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PubMed1987

The uptake of protons by heme-linked ionizable groups on azide binding to methemoglobin.

When azide ion reacts with methemoglobin in unbuffered solution the pH of the solution increases. This phenomenon is associated with increases in the pK values of heme-linked ionizable groups on the protein which give rise to an uptake of protons from solution. We have determined as a functional of pH the proton uptake, delta h+, on azide binding to methemoglobin at 20 degrees C. Data for methemoglobins A (human), guinea pig and pigeon are fitted to a theoretical expression based on the electrostatic effect of these sets of heme-linked ionizable groups on the binding of the ligand. From these fits the pK values of heme-linked ionizable groups are obtained for liganded and unliganded methemoglobins. In unliganded methemoglobin pK1, which is associated with carboxylic acid groups, ranges between 4.0 and 5.5 for the three methemoglobins; pK2, which is associated with histidines and terminal amino groups, ranges from 6.2 to 6.7. In liganded methemoglobin pK1 lies between 5.8 and 6.3 and pK2 varies from 8.1 to 8.5. The pH dependences of the apparent equilibrium constants for azide binding to the three methemoglobins at 20 degrees C are well accounted for with the pK values calculated from the variation of delta h+ with pH.

Azides↗

PubMed1994

Methemoglobin formation and reduction in canine erythrocytes characterized by inherited high Na+, K(+)-ATPase activity with normal and high glutathione concentrations.

The methemoglobin formation and methemoglobin reduction in canine erythrocytes characterized by inherited high potassium (K+) and normal reduced glutathione concentrations (HK-low GSH cells) were compared with those in canine erythrocytes with inherited high K+ and high GSH concentrations (HK-high GSH cells) and normal canine erythrocytes with low K+ and low (= normal) GSH concentrations (LK-low GSH cells). The rate of methemoglobin formation induced by sodium nitrite (NaNO2) was in the order; LK-low GSH > HK-low GSH > HK-high GSH cells, and the difference among groups was significant at 7 and 15 min. Methemoglobin reduction in a medium containing glucose occurred rapidly in both HK-high GSH and HK-low GSH cells, and the rate of reduction was 1.7-fold higher than in LK-low GSH cells. Accumulation of pyruvate equivalent to the amount of methemoglobin reduced indicated that methemoglobin was predominantly reduced by NADH-methemoglobin reductase coupled to glycolysis. HK-low GSH cells showed an increased glycolytic rate and high pyruvate kinase activity similar to the levels in HK-high GSH cells. It is therefore evident that HK-low GSH cells offer greater protection against oxidation of hemoglobin to methemoglobin than LK-low GSH cells because of the increased glycolytic rate in HK-low GSH cells attributable to high pyruvate kinase activity in these cells.

Animals↗

PubMed1987

Electrochemical reduction of methemoglobin either directly or with flavin mononucleotide as a mediator.

Electrochemical reduction of methemoglobin on a platinum electrode is studied by means of thin layer spectroelectrochemistry. For methemoglobin alone in solution, direct reduction is very slow even for potentials close to those of the reduction of the solvent. The reduction of a methemoglobin-oxyhemoglobin mixture with an imposed potential causes the electrochemical reduction of oxygen, the conversion of oxyhemoglobin into deoxyhemoglobin, and a simultaneous transformation of part of the molecules into methemoglobin. When fixed oxygen has disappeared, reduction of methemoglobin takes place. The reduction of methemoglobin and deoxyhemoglobin is catalyzed by the presence of flavin mononucleotide (FMN). For the oxyhemoglobin-methemoglobin mixture, flavin makes a fast deoxygenation of oxyhemoglobin without a change in the oxidation state of the iron. It also allows the rapid reduction of methemoglobin. In each case, the resulting deoxyhemoglobin solutions do not show any electrolysis-induced modification of the equilibrium curves for oxygen binding.

Electrochemistry↗

PubMed1981

Reduction of methemoglobin by ferredoxin and ferredoxin-NADP reductase system.

