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

Air Pollution and Perinatal Mental Health: A Comprehensive Overview

BACKGROUND

The aim of the present study was to summarise the available data about the link between air pollution exposure and the new-onset and severity of psychiatric disorders in pregnant women during the perinatal period.

MATERIALS AND METHODS

We selected articles published until June 2022 on PubMed and the Web of Science. Pollutants included were PM_2.5 (particulate matter 2.5 micrometres and smaller), PM₁₀ (particulate matter 10 micrometres and smaller), NO₂ (nitrogen dioxide), O₃ (ozone), SO₂ (sulphur dioxide), CO (carbon monoxide), PBDEs (polybrominated diphenyl ethers), PFAS (per- and polyfluoroalkyl substances), lead, and cadmium. The perinatal period was considered as the time of pregnancy until one year after childbirth.

RESULTS

Nine studies were included; most of them evaluated the association between exposure to air pollutants and the onset of Postpartum Depression (PPD). Two studies showed an association between, respectively, only PM_2.5 and both PM_2.5 and NO₂ exposure and PPD onset 12 months after childbirth, while another study found a significant association between NO₂ exposure and PPD occurrence 6 months after childbirth. PBDE blood levels were associated with more severe depressive symptoms. Lastly, one study observed a link between stressful symptoms and exposure to PM_2.5, PM₁₀ during pregnancy.

CONCLUSION

More comprehensive and uniform studies are required to make a roadmap for future interventions, given the growing relevance of issues such pollution and mental health, particularly during the perinatal period.

Black Urine and Methemoglobinemia in the Setting of Sepsis Due to Clostridium Perfringens

Clostridium Perfringens is an anaerobic gram-positive bacillus able to produce different types of toxins and can cause septicemia. The mechanism is through translocation from a previously colonized gastrointestinal or genital tract. Massive intravascular hemolysis induced by this bacterium is a rare presentation reported in only 7% to 15% of cases of Clostridium Perfringens bacteremia with a mortality rate reaching 90%.We present the case of a middle-aged man with metastatic melanoma having black-colored urine as the first sign of massive hemolysis along with mild methemoglobinemia. Despite timely management, the patient progressed into septic shock with severe hypoxia and passed away. Postmortem, blood cultures grew clostridium perfringens. Black-colored urine and blood samples, sepsis-induced mild methemoglobinemia and acute massive hemolysis should raise concern for Clostridium Perfringens sepsis in the appropriate clinical settings.

The Interplay between Molten Globules and Heme Disassociation Defines Human Hemoglobin Disassembly

Hemoglobin functions as a tetrameric oxygen transport protein, with each subunit containing a heme cofactor. Its denaturation, either in vivo or in vitro, involves autoxidation to methemoglobin, followed by cofactor loss and globin unfolding. We have proposed a global disassembly scheme for human methemoglobin, linking hemin (ferric protoporphyrin IX) disassociation and apoprotein unfolding pathways. The model is based on the evaluation of circular dichroism and visible absorbance measurements of guanidine-hydrochloride-induced disassembly of methemoglobin and previous measurements of apohemoglobin unfolding. The populations of holointermediates and equilibrium disassembly parameters were estimated quantitatively for adult and fetal hemoglobins. The key stages are characterized by hexacoordinated hemichrome intermediates, which are important for preventing hemin disassociation from partially unfolded, molten globular species during early disassembly and late-stage assembly events. Both unfolding experiments and independent small angle x-ray scattering measurements demonstrate that heme disassociation leads to the loss of tetrameric structural integrity. Our model predicts that after autoxidation, dimeric and monomeric hemichrome intermediates occur along the disassembly pathway inside red cells, where the hemoglobin concentration is very high. This prediction suggests why misassembled hemoglobins often get trapped as hemichromes that accumulate into insoluble Heinz bodies in the red cells of patients with unstable hemoglobinopathies. These Heinz bodies become deposited on the cell membranes and can lead to hemolysis. Alternatively, when acellular hemoglobin is diluted into blood plasma after red cell lysis, the disassembly pathway appears to be dominated by early hemin disassociation events, which leads to the generation of higher fractions of unfolded apo subunits and free hemin, which are known to damage the integrity of blood vessel walls. Thus, our model provides explanations of the pathophysiology of hemoglobinopathies and other disease states associated with unstable globins and red cell lysis and also insights into the factors governing hemoglobin assembly during erythropoiesis.

Blood-Related Toxicity after Traumatic Brain Injury: Potential Targets for Neuroprotection

Emergency visits, hospitalizations, and deaths due to traumatic brain injury (TBI) have increased significantly over the past few decades. While the primary early brain trauma is highly deleterious to the brain, the secondary injury post-TBI is postulated to significantly impact mortality. The presence of blood, particularly hemoglobin, and its breakdown products and key binding proteins and receptors modulating their clearance may contribute significantly to toxicity. Heme, hemin, and iron, for example, cause membrane lipid peroxidation, generate reactive oxygen species, and sensitize cells to noxious stimuli resulting in edema, cell death, and increased morbidity and mortality. A wide range of other mechanisms such as the immune system play pivotal roles in mediating secondary injury. Effective scavenging of all of these pro-oxidant and pro-inflammatory metabolites as well as controlling maladaptive immune responses is essential for limiting toxicity and secondary injury. Hemoglobin metabolism is mediated by key molecules such as haptoglobin, heme oxygenase, hemopexin, and ferritin. Genetic variability and dysfunction affecting these pathways (e.g., haptoglobin and heme oxygenase expression) have been implicated in the difference in susceptibility of individual patients to toxicity and may be target pathways for potential therapeutic interventions in TBI. Ongoing collaborative efforts are required to decipher the complexities of blood-related toxicity in TBI with an overarching goal of providing effective treatment options to all patients with TBI.

