Proposed Mechanism of Nitrite-Induced Methemoglobinemia

GKT137831 and hydrogen peroxide increase the release of 6-nitrodopamine from the human umbilical artery, rat-isolated right atrium, and rat-isolated vas deferens

Introduction: The human umbilical artery (HUA), rat-isolated right atrium, and rat-isolated vas deferens present a basal release of 6-nitrodopamine (6-ND). The basal release of 6-ND from these tissues was significantly decreased (but not abolished) when the tissues were pre-incubated with Nω-nitro-L-arginine methyl ester (L-NAME). Methods: In this study, the effect of the pharmacological modulation of the redox environment on the basal release of 6-ND was investigated. The basal release of 6-ND was measured using Liquid chromatography with tandem mass spectrometry (LC-MS/MS). Results and Discussion: Pre-incubation (30 min) of the tissues with GKT137831 (1 μM) caused a significant increase in the basal release of 6-ND from all tissues. In the HUA, pre-incubation with diphenyleneiodonium (DPI) (100 μM) also caused significant increases in the basal release of 6-ND. Preincubation of the HUA with hydrogen peroxide (H2O2) (100 μM) increased 6-ND basal release, whereas pre-incubation with catalase (1,000 U/mL) significantly decreased it. Pre-incubation of the HUA with superoxide dismutase (SOD) (250 U/mL; 30 min) also significantly increased the basal release of 6-ND. Preincubation of the HUA with either allopurinol (100 μM) or uric acid (1 mM) had no effect on the basal release of 6-ND. Pre-treatment of the HUA with L-NAME (100 μM) prevented the increase in the basal release of 6-ND induced by GKT137831, diphenyleneiodonium, and H2O2. The results obtained indicate a major role of endogenous H2O2 and peroxidases as modulators of 6- ND biosynthesis/release and a lack of peroxynitrite contribution.

Optimizing transcardial perfusion of small molecules and biologics for brain penetration and biodistribution studies in rodents

Efficiently removing blood from the brain vasculature is critical to evaluate accurately the brain penetration and biodistribution of drug candidates, especially for biologics as their blood concentrations are substantially higher than the brain concentrations. Transcardial perfusion has been used widely to remove residual blood in the brain; however, the perfusion conditions (such as the perfusion rate and time) reported in the literature are quite varied, and the performance of these methods on blood removal has not been investigated thoroughly. In this study, the effectiveness of the perfusion conditions was assessed by measuring brain hemoglobin levels. Sodium nitrite (NaNO₂ ) as an additive in the perfusate was evaluated at different concentrations. Blood removal was significantly improved with 2% NaNO₂ over a 20 min perfusion in mouse without disrupting the integrity of the blood-brain barrier (BBB). In mice, the optimized perfusion method significantly lowered the measured brain-to-plasma ratio (K_p,brain ) for monoclonal antibodies due to the removal of blood contamination and small molecules with a moderate-to-high BBB permeability and with a high brain-unbound-fraction (f_u,brain ) presumably due to flux out of the brain during perfusion. Perfusion with or without NaNO₂ clearly removed the residual blood in rat brain but with no difference observed in K_p,brain between the perfusion groups with or without 2% NaNO₂ . In conclusion, a perfusion method was successfully developed to evaluate the brain penetration of small molecules and biologics in rodents for the first time. The transcardial perfusion with 2% NaNO₂ effectively removed the residual blood in the brain and significantly improved the assessment of brain penetration of biologics. For small molecules, however, transcardial perfusion may not be performed, as small molecule compounds could be washed away from the brain by the perfusion procedure.

Preclusion of methemoglobinemia caused by nitrate drugs in diabetics and nondiabetics: Possible role of Vitamin C

The drugs containing nitrates like isosorbide dinitrate, isosorbide mononitrate and glyceryl trinitrate, etc., trigger the oxidation of hemoglobin which is manifested in the pathological disorder named methemoglobinemia. It was considered interesting to investigate the preventive roles of vitamin C towards the toxic effects of nitrate containing drugs used for the treatment of angina. The aim is to find whether these drugs need to be administered with special care to diabetic patients who are more prone to develop methemoglobinemia. Vitamin C (500 mg/day) was administered orally to reduce the methemoglobin (metHb) level in both the diabetic and nondiabetic patients consuming nitrate containing drugs regularly, keeping diabetic and nondiabetic patients not on nitrate drugs as control. Concentration of metHb and hemoglobin A (HbA) was estimated spectrophotometrically assuming the molar extinction coefficient values of metHb as 3.78 mM^--1 cm^--1 at 630 nm and HbA as 125,000 M ^--1 cm ^--1 at 415 nm. MetHb level was found to be lower after the treatment with vitamin C for 30 consecutive days than that before the trial with statistically significant two tailed p value. Additionally, fasting insulin level was also found to decrease after 4 weeks of consumption of vitamin C with moderate lowering of fasting serum glucose level as well, indicating a higher insulin sensitivity for the treated patients.

