective electrode system, which enabled us to measure NO in phosphate solution, continuously and quantitatively. When 500 microM nitrite was added in the solution of oxymyoglobin (65 microM in heme), oxymyoglobin was oxidized to metmyoglobin in a sigmoidal manner. During the reaction, NO was generated quickly at the initial lag phase, and reached its peak (NO approximated to 30 microM), before the burst oxidation of oxymyoglobin occurred. Then, NO content was gradually decreased. By the addition of the increased concentration of nitrite (1 mM or 2 mM) to oxymyoglobin solution (65 microM in heme), the production rate of NO was much accelerated and the amounts of NO were increased, in good accordance with the accelerated oxidation of oxymyoglobin by nitrite. These results suggest that NO is involved in the oxidation of oxymyoglobin by nitrite.

Methemoglobin production by nitric oxide in fresh sheep blood

As nitric oxide (NO) inhalation is used therapeutically, we studied the production of methemoglobin (metHb) by NO in fresh adult sheep blood. NO solutions were prepared by bubbling a 10% NO-90% N2 gas mixture in phosphate-buffered saline (pH = 7.41 at 20 degrees C) for at least 60 min. Fresh blood samples were obtained from catheterized femoral arteries or veins just prior to mixing with NO solution. Measurements of metHb were made at times 0, 30 sec, 2 min and 5 min after mixing of NO-containing buffer and blood using a Radiometer OSM-3 hemoximeter. Mixing was performed using two syringes connected via a stopcock. The reaction of NO with blood occurred rapidly after mixing since data values for each of the time points after 30 sec were unchanged for all mixtures. The mixing volume ratio of NO-containing buffer to blood was either 1:1 (protocol A) for comparisons of arterial vs venous blood, or the ratios were randomized (protocol B) to investigate effects of Hb oxygenation. Protocol A elicited only slight increases of metHb in arterial and venous blood which did not differ significantly. In protocol B, an increase of metHb was associated with a relative decline of deoxyhemoglobin (deoxyHb). A higher molar ratio of NO/deoxyHb yielded greater amounts of metHb. Therefore, in fresh sheep blood, deoxyHb is converted to metHb in the presence of NO. This reaction is not affected by the presence of oxyhemoglobin.

Curcumin inhibits nitrite-induced methemoglobin formation

Curcumin protects hemoglobin from nitrite-induced oxidation to methemoglobin. The protection is not observed when curcumin is added after the autocatalytic stage of the oxidation of hemoglobin by nitrite. The ability of curcumin to scavenge superoxide may be responsible since superoxide is implicated in promoting the autocatalytic stage of oxidation of hemoglobin by nitrite.

Formation of methemoglobin and metmyoglobin using 8-aminoquinoline derivatives or sodium nitrite and subsequent reaction with cyanide

The kinetics of the oxidation of hemoglobin (Hb) and myoglobin (Mb) by sodium nitrite, 8-[(4-amino-1-methylbutyl)amino]-6-methoxy-quinoline diphosphate (primaquine), 6-methoxy-8-(6-diethylaminohexylamino)-4-methyl-quinoline dihydrochloride (WR6026) and 8-[(4-amino-1-methylbutyl)amino]-2,6-dimethoxy-4-methyl- 5-[(3-trifluoromethyl)phenoxy]quinoline succinate (WR238,605) were studied at pH values ranging from 7.4 to 7.6 and at 37 +/- 1 degrees C. The reaction between Hb and primaquine, WR6026 and WR238,605 resulted in precipitation, as did the reaction between Mb and WR238,605. The reaction between nitrite ion (NO2-) and Hb showed a lag period followed by an autocatalytic phase. The data in this study are consistent with and substantiate the proposed mechanism for the Hb-NO2- oxidation reaction. The reaction between Mb and NO2- at higher NO2- concentrations also showed a lag period followed by an autocatalytic period, while at lower NO2- concentrations no lag period was seen. The data suggest a shift in rate constant at these lower NO2- concentrations. The reaction between Mb and both WR6026 and primaquine followed a two-term rate law with oxidant-dependent and -independent terms. Concentration-effect curve data, along with these results, suggest the presence of a catalytic pathway. The rates of formation of cyanomethemoglobin and cyanometmyoglobin complexes from cyanide ion and methemoglobin (MHb) and metmyoglobin (MMb), respectively, were followed in the presence of the heme oxidants. The rate constants were all within a narrow range and suggest that complexation of cyanide by MHb and MMb is not affected by the presence of oxidants.