Methaemoglobinaemia risk factors with inhaled nitric oxide therapy in newborn infants

BACKGROUND

Inhaled nitric oxide (iNO), commonly used for hypoxic neonates, may react with haemoglobin to form methaemoglobin (MetHb). MetHb monitoring during iNO therapy has been questioned since low doses of iNO are used.

AIM

To evaluate the incidence of and identify risk factors associated with elevated MetHb in neonates treated with iNO.

METHODS

Neonates who were treated with iNO and had at least one MetHb measurement were included. Demographic characteristics and methods of iNO administration (dosage, duration) at the time of each MetHb measurement were analysed.

RESULTS

Four hundred and fifty-two MetHb measurements from 81 premature and 82 term and near-term infants were analysed. MetHb was above 5% in one-term infant, and between 2.5-5% in 16 infants. A higher maximum dose of iNO (22.7 vs 17.7 p.p.m.), but not gestational age, was a significant risk factor for elevated MetHb. Significantly higher oxygen levels (75.5% vs 51.7%) were associated with higher MetHb in term infants. Preterm infants had no risk for high MetHb when iNO was kept below 8 p.p.m. These data suggest the possibility of limiting blood withdrawal when low doses iNO are used.

CONCLUSION

High MetHb is exceptional in neonates treated with low dose iNO. Associated risk factors are related to high iNO dose and the simultaneous use of high concentrations of oxygen.

Prevention of hemoglobin interference on the formazan reaction

BACKGROUND

Hb interference induces false test results and thus hinders the establishment of further applications that use the formazan method. We attempted to eliminate hemoglobin (Hb) interference on the formazan reaction.

METHODS

Total serum bile acid (TBA) reagent was constructed to elucidate the mechanism of Hb interference. We determined the interaction between formazan reagents and Hb using spectral analysis and investigated the optimal condition of the reagent to establish a practical technique for its prevention.

RESULTS

Hb caused overestimation during TBA measurement, and its influence was attributed to the errors of both the sample and reagent blank signals. These errors were triggered by the reaction between Hb and formazan reagents. The main finding was that the addition of both imidazole and sodium nitrite in the reagent considerably accelerated the oxidation of Hb, even at neutral pH. The overestimation of TBA concentration was reduced to <3 micromol/l even with Hb concentrations >or=500 mg/dl.

CONCLUSIONS

We elucidated the mechanism of Hb interference on the formazan reaction and succeeded in preventing its influence. This will help to solve the technical difficulties associated with utilizing formazan reactions as routine diagnostic tools.

The existence and significance of a mitochondrial nitrite reductase

The physiological functions of nitric oxide (NO) are well established. The finding that the endothelium-derived relaxing factor (EDRF) is NO was totally unexpected. It was shown that NO is a reaction product of an enzymatically catalyzed, overall, 5-electron oxidation of guanidinium nitrogen from L-arginine followed by the release of the free radical species NO. NO is synthesized by a single protein complex supported by cofactors, coenzymes (such as tetrahydrobiopterin) and cytochrome P450. The latter can uncouple from substrate oxidation producing O2*- radicals. The research groups of Richter [Ghafourifar P, Richter C. Nitric oxide synthase activity in mitochondria. FEBS Lett 1997; 418: 291-296.] and Boveris [Giulivi C, Poderoso JJ, Boveris A. Production of nitric oxide by mitochondria. J Biol Chem 1998; 273: 11038-11043.] identified a mitochondrial NO synthase (NOS). There are, however, increasing reports demonstrating that mitochondrial NO is derived from cytosolic NOS belonging to the Ca2+-dependent enzymes. NO was thought to control cytochrome oxidase. This assumption is controversial due to the life-time of NO in biological systems (millisecond range). We found a nitrite reductase in mitochondria which is of major interest. Any increase of nitrite in the tissue which is the first oxidation product of NO, for instance following NO donors, will stimulate NO-recycling via mitochondrial nitrite reductase. In this paper, we describe the identity and the function of mitochondrial nitrite reductase and the consequences of NO-recycling in the metabolic compartment of mitochondria.

The Role of Nitric Oxide Synthase Inhibition in the Adverse Effects of Etomidate in the Setting of Focal Cerebral Ischemia in Rats