Analyte-restrained silver coating of gold nanostructures: an efficient strategy to advance multicolorimetric probes

Visual detection based on gold nanorods (AuNRs) has gained tremendous attention in sensing applications owing to the potential for simple, inexpensive, instrument-free, and on-site detection. The proper selection of the mechanism involved in the interaction between the analyte and the nanostructure plays a significant role in designing a selective and multicolorimetric probe for visual purposes. A winning mechanism to develop multicolorimetric probes is the silver metalization of AuNRs. Herein, an unprecedented idea is presented to expand the variety of multicolorimetric sensors relying on the mechanism of silver deposition. We introduce the anti-silver deposition mechanism in which the analyte directly or indirectly restrains the silver coating of AuNRs. To ascertain the anti-silver deposition mechanism, we have exploited the proposed idea for the direct detection of nitrate. The presence of nitrate (as restrainer agent), which was firstly treated with ascorbic acid (as reducing agent), induced a decrease in the spectral blueshift of AuNRs along with diverse sharp color transitions from reddish-orange (blank) to maroon, wine, berry/purple, dark blue, teal, green, seafoam, and mint. The difference in the spectrum area of the probe in the absent (S_o) and presence (S) of nitrate were linearly proportional to nitrate concentration in the range of 0.5-5.5 mmol l^-1and the limit of detection was calculated to be 465μmol l^-1. Furthermore, the practicability of the multicolor probe was assessed by the determination of nitrate in complex environmental samples.

Thiol-mediated etching of gold nanorods as a neoteric strategy for room-temperature and multicolor detection of nitrite and nitrate

The excessive presence of nitrite and nitrate in environmental matrixes has raised concerns among the scientific communities due to their negative impacts on human health and living organisms. Considering the necessity of regular monitoring and rapid evaluation of nitrite and nitrate, it is of great interest to develop methods capable of on-site detection of these compounds. This study presents a non-aggregation colorimetric method based on etching gold nanorods (AuNRs) for visual detection of nitrite and nitrate. Instead of temperature, we propose using thiourea as a sulfur-containing compound to accelerate the rate of AuNR etching. Thiourea forms stable cationic species with Au⁺ ions and consequently speeds up the etching process by reducing the redox potential of Au⁺/Au. In the presence of thiourea, the AuNRs are etched by nitrite resulting in wide obvious color changes from brown to light brown, green, blue, purple, pink, and colorless. In addition to nitrite, the developed method is capable of nitrate determination by reducing nitrate to nitrite through acid-washed zinc powder and is the first report of colorful detection of nitrate. Under optimized conditions, a good linear relationship was found between nitrite/nitrate concentration and the colorimetric response in the range of 8.0 to 100 μmol L^-1 and 0.5 to 3.0 mmol L^-1 with a limit of detection (LOD) as low as 1.3 μmol L^-1 and 173.3 μmol L^-1 for nitrite and nitrate, respectively. Furthermore, the practical application of our developed probe was confirmed by accurate determination of nitrite and nitrate in various complex media including water samples, soil extracts, and food products such as salami and sausage.

Toxicity of sodium nitrite-based vertebrate pesticides for European starlings (Sturnus vulgaris)

In the 21st century, invasive animals rank second only to habitat destruction as the greatest threat to global biodiversity. Socially-acceptable and cost-effective strategies are needed to reduce the negative economic and environmental impacts of invasive animals. We investigated the potential for sodium nitrite (SN; CAS 7632-00-0) to serve as an avian toxicant for European starlings (Sturnus vulgaris L.). We also assessed the non-target hazard of an experimental formulation of SN that is being developed as a toxicant for invasive wild pigs (Sus scrofa L.). In gavage experiments with European starlings, we identified a lowest observed adverse effect level (LOAEL) for mortality of 2.40% technical SN (w/v; 120 mg SN/kg body mass) and a no observed adverse effect level (NOAEL) for mortality of 1.30% technical SN (65 mg/kg). The exposure of ten starlings to the experimental formulation of SN (10% SN pig toxicant) resulted in one starling mortality during four days of exposure to the toxic bait. Sodium nitrite toxicity presented a moderate hazard to European starlings; thus, the future development of SN as an avian toxicant is dependent upon its cost-effectiveness. We discuss the management of toxic effects and non-target hazards of SN for wild birds, including best practices for toxic baiting of vertebrate pests and management of invasive wild pigs.