The changes in absorption spectra between 450 and 650 nm during the reduction of methemoglobin A, (alpha 2+ beta 3+)2, and (alpha 3+ beta 2+)2 valency hybrids by the system including ferredoxin and ferredoxin-NADP reductase were studied under anaerobic conditions. During the reduction of methemoglobin A, the isosbestic points gradually shifted to different positions. These shifts were clearly observed in the presence of inositol hexaphosphate, i.e. the isosbestic points were initially observed at 525 and 603 nm, and these shifted to 529 and 599 nm, respectively, suggesting that the intermediate hemoglobins are produced during the process of the reaction. This was confirmed by the isoelectric focusing electrophoresis of the partially reduced methemoglobin solutions with ferredoxin-NADP reductase system on Ampholine-polyacrylamide gel plate. On the other hand, the absorption spectra of alpha 2+ beta 3+)2 and (alpha 3+ beta 2+)2 changed with excellent isosbestic points during th reductive reaction by ferredoxin-NADP reductase system, i.e. (alpha 2+ beta 3+)2 at 526 and 601 nm and (alpha 3+ beta 2+)2 at 529 and 599 nm. From these results, the mechanism of methemoglobin reduction by ferredoxin-NADP reductase system was suggested. 1) There are two pathways for the reduction of methemoglobin including (formula, see text). 2) The beta chains of methemoglobin may be more susceptible to the reduction than the alpha chains in tetrameric methemoglobin, and thereby the (alpha 3+ beta 2+)2 valency hybrid accumulated at the halfway point of the reaction. 3) The shift in isosbestic points of methemoglobin reduction (525 nm leads to 529 nm, 603 nm leads to 599 nm) is due to the accumulation of (alpha 3+ beta 2+)2, whose isosbestic points during the reduction by the ferredoxin-NADP reductase systems were 529 and 599 nm.

Ferredoxin-NADP Reductase↗

PubMed2000

A simple and rapid evaluation of methemoglobin toxicity of 8-aminoquinolines and related compounds.

Methemoglobin, a toxic ferric form of hemoglobin, is continuously formed in normal erythrocytes, but during abnormal situations in situ, the level is enhanced. 8-Amino-quinolines and related compounds are causative agents for methemoglobin formation. Employing oxyhemoglobin, methemoglobin toxicity was about six times higher with primaquine compared to CDRI Compound 80/53 at 10(-9) M concentration. Methemoglobin reductase activity was also completely inhibited by primaquine, whereas 24% inhibition was noted in the case of 80/53 at the same concentrations. Mastomys, a rodent animal model, was found to be equally good for comparative evaluation of methemoglobin toxicity. Further, with the use of primaquine transdermal tape on the Mastomys model, a rise in methemoglobin occurred with increase in time. In conclusion, the study presents simple, economical, less time-consuming methods for the evaluation of methemoglobin toxicity, in vitro and in vivo, without employing the conventional Beagle dog model.

Administration, Cutaneous↗

PubMed1985

Hexose monophosphate shunt-stimulated reduction of methemoglobin by divicine.

Reduced divicine (2,6-diamino-4,5-dihydroxypyrimidine), an aglycone implicated in the pathogenesis of favism, reduces methemoglobin efficiently in intact erythrocytes and in hemolysates. Oxidized divicine produces the same effect when glucose or an NADPH-generating system is added to intact erythrocytes or to hemolysates. Although NADPH, NADH, and GSH have no direct methemoglobin-reducing activity in vitro, they convert oxidized divicine to the reduced hydroquinone species, which is responsible for the electron transfer to methemoglobin. Reduction of methemoglobin is optimally observed under nitrogen since, in the presence of oxygen, reduced divicine undergoes autoxidation. Several lines of evidence rule out the reduction of methemoglobin by divicine through an enzyme-catalyzed process, although it is certainly sustained by the hexose monophosphate shunt activity of erythrocytes through the generation of both NADPH and GSH. Thus, the strong enhancing effect that glucose produces on the divicine-dependent methemoglobin reduction within intact normal erythrocytes is completely absent in erythrocytes from glucose-6-phosphate dehydrogenase-deficient subjects. This distinctive behavior might account for the enhanced methemoglobin levels that are found both in vitro in glucose-6-phosphate dehydrogenase-deficient erythrocytes exposed to divicine and in vivo as a typical feature of the acute hemolytic crisis of favic patients.

Catalase↗

PubMed1981

The influence of beta-93 sulfhydryl groups, organic phosphate and heme concentration on methemoglobin reduction.