We evaluated the effect of N(G)-nitro-L-arginine-methyl-ester (l-NAME, a nitric oxide synthase [NOS] inhibitor) and L-arginine (nitric oxide substrate) on cerebral mitochondrial dysfunction (hereafter referred to as "injury") after temporary middle cerebral artery occlusion (MCAo) during halothane or etomidate anesthesia in spontaneously hypertensive rats. Sixty minutes before MCAo, rats were randomized to 1 of 5 regimens (n = 8 per group): h/control, 1.2 minimum alveolar anesthetic concentration of halothane; h/L-NAME, 1.2 minimum alveolar anesthetic concentration of halothane and L-NAME (30 mg/kg); etomidate, an electroencephalographic (EEG) burst suppression dose of etomidate; e/L-NAME, an EEG burst suppression dose of etomidate and L-NAME (30 mg/kg); or e/L-NAME/arg, an EEG burst suppression dose of etomidate, L-NAME (30 mg/kg), and L-arginine (bolus of 300 mg/kg with an infusion at 35 mg x kg(-1) x min(-1)). After 180 min of MCAo and 120 min of reperfusion, volume of injury was determined using 2,3,5-triphenytetrazolium stain. Injury volume (mm(3), mean +/- sd) was larger in the etomidate group (153 +/- 17) than the halothane anesthetized h/control group (93 +/- 16) (P < 0.05) but did not differ between the e/L-NAME (162 +/- 17) and h/L-NAME groups (155 +/- 26). Injury volume in the e/L-NAME/arg group (88 +/- 15) was not different from the h/control group (93 +/- 16) and was less than that in either the etomidate or the e/L-NAME groups (P < 0.05). The data reproduce our previous observation that, relative to a halothane-anesthetized control state, etomidate has an adverse effect on ischemic injury in the setting of temporary focal cerebral ischemia. Prior inhibition of NOS with L-NAME resulted in no difference in the volume of injury between groups receiving etomidate or halothane (162 +/- 17 versus 155 +/- 26). Administration of a large dose of L-arginine prevented the adverse effect of etomidate. The data were obtained after only 2 h of reperfusion and therefore cannot be construed as representative of final neurologic outcome. They nonetheless suggest that etomidate produces an adverse effect on mitochondrial function early in the course of focal cerebral ischemia, in part, by inhibition of NOS.

Ascorbate enhances iNOS activity by increasing tetrahydrobiopterin in RAW 264.7 cells

Studies on the effect of ascorbic acid on inducible nitric oxide synthase (iNOS) activity are few and diverse, likely to be dependent on the species of cells. We investigated a role of ascorbic acid in iNOS induction and nitric oxide (NO) generation in mouse macrophage cell line RAW 264.7. Although interferon- (IFN-) gamma alone produced NO end products, ascorbic acid enhanced NO production only when cells were synergistically stimulated with IFN-gamma plus Escherichia coli lipopolysaccharide (LPS). Ascorbate neither enhanced nor decreased the expression of iNOS protein in RAW 264.7 cells, in contrast to the reports that ascorbic acid augments iNOS induction in a mouse macrophage-like cell line J774.1 and that ascorbate suppresses iNOS induction in rat skeletal muscle endothelial cells. Intracellular levels of tetrahydrobiopterin (BH4), a cofactor for iNOS, were increased by ascorbate in RAW 264.7 cells. However, ascorbate did not increase GTP cyclohydrolase I mRNA, the main enzyme at the critical steps in the BH4 synthetic pathway, expression levels and activity. Sepiapterin, which supplies BH4 via salvage pathway, more efficiently enhanced NO production if ascorbate was added. These data suggest that enhanced activation of iNOS by ascorbic acid is mediated by increasing the stability of BH4 in RAW 264.7 cells.

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.

Combined effects of inhaled nitric oxide (iNO) and oxidant agents on the production of methemoglobinemia in newborn piglets

OBJECTIVE

To investigate the effects of the association of inhaled nitric oxide (iNO) and oxidant drugs (acetaminophen, phytomenadione, and EMLA cream) on methemoglobinemia during the neonatal period.

DESIGN

Prospective, randomized, experimental study.

SETTING

University Experimental Pharmacology laboratory.

SUBJECTS

Sixty newborn piglets weighing 1.5-2.0 Kg.

INTERVENTIONS

Twelve groups of five piglets were anaesthetized, mechanically ventilated, and studied for 3 hrs. Eight groups received iNO (40 ppm or 80 ppm) alone or in association with a single intravenous dose of acetaminophen (120 mg/kg propacetamol), phytomenadione (5 mg vitamin K1) or EMLA cream (2.5 g) applied to the ventral lower abdomen for 3 hrs. Three other groups received, respectively, acetaminophen, phytomenadione, or EMLA cream without iNO. The last group (control group) received neither drugs nor iNO.

MEASUREMENTS AND MAIN RESULTS

Methemoglobinemia was measured before the beginning of each experiment, 30 mins later, and every hour for 3 hrs. There was no significant difference in methemoglobinemia at any time between groups receiving acetaminophen (0.90%+/-0.12%), phytomenadione (0.88%+/-0.11%), or EMLA cream alone (0.97%+/-0.11%) and the control group (0.92%+/-0.12%). At 3 hrs, methemoglobinemia was slightly but significantly increased in group receiving iNO alone (1.04%+/-0.17% at 40 ppm iNO and 1.14%+/-0.16% at 80 ppm iNO; p < .05). Conversely, methemoglobinemia increased as a function of time in groups in which iNO was associated to drug administration and was significantly greater than the control group at 3 hrs (80 ppm iNO + acetaminophen, 2.80%+/-0.47%; 80 ppm iNO + phytomenadione, 2.38%+/-0.45%; 80 ppm iNO + EMLA cream, 2.33%+/-046%; p < .001).

CONCLUSIONS

These results demonstrate that if oxidant drugs (acetaminophen, phytomenadione, or EMLA cream) did not increase blood methemoglobinemia in neonatal piglets, their association with iNO caused an increase in methemoglobin. Special care should be taken to monitor methemoglobinemia when iNO is combined to such drugs in newborn infants.