Plasmonic nanoparticles for colorimetric detection of nitrite and nitrate

Irregular and unknowingly use of chemical compounds is a serious threat to the environment, human health, and other living organisms attributable and intensified by the growing population and increasing demand for food. Nitrite and nitrate are among those compounds that are widely used in agricultural and industrial products. Therefore on-site, rapid, simple, and accurate monitoring of nitrite/nitrate is highly desirable. In this review, while emphasizing the importance of nitrite and nitrate in food chain safety and health of living organisms, their measurement methods, in particular, nanoplasmonic colorimetric sensors are comprehensively discussed based on the researches in this field. Nanoplasmonic-based sensors have proved to be successful in comparison with traditional methods due to their low cost, biocompatibility, high sensitivity and selectivity, and most importantly, the ability to visually detect and be used on-site to measure nitrite and nitrate. The design principle of nanoplasmonic sensors will be presented into two categories of aggregation- and etching-based detection followed by their applications in nitrite detection. The nitrate measurement will be discussed based on either direct detection of nitrate or indirect strategy in which nitrate is reduced to nitrite by enzymes or metals. Finally, the remaining challenges and prospects in this topic will be described and outlined.

Risk assessment of nitrate and nitrite in feed

The European Commission asked EFSA for a scientific opinion on the risks to animal health related to nitrite and nitrate in feed. For nitrate ion, the EFSA Panel on Contaminants in the Food Chain (CONTAM Panel) identified a BMDL 10 of 64 mg nitrate/kg body weight (bw) per day for adult cattle, based on methaemoglobin (MetHb) levels in animal's blood that would not induce clinical signs of hypoxia. The BMDL 10 is applicable to all bovines, except for pregnant cows in which reproductive effects were not clearly associated with MetHb formation. Since the data available suggested that ovines and caprines are not more sensitive than bovines, the BMDL 10 could also be applied to these species. Highest mean exposure estimates of 53 and 60 mg nitrate/kg bw per day in grass silage-based diets for beef cattle and fattening goats, respectively, may raise a health concern for ruminants when compared with the BMDL 10 of 64 mg nitrate/kg bw per day. The concern may be higher because other forages might contain higher levels of nitrate. Highest mean exposure estimates of 2.0 mg nitrate/kg bw per day in pigs' feeds indicate a low risk for adverse health effects, when compared with an identified no observed adverse effect level (NOAEL) of 410 mg nitrate/kg bw per day, although the levels of exposure might be underestimated due to the absence of data on certain key ingredients in the diets of this species. Due to the limitations of the data available, the CONTAM Panel could not characterise the health risk in species other than ruminants and pigs from nitrate and in all livestock and companion animals from nitrite. Based on a limited data set, both the transfer of nitrate and nitrite from feed to food products of animal origin and the nitrate- and nitrite-mediated formation of N-nitrosamines and their transfer into these products are likely to be negligible.

Detecting Methemoglobinemia in Animals with a Drop of Blood

A major concern during pesticide development and use is the impact on non-target species, such as raptors or domestic cats and dogs. Sodium nitrite and para-aminopropiophenone (PAPP) are two toxicants currently being studied for the control of invasive species, such as starlings and feral swine. When given to an animal these compounds oxidize hemoglobin, which renders it unable to carry oxygen resulting in methemoglobinemia. This study developed a method to estimate methemoglobin levels in mammals and birds by examining the efficacy of sodium nitrite to induce the conversion of hemoglobin to methemoglobin. Varying concentrations of sodium nitrite were added to aliquots of coyote, vole, feral swine, starling, and duck blood, collected from captive animals. The blood samples were analyzed spectrophotometrically to determine percent methemoglobin and digitally to determine red color values (RCV) associated with different methemoglobin levels. The avian and mammalian blood reached 100% methemoglobin levels at 200 mM and 15 mM sodium nitrite, respectively. All animals had similar RCV for a given percent methemoglobin. In conclusion, this study developed a procedure to quickly determine methemoglobin levels in mammals and birds. Furthermore, percent methemoglobin can be estimated with one standard curve from any animal species and an image of a blood spot. The technique will be useful during field studies, in agricultural areas, or in a veterinarian’s office for the rapid diagnosis of methemoglobinemia in non-target animals that have eaten toxicants/baits or baited animals.