The interaction between nitrogen oxides and hemoglobin and endothelium-derived relaxing factor

Among nitrogen oxides, NO and NO2 are free radicals and show a variety of biological effects. NO2 is a strongly oxidizing toxicant, although NO, not oxidizing as NO2, is toxic in that it interacts with hemoglobin to form nitrosyl- and methemoglobin. Nitrosylhemoglobin shows a characteristic electron spin resonance (ESR) signal due to an odd electron localized on the nitrogen atom of NO and reacts with oxygen to yield nitrate and methemoglobin, which is rapidly reduced by methemoglobin reductase in red cells. NO was found to inhibit the reductase activity. Part of NO inhaled in the body is oxidized by oxygen to NO2, which easily dissolves in water and converts to nitrite and nitrate. The nitrite oxidizes oxyhemoglobin autocatalytically after a lag. The mechanism of the oxidation, particularly the involvement of superoxide, was controversial. The stoichiometry of the reaction has now been established using nitrate ion electrode and a methemoglobin free radical was detected by ESR during the oxidation. Complete inhibition of the autocatalysis by aniline or aminopyrine suggests that the radical catalyzes conversion of nitrite to NO2, which oxidizes oxyhemoglobin. Recently NO was shown to be one of endothelium-derived relaxing factors and the relaxation induced by the factor was inhibited by hemoglobin and potentiated by superoxide dismutase.

Mechanism of autocatalytic oxidation of oxyhemoglobin by nitrite

Oxidation of oxyhemoglobin by nitrite is characterized by the presence of a lag phase followed by autocatalysis. The stoichiometry of the overall reaction is described by the following equation: 4HbO2 + 4NO2- + 4H+ = 4Hb+ + 4NO3- + O2 + 2H2O (Hb denotes hemoglobin monomer). During the oxidation, we detected a free radical at g = 2.005, which is very similar to the methemoglobin free radical generated by the reaction with hydrogen peroxide. Nitrosylhemoglobin was not detected. The oxidation was delayed by the addition of KCN or catalase, but was not modified by superoxide dismutase in phosphate buffer. In bistris buffer, however, superoxide dimutase markedly prolonged the lag phase. The results suggest that during the oxidation, the methemoglobin peroxide compound is generated and converts nitrite into nitrogen dioxide by its peroxidatic activity. Nitrogen dioxide oxidizes oxyhemoglobin to methemoglobin and nitrite, yielding the autocatalytic phase.

Inhibition by amines indicates involvement of nitrogen dioxide in autocatalytic oxidation of oxyhemoglobin by nitrite

Oxidation of oxyhemoglobin by nitrite is characterized by a lag period followed by an autocatalytic phase. The oxidation can be inhibited by the addition of morpholine, piperidine, triethanolamine or triethylamine (6 mM each). These amines are known to react with nitrogen dioxide to yield nitrosamine. Unexpectedly, aniline or aminopyrine (120 microM each) markedly inhibited the oxidation. These compounds, but not the other amines given above, inhibited the peroxide compound formation from methemoglobin and hydrogen peroxide. The results establish that, during the oxidation, the peroxide compound is generated and converts nitrite into nitrogen dioxide by its peroxidatic activity, resulting in an autocatalytic phase.