Effects of oxidised alpha-lipoic acid and alpha-tocopherol on xenobiotic-mediated methaemoglobin formation in diabetic and non-diabetic human erythrocytes in-vitro

The effects of oxidised alpha-lipoic acid and alpha-tocopherol were investigated on a human erythrocytic in vitro model of diabetic metabolic stress. Preincubation of non-diabetic and diabetic erythrocytes with oxidised alpha-lipoic acid or alpha-tocopherol resulted in marked increases in nitrite-mediated methaemoglobin formation. In contrast, oxidised alpha-lipoic acid resulted in considerable reductions in 4-aminophenol-mediated methaemoglobin formation in both diabetic and non-diabetic cells. alpha-Tocopherol showed an increase only in diabetic cells, at one time point. Monoacetyl dapsone hydroxylamine (MADDS-NHOH)-mediated methaemoglobin formation was reduced by oxidised alpha-lipoic acid in non-diabetic and diabetic cells at all three time points, although alpha-tocopherol had no effect with MADDS-NHOH. In diabetic cells only, alpha-tocopherol incubation caused a reduction in GSH levels compared with non-diabetic cells. As the agents showed pro- as well as anti-oxidant effects in this study, further studies are required to demonstrate potential diabetic benefit from alpha-lipoic acid adminstration.

PEG-Hemoglobin as a resuscitation solution in the treatment of hypovolemic shock in the anesthetized rat

This study was designed to determine the advantages of using the hemoglobin-based oxygen carrier, polyethylene glycol conjugated bovine hemoglobin (PEG-Hb), as an additive to Ringer's lactate solution (RLS) for the treatment of acute hemorrhage in anesthetized female rats. Different compositions of PEG-Hb and RLS were administered intravenously in a paradigm that provided 30 ml/kg of resuscitation fluid following an episode of 15 min of hypotension. Hypotension was achieved by the removal of blood (1 ml/min) from the femoral vein until the mean arterial pressure was lowered to or below 50 mmHg and subsequently maintained until resuscitation. Short-term cardiovascular assessment showed that resuscitation fluids containing PEG-Hb resulted in higher mean arterial pressure, aortic blood flow, renal blood flow, and less dramatic shifts in arterial base excess and respiratory blood gases than plain RLS. The long-term survival experiment showed lower lactate dehydrogenase, alkaline phosphatase, and serum glutamic pyruvic transaminase levels in most groups resuscitated with solutions containing PEG-Hb, but no differences in survival (100%) were observed. The data suggest that the addition of PEG-Hb to RLS improves its resuscitative effects. Specifically, a solution of 50% RLS:50% PEG-Hb appeared to have the most favorable cardiovascular and metabolic effects in this anesthetized rat hypovolemic shock resuscitation model. Presumably, the improved effects seen with the addition of PEG-Hb were due to its innate plasma expansion and oxygen-delivery capabilities.

Hemoglobin protects from streptococcal cell wall-induced arthritis

OBJECTIVE

To investigate the ability of hemoglobin (Hgb), a nitric oxide (NO) scavenger, to deplete excess NO and reduce inflammation and injury in synovial tissue from joints with inflammatory arthritis.

METHODS

The severity of streptococcal cell wall-induced arthritis was monitored following administration of Hgb. Plasma nitrite and nitrate levels were measured, and inducible NO synthase (iNOS) and cytokine messenger RNA (mRNA) expression in peripheral blood mononuclear cells (PBMC) and joint tissue were evaluated.

RESULTS

Following systemic administration of Hgb to arthritic rats, plasma levels of nitrite and nitrate as well as iNOS mRNA expression in the joints and PBMC were significantly reduced. Moreover, inflammatory cell accumulation and disease pathology in the joint tissue were dramatically attenuated without obvious side effects. Consistent with this reduction in the inflammatory response, cytokine gene expression was decreased in the synovium of Hgb-treated rats.

CONCLUSION

Modulation of NO levels through the use of a NO scavenger, Hgb, influences the development and severity of arthritis. These findings suggest that depletion of excess NO by NO scavengers provides a prototype for further exploration of potential treatments for chronic arthritis.

Nitric oxide metabolism and breakdown

The steady-state concentration and thus the biological effects of NO are critically determined not only by its rate of formation, but also by its rate of decomposition. Bioreactivity of NO at physiological concentrations may differ substantially from that suggested by in vitro experiments. The charge neutrality and its high diffusion capacity are hallmarks that characterize NO bioactivity. Reactive oxygen derived species are major determinants of NO breakdown. Biotransformation of NO and its related N-oxides occurs via different metabolic routes within the body. S-Nitrosothiols formed upon reaction of NO with redox-activated thiols represent an active storage pool for NO. The major oxidative metabolites represent nitrite and nitrate, the ratio of both is determined by the microenvironmental redox conditions. In humans, circulating nitrite represents an attractive estimate of regional endothelial NO formation, whereas nitrate, with some caution, appears useful in estimating overall nitrogen/NO turnover. Within the near future, more specific biochemical tools for diagnosis of reduced NO bioactivity will become available. Increasing knowledge on the complex metabolism of NO in vivo will lead to the development of new therapeutic strategies to enhance bioactivity of NO via modulation of its metabolism.