Nitrite and nitroso compounds can serve as specific catalase inhibitors

OBJECTIVE

We present evidence that nitrite and nitrosothiols, nitrosoamines and non-heme dinitrosyl iron complexes can reversibly inhibit catalase with equal effectiveness.

METHODS

Catalase activity was evaluated by the permanganatometric and calorimetric assays.

RESULTS

This inhibition is not the result of chemical transformations of these compounds to a single inhibitor, as well as it is not the result of NO release from these substances (as NO traps have no effect on the extent of inhibition). It was found that chloride and bromide in concentration above 80 mM and thiocyanate in concentration above 20 μM enhance catalase inhibition by nitrite and the nitroso compounds more than 100 times. The inhibition degree in this case is comparable with that induced by azide.

DISCUSSION

We propose that the direct catalase inhibitor is a positively charged NO-group. This group acquires a positive charge in the active center of enzyme by interaction of nitrite or nitroso compounds with some enzyme groups. Halides and thiocyanate protect the NO⁺ group from hydration and thus increase its inhibition effect. It is probable that a comparatively low chloride concentration in many cells is the main factor to protect catalase from inhibition by nitrite and nitroso compounds.

Involvement of ferryl in the reaction between nitrite and the oxy forms of globins

The reaction between nitrite and the oxy forms of globins has complex autocatalytic kinetics with several branching steps and evolves through chain reactions mediated by reactive species (including radicals) such as hydrogen peroxide, ferryl and nitrogen dioxide, starting with a lag phase, after which it proceeds onto an autocatalytic phase. Reported here are UV-Vis spectra collected upon stopped-flow mixing of myoglobin with a supraphysiological excess of nitrite. The best fit to the experimental data follows an A → B → C reaction scheme involving the formation of a short-lived intermediate identified as ferryl. This is consistent with a mechanism where nitrite binds to oxy myoglobin to generate an undetectable ferrous-peroxynitrate intermediate, whose decay leads to nitrate and ferryl. The ferryl is then reduced to met by the excess nitrite. DFT calculations reveal an essentially barrierless reaction between nitrite and the oxy heme, with a notable outer-sphere component; the resulting metastable ferrous-peroxynitrate adduct is found to feature a very low barrier towards nitrate liberation, with ferryl as a final product-in good agreement with experiment.

A case of almagel-induced methemoglobinemia after pancreatoduodenal resection

We are reporting the case of severe methemoglobinemia resulting from Almagel which was given for treatment of gastrointestinal ulcers after pancreatoduodenal resection. A 64 year old man was treated with Almagel A postoperatively. One day after treatment began, the patient reported what appeared to be cyanotic discolorations of his lips and fingers. (SpO2 80%, normal pO2). Two days later, the patient was diagnosed with methemoglobinemia via co-oximetry and treated with methylene blue. The causes of methemoglobinemia remained unclear, and Almagel A was reinitiated until the second episode of methemoglobinemia, was diagnosed and treated promptly. The dose of benzocaine in Almagel A (100 mg/5 ml) was sufficient enough to cause methemoglobinemia, confirmed using multiwavelength co-oximeter and successfully treated with methylene blue.

Crystallographic trapping of heme loss intermediates during the nitrite-induced degradation of human hemoglobin

Heme is an important cofactor in a large number of essential proteins and is often involved in small molecule binding and activation. Loss of heme from proteins thus negatively affects the function of these proteins but is also an important component of iron recycling. The characterization of intermediates that form during the loss of heme from proteins has been problematic, in a large part, because of the instability of such intermediates. We have characterized, by X-ray crystallography, three compounds that form during the nitrite-induced degradation of human α(2)β(2) hemoglobin (Hb). The first is an unprecedented complex that exhibits a large β heme displacement of 4.8 Å toward the protein exterior; the heme displacement is stabilized by the binding of the distal His residue to the heme Fe, which in turn allows for the unusual binding of an exogenous ligand on the proximal face of the heme. We have also structurally characterized complexes that display regiospecific nitration of the heme at the 2-vinyl position; we show that heme nitration is not a prerequisite for heme loss. Our results provide structural insight into a possible pathway for nitrite-induced loss of heme from human Hb.