Effects of nine synthetic putative metabolites of primaquine on activity of the hexose monophosphate shunt in intact human red blood cells in vitro

Suspensions of washed human red blood cells were treated with nine synthetic putative metabolic derivatives of primaquine (PQ'), and their individual effects on activity of the hexose monophosphate shunt (HMS) were quantitated by radiometric analysis of 14CO2 from [14C] glucose. The most potent HMS stimulant was 5-hydroxy-6-methoxy-8-aminoquinoline (5H6MQ), which caused 10-fold elevation of HMS activity at an estimated concentration of 0.004 mM. Ten millimolar primaquine (PQ) was required to achieve the same effect. Thus, 5H6MQ was approximately 2500-fold more reactive with the HMS than PQ. Other analogs achieved less than 0.4- to 154-fold increases in HMS reactivity. Patterns of effects on HMS activity indicated that 5-hydroxylation and/or N-dealkylation of PQ strongly enhanced HMS reactivity. In contrast, none of the putative metabolites of PQ activated the proteolytic system known to degrade oxidized protein in red cells, indicating that stimulation of the HMS by the PQ analogs was not related to an injurious oxidative stress. Red cells pretreated with 1.0 mM N-ethylmaleimide (NEM) or with 1.0% (w/v) sodium nitrite to cause glutathione sulfhydryl blockage and conversion of red cell hemoglobin to methemoglobin (metHb), respectively, also showed elevation of HMS activity when exposed to 5H6MQ. These observations suggested that 5H6MQ-induced elevation of HMS activity was at least partially independent of glutathione redox reactions, hydrogen peroxide accumulation and reaction with oxyhemoglobin. The relevance of these observations to proposed mechanisms of hemolytic toxicity of PQ is discussed.

Oxidative activity of hydroxylated primaquine analogs. Non-toxicity to glucose-6-phosphate dehydrogenase-deficient human red blood cells in vitro

The individual effects of two putative metabolites of primaquine (5,6-dihydroxyprimaquine and 5,6-dihydroxy-8-aminoquinoline) on the hexose monophosphate shunt (HMS) and on the ATP-dependent proteolytic system which rapidly degrades oxidized erythrocyte protein were measured in intact red blood cells in vitro from two blood donors. In red cells treated with nitrite (1-40 mM) or phenylhydrazine (0.01-10 mM), proteolytic activity was detected only with concentrations (7.5 mM NaNO2 and 0.25 mM phenylhydrazine) causing greater than 15-fold elevation of HMS activity, and glucose-6-phosphate dehydrogenase (G6PD)-deficient (25% of normal activity) red cell suspensions thus treated showed approximately 30% greater proteolysis. G6PD-normal and deficient red cells treated with the primaquine analogs, however, did not experience proteolysis with concentrations (0.25 mM) in excess of those causing 17-fold elevation of HMS activity. Stimulation of the HMS by the primaquine analogs thus appears unrelated to an erythrotoxic oxidative stress. Methylene blue is known to cause an elevation of HMS activity through direct and diaphorase II-dependent oxidation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) which is independent of injurious oxidative stress. It was found that the putative primaquine metabolites also caused direct and diaphorase II-dependent oxidation of NADPH in dilute hemolysate, thus suggesting that the putative primaquine metabolites have a methylene blue-like redox disposition in red blood cells. Results obtained in this study suggest that the hemolytic toxicity of primaquine may be unrelated to processes which lead to oxidative deterioration of red cell protein.