The ability of polyethylene glycol conjugated bovine hemoglobin (PEG-Hb) to adequately deliver oxygen in both exchange transfusion and top-loaded rat models

The purpose of this study was to determine whether a six gram percent (g%) solution of the hemoglobin based oxygen carrier, polyethylene glycol conjugated bovine hemoglobin (PEG-Hb) could adequately deliver oxygen in both partial exchange transfusion and top-loaded rat models. This study measured tissue oxygen tension, circulatory retention and cardiovascular effects following both 30% exchange transfusion and 20 to 25 mL/kg top-loaded infusions of PEG-Hb. Oxygen delivery to rat tissues was determined using an oxygen dependent phosphorescence quenching method (Oxyspot). Telemetric intravascular blood pressure probes monitored heart rate and mean arterial pressure. In both models, six g% PEG-Hb (P50-15 torr) was shown to oxygenate tissue better than stroma-free bovine Hb (P50-26 torr), cross-linked bovine Hb (P50-48 torr) or simple plasma expanders. The mean circulatory half life of PEG-Hb was 15.0 +/- 2.3 hours and 17.4 +/- 1.6 hours for exchange transfusion and 25 mL/kg top-loaded rat models, respectively. Mean arterial pressure (MAP) in PEG-Hb treated rats was insignificantly different from sham controls undergoing a 30% exchange transfusion or following a top-loaded infusion. In conclusion, the PEG conjugated form of bovine Hb with its relatively long vascular persistence may possess characteristics that facilitate tissue oxygenation in the rat.

The effects of stroma-free and dextran-conjugated hemoglobin on hemodynamics and carotid blood flow in hemorrhaged guinea pigs

Hemoglobin solutions are potential resuscitative fluids with volume expanding and oxygen delivery abilities developed to reduce the use of blood transfusion. Most hemoglobin solutions in clinical trials increase transiently arterial pressure by inhibiting nitric oxide-dependent vasodilation. Our objective was to compare the effects on central hemodynamics and carotid blood flow of two hemoglobin solutions after resuscitation from hemorrhage in anesthetized guinea pigs. After anesthesia and instrumentation, severe hemorrhage was induced by withdrawing 50% of the blood volume. Resuscitation was performed after 15 min of hypovolemia with 5% albumin, stroma-free hemoglobin, or hemoglobin conjugated to dextran-benzenetetracarboxylate (Dex-BTC-Hb). The mean arterial pressure (MAP), carotid blood flow (CBF), vascular resistance index and heart rate (HR) were monitored for 3 hours after resuscitation. After hemorrhage, MAP and CBF dropped to 57.6 +/- 4.4% and 58.9 +/- 3.7% of control values respectively. Albumin failed to maintain hemodynamics in the decompensatory phase of shock. Both hemoglobin solutions gave rise to a transient increase in MAP (35%); stroma-free hemoglobin increased the CBF (150%) and resistance index (24%) whereas Dex-BTC-Hb had no effect on CBF and vascular resistances. None of the solutions affected the HR. Modified hemoglobin has attenuated effects on CBF and resistance index compared to stroma-free hemoglobin. This may be due to a balance between the stimulation of nitric oxide synthesis by shear-stress and the inhibition of vasodilation by nitric oxide trapping.

Inhaled nitric oxide in primary pulmonary hypertension: a safe and effective agent for predicting response to nifedipine

OBJECTIVES

The purpose of this study was to assess the utility of inhaled nitric oxide (NO), a selective pulmonary vasodilator, for predicting the safety and acute hemodynamic response to high-dose oral nifedipine in primary pulmonary hypertension (PPH).

BACKGROUND

A significant decrease in pulmonary vascular resistance with an oral nifedipine challenge is predictive of an improved prognosis, and potential clinical efficacy in PPH. However, the required nifedipine trial carries significant first-dose risk of hypotension. While inhaled NO has been recommended for assessing pulmonary vasodilator reserve in PPH, it is not known whether it predicts the response to nifedipine.

METHODS

Seventeen patients with PPH undergoing a nifedipine trial were assessed for hemodynamic response to inhaled NO at 80 parts per million for 5 minutes. The nifedipine trial consisted of 20 mg of nifedipine hourly for 8 hours unless limited by hypotension or intolerable side effects. Patients were classified as responders and nonresponders with positive response defined as > or =20% reduction in mean pulmonary artery pressure (mPA) or pulmonary vascular resistance (PVR) with the vasodilator administration.

RESULTS

NO was safely administered to all participants. Seven of 17 (41.2%) responded to NO, and 8 of the 17 to nifedipine (47.1%). Nifedipine was safely administered in 14 of the 17. Three suffered either mild or severe hypotension, including one death. All NO responders also responded to nifedipine, and 9 of the 10 NO nonresponders were nifedipine nonresponders, representing a sensitivity of 87.5%, specificity of 100%, and overall predictive accuracy of 94%. All NO responders tolerated a full nifedipine trial without hypotension. There was a highly significant correlation between the effects of NO and nifedipine on PVR (r=0.67, p=0.003).

CONCLUSIONS

The pulmonary vascular response to inhaled NO accurately predicts the acute hemodynamic response to nifedipine in PPH, and a positive response to NO is associated with a safe nifedipine trial. In patients comparable with those evaluated, a trial of nifedipine in NO nonresponders appears unwarranted and potentially dangerous.