Structure of human R-state aquomethemoglobin at 2.0 Å resolution

The crystal structure of tetrameric (αβ)(2) R-state human adult aquomethemoglobin is reported at 2.0 Å resolution. The asymmetric unit contained one αβ subunit pair. The R-state crystal belonged to space group P4(1)2(1)2, with unit-cell parameters a = b = 53.6, c = 192.8 Å. An Fe-bound water molecule was modeled into the heme distal pockets of each of the α and β subunits. In the α subunit, a highly ordered liganded water was modeled with an Fe-O(water) distance of 2.2 Å and appears to be protected against escape from the distal pocket by the conformation of the heme propionate groups, which point upwards towards the distal His58 residue aided by a hydrogen-bonding network involving the solvent. In the β subunit, the liganded water exhibited greater motion and was modeled with a longer Fe-O(water) distance of 2.5 Å; in this subunit both propionate groups point downwards away from the distal His63 residue, presumably allowing greater motion of the liganded water in and out of the distal pocket.

The nitrite anion binds to human hemoglobin via the uncommon O-nitrito mode

The nitrite anion is known to oxidize and degrade hemoglobin (Hb). Recent literature reports suggest a nitrite reductase activity for Hb, converting nitrite into nitric oxide. Surprisingly, no structural information about Hb-nitrite interactions has been reported. We have determined the crystal structure of the ferric Hb-nitrite complex at 1.80 A resolution. The nitrite ligand adopts the uncommon O-nitrito binding mode. In addition, the nitrito conformations in the alpha and beta subunits are different, reflecting subtle effects of the distal His in orienting the nitrite ligand in the O-nitrito binding mode.

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.

The reaction between nitrite and oxyhemoglobin: a mechanistic study

The nitrite anion (NO(-)(2)) has recently received much attention as an endogenous nitric oxide source that has the potential to be supplemented for therapeutic benefit. One major mechanism of nitrite reduction is the direct reaction between this anion and the ferrous heme group of deoxygenated hemoglobin. However, the reaction of nitrite with oxyhemoglobin (oxyHb) is well established and generates nitrate and methemoglobin (metHb). Several mechanisms have been proposed that involve the intermediacy of protein-free radicals, ferryl heme, nitrogen dioxide (NO(2)), and hydrogen peroxide (H(2)O(2)) in an autocatalytic free radical chain reaction, which could potentially limit the usefulness of nitrite therapy. In this study we show that none of the previously published mechanisms is sufficient to fully explain the kinetics of the reaction of nitrite with oxyHb. Based on experimental data and kinetic simulation, we have modified previous models for this reaction mechanism and show that the new model proposed here is consistent with experimental data. The important feature of this model is that, whereas previously both H(2)O(2) and NO(2) were thought to be integral to both the initiation and propagation steps, H(2)O(2) now only plays a role as an initiator species, and NO(2) only plays a role as an autocatalytic propagatory species. The consequences of uncoupling the roles of H(2)O(2) and NO(2) in the reaction mechanism for the in vivo reactivity of nitrite are discussed.

Methemoglobin--it's not just blue: a concise review

Hemoglobin has functions besides carrying oxygen to the tissues, and regulates vascular tone and inflammation via a redox couple with methemoglobin. Hemoglobin has iron in the reduced valance Fe(II) and methemoglobin has iron in the oxidized valance Fe (III), with a free energy capable of producing water from oxygen. In generating methemoglobin the couple functions as a nitrite reductase. The degree of oxidation of hemoglobin senses the oxygen level in the blood and uses its ability to produce nitric oxide from nitrite to control vascular tone, increasing blood flood when the proportion of oxygenated hemoglobin falls. Additional cardiovascular damage is produced by methemoglobin mediated oxidation of light density lipoproteins, accelerating arteriosclerosis. In addition, the release of heme from methemoglobin is an important factor in inflammation. These physiologic functions are paralleled by the well-described role in the oxidation of various drugs resulting in methemoglobinemia.

Methemoglobinemia: An Industrial Outbreak Among Rubber Molding Workers

OBJECTIVE

This case series documents an occupational outbreak of methemoglobinemia among five steam press operators at a rubber plant. Investigative findings identified the cause as repeated exposure (through manual handling) to an adhesive containing dinitrobenzene.

RESULTS

The workers presented with yellow-stained hands and a variety of clinical manifestations. Methemoglobinemia levels obtained in the emergency room ranged from 3.8% to 41.2%. Methylthioninium chloride (methylene blue) rapidly reversed the cyanosis and alleviated associated symptoms in the rubber molding workers requiring treatment.

CONCLUSIONS

Prompt action by plant officials and subsequent investigation by National Institute of Occupational Safety and Health/Occupational Safety & Health Administration personnel resulted in the timely and successful resolution of the problem. Individual presentations, assessment, and management are discussed along with recommendations for occupational investigation and referral.