Native and modified methemoglobin (beta-93-SH groups blocked) were reduced by NADH-dependent methemoglobin reductase in the absence and the presence of organic phosphate (inositol hexaphosphate). These experiments were performed with dilute as well as concentrated methemoglobin solutions (physiological heme concentration). It is shown that: (a) in dilute solutions the blockage of beta-93-SH groups lowers the rate of methemoglobin reduction in the absence of organic phosphate but the rates of native and modified methemoglobin reduction are similar in the presence of organic phosphate; (b) at physiological heme concentration the blockage of beta-93-SH groups does not affect the rate of reduction but the organic phosphate stimulates the reduction of both native and modified methemoglobins in a similar fashion, as it does in dilute solutions. It is concluded that, although in dilute solutions the blockage of beta-93-SH groups alters the reduction rate, at physiological heme concentration the presence of free beta-93-SH groups does not have any significant effect on methemoglobin reduction. On the contrary, the organic phosphates do accelerate the rate of reduction at all ranges of heme concentration.

Cystine↗

PubMed1985

Relationship between red blood cell uptake and methemoglobin production by nitrobenzene and dinitrobenzene in vitro.

Nitrobenzene increases methemoglobin formation when incubated with native hemoglobin but not when incubated with red blood cell suspensions. These experiments were designed to determine if transport of nitrobenzene across the red blood cell membrane is a limiting factor for methemoglobin production by red blood cell suspensions. Incubation of [14C]-m-, o- or p-dinitrobenzene, but not mononitrobenzene, with red blood cell suspensions caused a time-dependent increase in methemoglobin. All three dinitrobenzenes and mononitrobenzene crossed the red blood cell membrane and accumulated in the erythrocytes after only 1 min of incubation. Incubation of mononitrobenzene with hemolysates did not result in methemoglobin production. Incubation of red blood cells with the dinitrobenzenes or mononitrobenzene for 1 and 10 min at 4 degrees C did not influence red blood cell uptake of the nitrobenzenes, suggesting that these compounds do not enter the red blood cell by an active process. Dinitrobenzene-induced methemoglobin production was markedly inhibited at 4 degrees C, and may be a result of decreased interaction with hemoglobin and/or decreased metabolism to reactive intermediates which mediate methemoglobin production. These data indicate that red blood cell transport of nitrobenzene is not the limiting factor in methemoglobin production in vitro.

Animals↗

PubMed1980

PH dependent volume changes accompanying the binding reactions of human and pigeon methemoglobins.

Dilatometric measurements of the volume changes accompanying the binding reactions of azide ion to human adult and pigeon methemoglobins as a function pH at 25 degrees C demonstrate pH values of maximum volume change (pH delta Vmax) which are different for the different hemoglobins. pH delta Vmax occurs at pH 6.7 for human methemoglobin A and at pH 7.7 for pigeon methemoglobin. The pH delta Vmax occurs near the characteristic pH (pHch) of maximum enthalpy of the same binding reaction. It is shown that the large pH variation in delta V can arise if the configuration of charged groups on the surface of the molecule is different in methemoglobin and methemoglobin complex. When such a difference in configuration exists the addition of the same number of protons to methemoglobin and methemoglobin complex will give rise to different changes in the partial molar volume of the two species.

Animals↗

PubMed1996

N-acetylcysteine reduces methemoglobin in vitro.

STUDY OBJECTIVE: To determine whether N-acetylcysteine (NAC reduces methemoglobin. METHODS: We carried out an in vitro laboratory experiment in which five healthy adult volunteers donated blood. Each sample was divided equally among three test tubes. Tube 1 served as a negative control. Sodium nitrite .18 mol/L with dextrose .23 mol/L was added to tube 2 and to tube 3. Next, phosphate-buffered saline solution (PBS) was added to tube 2 and NAC (200 mg/mL) to tube 3. Serial methemoglobin levels were measured over 5.5 hours. RESULTS: Maximum methemoglobin levels were observed at 1.5 hours for both the NAC-nitrite and the PBS-nitrite sample (62.7% +/- 8.1% and 65.1% +/- 7.0%, respectively; data expressed as mean +/- SD). The mean difference in methemoglobin between NAC-nitrite and PBS-nitrite was significant at 4.5 hours (29.3% +/- 23.0%, P = .046). The mean rate of methemoglobin decline in NAC-nitrite samples was also different from that of PBS-nitrite samples (10.7% +/- 1.0% versus 2.9% +/- 2.3%, P = .002). The rate of decline was linea (zero order) in the NAC nitrite samples and represented by the equation: % change methemoglobin = .18 x time in minutes. Area under the concentration-time curve was also different among groups (P < .05). CONCLUSION: In this in vitro model, NAC reduced chemically induced methemoglobin.