Renal response to hemodilution with albumin or crosslinked bovine hemoglobin: role of nitric oxide

The decreased hematocrit that occurs with hemodilution leads to a decrease in peripheral resistance while venous return and cardiac output increase. We determined systemic and renal responses to hemodilution with a solution of albumin or a crosslinked hemoglobin-based oxygen carrier (XLHb) and the effect of inhibition of NO synthesis on the responses to albumin. Clearance experiments were done on anesthetized rats to determine mean arterial pressure (MAP), glomerular filtration rate (GFR), effective renal plasma flow (ERPF), and sodium excretion before and after isovolemic exchange transfusion (2 ml per 100 gm body weight) with either (1) 5% albumin (n = 5), (2) 5% albumin plus N omega-nitro-L-arginine methyl ester (L-NAME, 3.5 mg/kg; n = 6), or (3) 6% XLHb (n = 7) and after administration of L-NAME alone (n = 4). Hematocrit decreased similarly in all exchange groups (from 42 +/- 1.0 to 29 +/- 1.3). MAP decreased with albumin exchange, increased with L-NAME, and remained unchanged with albumin+L-NAME or XLHb. GFR, ERPF, and renal blood flow increased while filtration fraction and renal resistance decreased with albumin exchange; responses were the opposite with L-NAME, and with albumin+L-NAME and XLHb these parameters remained approximately the same as control values. Red cell delivery decreased with L-NAME, albumin+L-NAME, and XLHb but remained at control levels with albumin. In conclusion, renal effects of decreased hematocrit can be offset by decreased NO availability. The similarity of results with XLHb and albumin+L-NAME is consistent with NO scavenging by hemoglobin. Increased renal vascular tone with XLHb limits oxygen delivery.

Elevated methemoglobin in patients with sepsis

BACKGROUND

It has been reported that large amounts of nitric oxide (NO) are released in patients with sepsis. NO is converted to methemoglobin and nitrate. This study was designed to determine whether blood methemoglobin levels were increased in patients with sepsis or septic shock.

METHODS

Forty-five critically ill patients including 8 with sepsis but without shock, 6 with septic shock and 31 non-septic patients were enrolled in the study. For septic and septic shock patients, blood methemoglobin concentrations were measured during sepsis or septic shock and at the time of recovery or just before the onset of sepsis. For the remaining non-septic patients, methemoglobin concentrations were measured at ICU admission and discharge.

RESULTS

Blood methemoglobin levels in the presence of sepsis or septic shock were significantly (P < 0.05) higher than those in non-septic patients and those at recovery or just before the onset of sepsis in both septic and septic shock patients.

CONCLUSIONS

Blood methemoglobin concentration may be useful as a marker of the onset of sepsis or septic shock.

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.

Intracellular Ca2+ dependence of nitric oxide mediated enhancement of interleukin-8 secretion in human endothelial cells

Nitric oxide (NO.) can induce transient [Ca2+] changes in endothelial cells not different from receptor mediated signalling. Whether this Ca2+ signal may provide a link with IL-8 secretion induced by NO. donors was investigated in human endothelial cells. Sodium nitroprusside (SNP) and S-nitroso-N-acetyl-DL-penicillamine (SNAP) dose dependently increased IL-8 production in this cell type. Additive IL-8 secretion was found with TNFalpha. Buffering intracellular Ca2+ with MAPT/AM suppressed NO. induced [Ca2+]i changes and reduced subsequent IL-8 secretion. The additive effect of both NO. donors on TNFalpha induced IL-8 secretion was completely blocked in the presence of MAPT/AM. SKF 96365, which has been shown to block receptor mediated Ca2+ entry, and TMB-8, which blocks intracellular Ca2+ release, both inhibited IL-8 secretion, particularly when TNFalpha was used as a costimulator, indicating that [Ca2+]i changes are important components of IL-8 induction by NO..

Physiological effect of polyethylene glycol conjugation on stroma-free bovine hemoglobin in the conscious dog after partial exchange transfusion

This study was designed to determine the effect of polyethylene glycol (PEG) conjugation on stroma-free bovine hemoglobin. This was accomplished by examining the effects of unmodified stroma-free bovine hemoglobin (bovine Hb), PEG modified bovine hemoglobin (PEG-Hb) and dextran 70 on heart rate (HR), mean arterial pressure (MAP), gross renal morphology, blood chemistry, and hemoglobinuria development in conscious beagle dogs following a 30% exchange transfusion. Dogs were implanted with telemetric blood pressure probes and after 2 weeks underwent an isovolumic 30% blood volume exchange transfusion. Dogs treated with bovine Hb displayed a significant increase in MAP for 2 h following the exchange transfusion with no effect on HR. These animals exhibited significant levels of hemoglobinuria (> 20% of infused dose) within 24 h. Histopathologically, all bovine Hb infused dogs displayed renal tubular vacuolization, with 2 dogs showing regions of tubular casts and tubular cell regeneration. PEG-Hb was shown to have a circulatory half-life of 58.3 +/- 2.4 h and caused no significant changes in MAP or HR throughout the study period. Dogs excreted less than 0.1% of infused PEG-Hb within 24 h and displayed only renal tubular epithelial cell vacuolization. Dextran 70 caused a slight but insignificant decrease in MAP, elevated the HR, and exhibited only slight renal vacuole formation. Blood chemistry remained essentially stable following exchange transfusion with all the test articles. The conjugation of PEG to hemoglobin greatly increased the parent protein's vascular retention while attenuating some of its less favorable attributes.