Kinetics of amyl nitrite-induced hemoglobin oxidation in cord and adult blood

The effect of amyl nitrite on the erythrocytes of adult and cord hemoglobin was examined in vitro. This study revealed that amyl nitrite caused oxyhemoglobin to become oxidized to methemoglobin wherein a rectangular hyperbolic curve was generated as the reaction progressed. This curve consisted of a reactionary log phase, and a terminal asymptotic phase only, with no inductionary lag phase. A comparative study of human cord blood oxidation times and adult blood was undertaken. It was revealed that cord blood erythrocytes were oxidized by amyl nitrite at a 5-6-fold greater rate than adult blood erythrocytes. Based on an independent Student's t-test, the time taken for cord blood erythrocytes to undergo oxidation was significantly shorter (P less than 0.05) than adult controls. This greatly enhanced reactivity of cord blood erythrocytes parallels earlier findings when sodium nitrite was used instead of amyl nitrite. However, this difference defies a simple explanation and must be attributed to many factors which may include pH, structural differences, and solubility phenomenon.

Autocatalytic oxidation of hemoglobin induced by nitrite: activation and chemical inhibition

The nitrite ion is a direct causative agent for methemoglobinemia. Oxidation of hemoglobin to methemoglobin under aerobic conditions is induced by nitrite, catalyzed by methemoglobin in the presence of hydrogen peroxide, and inhibited by chemical reagents ranging from cysteine and ascorbic acid to sulfite. The stoichiometry of nitrate production is dependent on the initial [NO2-]/[HbO2] ratio, but reaches a limiting value of 1:1 [NO3-]: [Hb+] when [NO2-]/[HbO2] greater than 8. Ascorbic acid is an exceptionally effective inhibitor for the autocatalytic oxidation, but its use does not affect the stoichiometry of nitrate formation. Sulfite reduces nitrate production to a level that is half that observed in its absence. These chemical inhibitors act upon the rapid autocatalytic stage for hemoglobin oxidation, but they do not influence the slow direct oxidation of hemoglobin by nitrite. The autocatalytic stage for hemoglobin oxidation results from nitrogen dioxide formed from nitrite through the peroxidase activity of methemoglobin. Peroxide and methemoglobin are formed during the initiation stage by electron transfer from nitrite that is kinetically first order in oxyhemoglobin and in nitrite.

Methylene blue-mediated hexose monophosphate shunt stimulation in human red blood cells in vitro: independence from intracellular oxidative injury

The red blood cell hexose monophosphate shunt (HMS) and proteolytic responses to several concentrations of Methylene Blue or sodium nitrite were measured. The results suggested two distinct mechanisms for activation of the HMS: (1) nitrite treatment increased HMS activity in response to oxidative challenge to red cell protein; (2) Methylene Blue treatment activated HMS without injurious oxidative challenge. Nitrite-treated cells actively degraded protein, whereas Methylene Blue-treated red cells did not activate proteolytic systems that degrade oxidized red cell protein. These observations are relevant to proposed in vitro systems for evaluation of drug hemolytic toxicity potential on the basis of HMS stimulation capacity.

Neurochemical effects of ethylhexyl nitrate in rats

Rats injected i.p. with 100 mg ethylhexyl nitrate/kg body wt excreted 0.3% of the dose in the urine within 24 h. No urinary nitrite was found within 5 postinjection hours while an output of 26.9 +/- 15.8 micrograms nitrite/h X kg body wt. was detected between 5-24 h. Cerebral glutathione concentration was below the control level after 1 day but returned to control value after 3 and 7 days postinjection. Brain acetylcholine esterase activity was marginally decreased after 1 day while RNA and succinate dehydrogenase assay excluded major structural damage. It seems that the mechanism of clinical symptoms experienced by exposed workers are comparable to those exposed to dynamite and are largely functional.

Production of superoxide anion by N,N-bis(2-hydroxyethyl)-iminotris(hydroxymethyl)methane buffer during oxidation of oxyhemoglobin by nitrite and effect of inositol hexaphosphate on the oxidation

Oxidation of oxyhemoglobin by nitrite is characterized by the presence of a lag phase followed by an autocatalysis. As reported previously (Kosaka, H., Imaizumi, K. and Tyuma, I. (1982) Biochim. Biophys. Acta 702, 237-241), in phosphate buffer nitrite produced an ESR signal at g 2.005 (hereafter referred to as the g 2 radical). The g 2 radical produced NO.2 from NO-2, then NO.2 oxidized oxyhemoglobin. Superoxide dismutase did not modify the oxidation. On the other hand in N,N-bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane (bistris) buffer, superoxide dismutase markedly elongated the lag phase and accelerated the autocatalysis, indicating O-2 production. Bistris scavenged the g 2 radical. O-2 was generated by the reduction of O2 by a radical derived from bistris. Inositol hexaphosphate inhibited the oxidation by decreasing H2O2 production from oxyhemoglobin.