Acetylcysteine↗

PubMed1997

Characterization of methemoglobin formation induced by 3,5-dichloroaniline, 4-amino-2,6-dichlorophenol and 3,5-dichlorophenylhydroxylamine.

3,5-Dichloroaniline is an intermediate in the production of certain fungicides. This study characterized the capacity of 3,5-dichloroaniline and two putative metabolites to induce methemoglobin formation. In vivo intraperitoneal (i.p.) administration of 0.8 mmol/kg 3,5-dichloroaniline resulted in elevated (P < 0.05) methemoglobin levels at 2 and 4 h after injection and returned to control values within 8 h. In vitro methemoglobin generation was monitored in washed erythrocytes incubated for 60 min at 37 degrees C with 4 and 8 mM 3,5-dichloroaniline. Methemoglobin generation in vitro was higher (P < 0.05) than control values in erythrocytes incubated for 30 min with 0.2-0.6 mM 4-amino-2,6-dichlorophenol or 5-100 microM 3,5-dichlorophenylhydroxylamine. The in vitro methemoglobin generating capacity in decreasing order was: 3,5-dichlorophenylhydroxylamine > 4-amino-2,6-dichlorophenol > > 3,5-dichloroaniline. The results of the in vitro studies further indicated that none of the compounds tested induced lipid peroxidation. Erythrocytes incubated with 5-100 microM 3,5-dichlorophenylhydroxylamine in vitro were associated with depletion of glutathione. These results indicated that: (a) 3,5-dichloroaniline and its metabolites can induce methemoglobin formation; (b) the N-hydroxy metabolite was the most potent inducer of hemoglobin oxidation and (c) glutathione depletion was associated with methemoglobin formation by 3,5-dichlorophenylhydroxylamine.

Aniline Compounds↗

PubMed1985

Spectroscopic studies of protein-heme interactions accompanying the allosteric transition in methemoglobins.

Resonance Raman, optical absorption, and circular dichroism spectroscopic techniques have been used to examine the effect of the addition of inositol hexaphosphate (IHP) to a series of carp and human methemoglobin derivatives. Markers of spin equilibrium in the high-frequency region (1450-1650 cm-1) of the resonance Raman spectrum yield high/low-spin ratios consistent with direct magnetic susceptibility measurements. Changes in the low-frequency region (100-600 cm-1) of the resonance Raman spectrum appear to correlate with the quaternary structure transition. Changes in the ultraviolet absorption spectra and the circular dichroism spectra also appear to be related to the quaternary structure change. By using the resonance Raman spin markers, we find that those derivatives of carp methemoglobin which are in spin equilibrium have a larger ratio of high-spin to low-spin populations than the corresponding derivatives of human methemoglobin. Upon the addition of IHP to the methemoglobins the spin equilibrium is shifted toward a larger high-spin population. This change in equilibrium is larger for the carp protein than for the human protein. We obtain an IHP-induced change in the free energy difference between the high-spin and low-spin states of 300 cal/mol for those human methemoglobins in which a quaternary structure change occurs and 600 cal/mol for carp methemoglobins. Our data are consistent with a quaternary structure change induced by IHP in all the carp methemoglobins studied (F-, H2O, SCN-, NO2-, N3-, and CN-) and in the F-, H2O, and SCN- derivatives of the human protein but not in the NO2-, N3-, and CN- derivatives. The Fe-CN stretching mode has been identified by isotopic substitution and found to be unchanged in frequency in carp CN- metHb when the quaternary structure is changed. On the basis of our results we conclude that the protein forces at the heme due to the addition of IHP do not significantly affect the position of the iron atom with respect to the heme plane. Rather, the changes in spin equilibrium may be caused by protein-induced changes in the orientation of the proximal histidine or tertiary structure changes in the heme pocket which affect the porphyrin macrocycle. Either of these changes, or a combination thereof, leads to changes in the iron d orbital energies and concomitant changes in the spin equilibrium.

Allosteric Regulation↗

PubMed2001

Hemoglobin and methemoglobin concentrations after large-dose infusions of diaspirin cross-linked hemoglobin.