Inhibition of nitric oxide generation and lipid peroxidation attenuates hemolysate-induced injury to cerebrovascular endothelium

The mechanisms of hemolysate-induced cerebral injury following subarachnoid hemorrhage are just beginning to be clarified. This study examined the injurious effects of hemolysate on endothelial cells derived from bovine middle cerebral arteries, and evaluated the roles of lipid peroxidation and nitric oxide production in this type of damage. Cultured endothelial cells were grown to confluency on gelatin-coated plates. The cells were characterized as endothelial cells on the basis of morphology. Factor VIII-related antigen staining, and low density lipoprotein (LDL) uptake. Additional cells were grown to confluency on collagen-coated well inserts, and were treated with hemolysate for 24 hours. Prior to hemolysate exposure, cells were treated with: a) an inhibitor of iron-dependent lipid peroxidation (tirilazad mesylate 100 microM), or b) an inhibitor of nitric oxide synthase (either N-nitro-L-arginine: NLA 300 microM, or aminoguanidine: AG at 1.5, 7.5, 15 or 150 microM). Permeability of the tracer, U-14C-sucrose, across the layer of endothelial cells was examined over a 24 hour period. Hemolysate induced a significant increase in the permeability across the endothelial cell layer. Pretreatment with tirilazad mesylate, NLA, or AG attenuated significantly hemolysate-induced changes in the endothelial cell barrier. These findings indicate that free radical generation and lipid peroxidation are critical participants in hemolysate-induced injury to the barrier function of the cerebrovascular endothelium. In addition, the results indicate that endothelial cells provide an adequate source of nitric oxide to damage their own cellular function. Finally, these findings strongly implicate free radical mechanisms in endothelial damage associated with subarachnoid hemorrhage.

Induction of endothelial cell injury by cigarette smoke

Cigarette smoke contains different populations of free radicals which may be responsible for endothelial cell (EC) injury of smokers. The purpose of this study was to examine the effects of gas-phase cigarette smoke on EC endothelium-derived relaxing factor (EDRF)/NO-guanylate cyclase (GC)-cGMP pathway and on EC detachment-type injury after incubation with smoke. Furthermore, we examined whether different kind of antioxidants can prevent smoke-caused EC injury. We measured cGMP pathway using direct (sodium nitroprusside, SNP) and indirect (A23187, the calcium ionophore and bradykinin, BK) activators of GC. Directly and indirectly stimulated EC cGMP production dose-dependently decreased and EC detachment increased after incubation with smoke. Externally added thiols (glutathione, GSH; D-Penicillamine, DP; N-acetylcysteine, NAC) protected EC from damage of cGMP production and cell detachment. Other antioxidants (catalase, deferoxamine and superoxide dismutase) were ineffective. These results suggest that the thiol containing GC in EC is destroyed or inactivated or thiol like species responsible for activation of GC is incomplete in EC after incubation with smoke. It is also possible that externally added thiols bind an unknown component of smoke and this way, EC is protected. EC injury may contribute to vascular diseases associated with cigarette smoking.

Cardiovascular effects of an ultra-short-acting nitric oxide-releasing compound, zwitterionic diamine/NO adduct, in dogs

BACKGROUND

This study was conducted to clarify the cardiovascular effects of a new NO-releasing compound, NOC-7, and to compare it with other nitrovasodilators, sodium nitroprusside (SNP) and nitroglycerin, in dogs anesthetized with pentobarbital.

METHODS AND RESULTS

A bolus injection of NOC-7 decreased mean aortic blood pressure in a dose-dependent manner. The onset was rapid and the recovery quick. Continuous infusion of NOC-7 decreased mean aortic pressure from 115 +/- 3.9 to 84 +/- 2.9 mm Hg and infusion of SNP, from 118 +/- 3.8 to 87 +/- 3.1 mm Hg. The optimum doses of NOC-7 and SNP were determined to be 2.73 +/- 0.77 and 11.5 +/- 6.1 micrograms.kg-1.min-1, respectively. During infusion of NOC-7, heart rate and cardiac output were increased (P < .05), pulmonary artery pressure was not changed, and systemic and pulmonary vascular resistances were decreased (P < .05). Electromagnetic flowmetry showed that portal venous and internal carotid arterial blood flow were increased (P < .05) and that hepatic and renal arterial blood flows were not changed. These hemodynamic changes during NOC-7 infusion were similar to those with SNP. The plasma level of NO2-/NO3 did not change, but methemoglobin increased slightly (P < .05). Comparison between hypotensive responses before and after a 3.5-hour infusion of NOC-7 or nitroglycerin showed that acute tolerance developed to nitroglycerin but not to NOC-7.

CONCLUSIONS

The results indicate that NOC-7 may be useful as an ultra-short-acting nitrovasodilator that has no major adverse effect or tolerance.

The influence of polyethylene glycol conjugation on bovine hemoglobin's intrinsic effect on the gastrointestinal system of the rat

The purpose of this study was to determine the impact of polyethylene glycol (PEG) conjugation on bovine hemoglobin's effect on gastrointestinal (GI) blood flow and motility in the Sprague Dawley rat. This study was divided into two parts: part one assessed blood flow, while the other evaluated bolus transit time through the GI. To examine blood flow, thirty-two rats were divided into four experimental groups (PEG-hemoglobin, bovine hemoglobin, Ringer's Lactate and autologous blood sham). Blood flow within the superior mesenteric artery was monitored during graduated isovolemic hemodilution. In the second part of the study, GI motility was estimated by bolus transit time. Thirty-six rats were assigned to four groups (PEG-hemoglobin, bovine hemoglobin, Ringer's Lactate and no treatment sham) and following an overnight fast, the rats were given a bolus injection (25 mL/kg) of test article. Three hours following injection, they received an oral 0.3 mL gavage of a charcoal/arabic gum mixture and were later sacrificed and their GI tract evaluated. Results indicated that the infusion of bovine hemoglobin reduced both baseline blood flow through the mesenteric artery and gastrointestinal transit time. In contrast, PEG-hemoglobin maintained baseline blood flow through the mesenteric artery and had no effect on GI transit time or morphology. Therefore, PEG conjugation of bovine hemoglobin significantly attenuated its intrinsic effect on the GI system of the rat.