Studies on the interaction of barbiturates with reactive oxygen radicals: implications regarding barbiturate protection against cerebral ischaemia

Although the molecular basis of ischaemic damage of the brain is as yet unknown, it has been postulated that the uncontrolled production of reactive oxygenated species derived from molecular oxygen (including hydroxyl radicals, superoxide radicals and singlet oxygen) may play a major role in the production of such injury. The ability of various barbiturates to modify the nature and extent of membrane damage produced by various oxygen radicals generated under well-defined conditions in vitro has been directly examined using the human erythrocyte as model membrane system. Our results indicate that barbiturates are unlikely to exert their protective effects by directly scavenging singlet oxygen, superoxide or hydroxyl radicals. The highly lipophilic barbiturate thiopentone is capable of decreasing the susceptibility of membranes to oxidative degradation by a direct membrane action, a property shared by amphipathic membrane stabilizers such as propranolol. The barbiturates were found to stabilize the haeme moiety of haemoglobin preventing its conversion to methaemoglobin in the presence of hydrogen peroxide. It is postulated that a major aspect of barbiturate action in decreasing ischaemic injury to the brain may involve the stabilization of haeme-coordinated iron complexes, thereby preventing the participation of these ubiquitous substances in initiating and potentiating free radical-mediated processes which have been implicated in the production of such injury.

Mechanism of autocatalytic oxidation of oxyhemoglobin by nitrite. An intermediate detected by electron spin resonance

Oxidation of oxyhemoglobin by nitrite is characterized by the presence of a lag phase followed by the autocatalysis. Just before the autocatalysis begins, an asymmetric ESR signal is detected which is similar to that of the methemoglobin radical generated from methemoglobin and H2O2 in shape, g value (2.005), peak-to-peak width (18 G) and other properties, except the difference in the dependence on temperature. Generation of H2O2 is indicated by the prolongation of the lag phase by the addition of catalase. On the other hand, the oxidation is modified by neither superoxide dismutase nor Nitroblue tetrazolium. The oxidation is prolonged in the presence of KCN. The present results indicate a free-radical mechanism for the oxidation in which the asymmetric radical catalyzes the formation of NO2 from NO2- by a peroxidase action and NO2 oxidizes oxyhemoglobin in the autocatalytic phase.

Kinetic analysis of myoglobin autoxidation by isoelectric-focusing electrophoresis

The autoxidation of horse myoglobin was studied in the presence or absence of catalase (EC 1.11.1.6) and/or superoxide dismutase (EC 1.15.1.1) at various pH values (6.6-7.8). Changes in the percentages of oxymyoglobin and metmyoglobin during the reaction were analysed by means of isoelectric focusing on Ampholine gel plates. Oxymyoglobin was decreased in a first-order manner, with an accompanying increase in metmyoglobin, under the various conditions studied. The observed reaction rate constants obtained under these conditions were pH-dependent; however, they were also greatly affected by the presence of the enzymes. The pH-dependence of the overall reaction was explained by the acid-base three-state model of myoglobin proposed by Shikama & Sugawara [(1978) Eur. J. Biochem. 91, 407-413]. The reaction process of myoglobin autoxidation was explained by the model suggested by Winterbourn, McGrath & Carrell [(1976) Biochem. J. 155, 493-502], indicating that superoxide anion and hydrogen peroxide are involved in the reaction mechanism.