UNLABELLED: Diaspirin cross-linked hemoglobin (DCLHb) solution is a purified human hemoglobin product chemically stabilized to deliver oxygen to tissues. We determined the peak plasma hemoglobin concentration and assessed changes in methemoglobin concentration after the infusion of 1 g/kg DCLHb in large blood loss surgical patients. This prospective, randomized study included 26 surgical patients who were either infused with up to three 250-mL units of 10% DCLHb or transfused with up to three units of packed red blood cells during the study infusion period. Serial plasma hemoglobin, plasma methemoglobin, and whole blood methemoglobin levels were measured before and at intervals up to 48 h after the study infusion period. Plasma hemoglobin and blood methemoglobin concentrations increased during the infusion of DCLHb. The plasma hemoglobin values in the DCLHb group continued to increase during each of the infusion periods to reach a peak plasma concentration of 1450 +/- 176 mg/dL. The fraction of whole blood methemoglobin increased from 0.84 +/- 0.77% at baseline to 4.08 +/- 1.36%. With a median DCLHb dose of 936 mg/kg (range 658-1500 mg/kg), the harmonic mean half-life was 10 h, and the increased whole blood methemoglobin reached a range not associated with complications. IMPLICATIONS: The dose of diaspirin cross-linked hemoglobin (DCLHb) (936 +/- 276 mg/kg) used in this study was one of the largest reported in humans to date. The DCLHb mean half-life was 10 h. The half-life observed was 2-4 times that found at smaller doses in previous studies. Whole blood methemoglobin fraction increased during DCLHb infusion but did not reach a range associated with complications.

Aspirin↗

PubMed1988

The reduction of methemoglobin levels by antioxidants.

Preventing the oxidation of hemoglobin in solution is one of the major requirements for the successful production and long-term storage of hemoglobin-based blood substitutes. To this end we have studied the effects of antioxidants on the rate of methemoglobin formation and disappearance in solutions of human and bovine hemoglobin at 4 degrees C and 37 degrees C. Ascorbate and desferal (5 mM) were observed to act as prooxidants, increasing the rate of methemoglobin formation at 37 degrees C. Trehalose, mannitol, glucose, and EDTA (5 mM) had no significant effect. Glutathione and NADH (10 mM) were the most effective antioxidants tested, causing a significant decrease in the rate of methemoglobin formation at 37 degrees C for periods of up to 50 hours. The combination of these antioxidants in bovine hemoglobin at 4 degrees C resulted in the reduction of methemoglobin levels to nearly undetectable levels in approximately 150 hours. In addition, NADH and glutathione were found to reduce methemoglobin levels to 10% over a period of 100 hours in a sample of human hemoglobin that had been stored at 4 degrees C for one year and had 60% methemoglobin. These results suggest that the prevention and reversal of methemoglobin formation during the long-term storage of hemoglobin solutions and hemoglobin-based blood substitutes may now be possible.

Animals↗

PubMed1976

Effect of organic phosphates on methemoglobin reduction by ascorbic acid.

The rate of methemoglobin reduction by ascorbic acid was accelerated in the presence of ATP,2,3-diphosphoglycerate (2,3-DPG), and inositol hexaphosphate (IHP). The acceleration was as much as three times, four times, and ten times in the presence of ATP, 2.3-DPG, and IHP at pH 7.0, respectively. The changes of the concentrations of methemoglobin and ascorbic acid during the methemoglobin reduction were determined, and the reaction was found to proceed stoichiometrically in the presence of IHP. The reduction rate of methemoglobin by ascorbic acid was compared at different concentrations of organic phosphates (ATP,2,3-DPG, and IHP) at various pH values (6.3, 7.0, 7.7). From the changes in the reduction rate under different concentrations of organic phosphates, the dissociation constants of ATP, 2,3-DPG, and IHP to methemoglobin could be determined and were estimated to be 3.3 X 10(-4) M, 2 X 10(-3) M, and 8 X 10(-6) M at pH 7.0, respectively. On the basis of these results, the acceleration mechanism of methemoglobin reduction by ascorbic acid due to the presence of organic phosphates was described. The physiological role of 2,3-DPG in human red cells was discussed in relation to the reduction of methemoglobin by ascorbic acid.

Adenosine Triphosphate↗

PubMed1975

Conformation and spin state in methemoglobin.

The properties of human methemoglobin have been investigated under a wide variety of conditions to determine its conformation and to test for evidence of the T state conformation which has been proposed by Perutz to exist in the presence of high spin ligands and inositol hexaphosphate (IHP). Subunit dissociation was measured as a criterion for the T state since marked differences in the tetramer-dimer equilibrium exist for oxyhemoglobin (R state) and deoxyhemoglobin (T state). In the absence of IHP, complexes of methemoglobin with both high spin ligands (water, fluoride) or low spin ligands (azide, cyanide) show extensive dissociation in 2,2-bis(hydroxymethyl)-2,2',2"-nitriloethanol buffers, pH 6, 0.1 M NaCl, with values of the tetramer-dimer dissociation constant (K4,2) near 10-5 M. The addition of IHP lowers K4,2 to a value near 10-5 M for all forms of methemoglobin. Combination of IHP with methemoglobin promotes a conformational change, but the change is apparently independence of spin state. The conformation acquired in the presence of IHP is not identical with the T state (K4,2 similar to 10-12 M) and can also occur with hemoglobin in the ferrous form, as revealed by a substantial reduction in K4,2 for CO-hemoglobin upon addition of IHP. Subunit dissociation has also been measured using the haptoglobin reaction, since haptoglobin binds only to hemoglobin dimers. The haptoglobin experiments give results that are qualitatively in agreement with the conclusions reached by ultracentrifuge measurements. Similar results are also obtained by estimating the degree of dissociation on the basis of the material which aggregates following mixing with dithionite. The effect of IHP on azide-binding kinetics with methemoglobin has also been examined. Changes in reactivity is observed upon addition of IHP, but the principal effect is observed upon addition of IHP, but the principal effect is an enhancement of the rate of reaction of the beta chains. Changes in the reactivity of the beta93 sulfhydryl group of methemoglobin also accompany addition of IHP, but in a manner which is largely independent of the spin state of the iron. Similar changes are again found with CO-hemoglobin upon addition of IHP. The rate of binding of bromthymol blue also shows some changes upon addition of IHP, but the changes are more pronounced for deoxyhemoglobin than for methemoglobin. Since the results obtained did not appear to indicate a significant role for spin state in the changes observed, additional studies were undertaken using EPR spectroscopy.

Azides↗

PubMed1987

Determining methemoglobin in blood by zero-crossing-point first-derivative spectrophotometry.

We determined methemoglobin in blood by zero-crossing-point first-derivative spectrophotometry. After lysis of erythrocytes, hemoglobin was converted into oxyhemoglobin and the first derivative spectrum was recorded between 405 and 425 nm. At the exact point where the first-derivative spectrum of oxyhemoglobin was zero ("zero-crossing point"), the first-derivative value of oxyhemoglobin and methemoglobin mixture was proportional to the methemoglobin concentration. The standard curve was linear for all proportions of methemoglobin. Within-assay precision (CV) was 3.4% for a 20% methemoglobin content. Correlation with results by the Evelyn and Malloy method was very good for high proportions of methemoglobin (greater than 10%), but the proposed technique was far better for low methemoglobin percentages because of its linearity, its high sensitivity, and its low detection limit.

Humans↗

PubMed1978

Mechanism of methemoglobin reduction by ascorbic acid under anaerobic conditions.

The time course of methemoglobin reduction by ascorbic acid under anaerobic conditions was analyzed by using isoelectric focusing on Ampholine plate gel in order to compare results obtained by studies of the changes in absorption during the reaction. The intermediate hemoglobin which appeared all through the reaction was single and identified as the alpha3+beta2+ valency hybrid. In the presence of inositol hexaphosphate, reduction of methemoglobin was considerably accelerated and this acceleration was restricted to the step in which methemoglobin is reduced to the alpha3+beta2+ valency hybrid. The phase containing alpha3+beta2+ valency hybrid reduction to deoxyhemoglobin was not affected by the presence of this organic phosphate. The reaction rate constant of each phase was estimated by the analysis of the changes in three components such as methemoglobin, alpha3+beta2+ valency hybrid, and deoxyhemoglobin. The specific attack of beta-methemoglobin chains in methemoglobin tetramer by ascorbic acid was explained by the functional differences in alpha- and beta-methemoglobin chains in the protein.

Anaerobiosis↗