Focal cerebral ischemia during anesthesia with etomidate, isoflurane, or thiopental: a comparison of the extent of cerebral injury

An investigation was performed to compare the cerebral protective properties of etomidate, isoflurane, and thiopental. In separate groups of spontaneously hypertensive rats, etomidate, isoflurane, or thiopental was administered to achieve and maintain burst-suppression of the electroencephalogram (3-5 bursts/min) for the duration of the experiment. A fourth group received 1.2 minimal alveolar concentration halothane. All groups underwent 3 hours of middle cerebral artery occlusion and then 2 hours of reperfusion. Thereafter, the animals were killed and the volume of injured brain was determined by staining with 2,3,5-triphenyltetrazolium. Physiological parameters did not differ among the four groups during the investigation, with the exception that hemolysis occurred in the etomidate group (free hemoglobin levels, approximately 0.4 g.dl-1). The volume of injured brain in the thiopental group (56 +/- 10 mm3) was significantly smaller than that in the halothane control group (99 +/- 13 mm3). The volumes of injured brain in the etomidate and isoflurane groups (145 +/- 11 mm3 and 139 +/- 14 mm3, respectively) were significantly larger than those in the control and thiopental groups. We speculate that the apparently detrimental effect of etomidate may be the result of the binding of nitric oxide of cerebral endothelial origin by the iron component of free hemoglobin. Intracranial pressure was not recorded, and in the isoflurane group, there may have been adverse effects on cerebral perfusion pressure associated with vasodilation caused by high concentrations of isoflurane. The results are consistent with a protective effect by barbiturates.

[Pulsed Doppler ultrasonography to measure the vasoactive effects of hemoglobin-dextran 10-benzene-tetracarboxylate, a potential erythrocyte substitute]

The effects of Dextran-Benzene-Tetracarboxylate-Hemoglobin (Dex-BTC-Hb), a chemically-modified hemoglobin-based oxygen carrier, on the vascular tone were compared to those of standard solutions, i.e. the animal's own blood and a 50 milligrams albumin solution, by measuring the carotid blood flow velocity, the mean arterial pressure, the heart rate and respiratory frequency, in anesthetized Hartley guinea pigs after a hemorragic shock. Stroma-free hemoglobin induced 40% hypertension and a 110% rise in blood flow velocity immediately after injection. The velocity was still increased 38%, 3 hours after injection. The calculations of the vascular resistances showed an increase in carotid vascular tone. Dex-BTC-Hb brought about 35% hypertension for two hours with no significant modifications of the vascular tone. These effects are similar to those of the albumin solution. These results indicate that, unlike stroma-free hemoglobin, Dex-BTC-Hb does not significantly affect the vascular tone, probably because of its slight interaction with the factors that regulate vascular tone.

Oxygen and CO binding to triply NO and asymmetric NO/CO hemoglobin hybrids

The bimolecular and geminate CO recombination kinetics have been measured for hemoglobin (Hb) with over 90% of the ligand binding sites occupied by NO. Since Hb(NO)4 with inositol hexaphosphate (IHP) at pH below 7 is thought to take on the low affinity (deoxy) conformation, the goal of the experiments was to determine whether the species IHPHb-(NO)3(CO) also exists in this quaternary structure, which would allow ligand binding studies to tetramers in the deoxy conformation. For samples at pH 6.6 in the presence of IHP, the bimolecular kinetics show only a slow phase with rate 7 x 10(4) M-1 s-1, characteristic of CO binding to deoxy Hb, indicating that the triply NO tetramers are in the deoxy conformation. Unlike Hb(CO)4, the fraction recombination occurring during the geminate phase is low (< 1%) in aqueous solutions, suggesting that the IHPHb(NO)3(CO) hybrid is also essentially in the deoxy conformation. By mixing stock solutions of HbCO and HbNO, the initial exchange of dimers produces asymmetric (alpha NO beta NO/alpha CO beta CO) hybrids. At low pH in the presence of IHP, this hybrid also displays a high bimolecular quantum yield and a large fraction of slow (deoxy-like) CO recombination; the slow bimolecular kinetics show components of equal amplitude with rates 7 and 20 x 10(4) M-1 s-1, probably reflecting the differences in the alpha and beta chains. Samples of symmetric hybrids (a2NOI32Co or a2Co922NO) showed a lower (R-like) bimolecular yield and less slow phase for the CO bimolecular recombination, relative to the asymmetric hybrid or the triply NO species. The slower (T state) bimolecular rate of 7 x 104 M-1 s-1 was observed for CO rebinding to a chain.While oxygen equilibrium studies with 'HPHb(NO)3 were hampered by a high oxidation rate, it was possible to perform experiments with samples equilibrated with a mixed CO/oxygen atmosphere. Photodissociation of CO allows a temporary exposure of the binding sites to oxygen. The results confirm that IHPHb(NO)3 has a low oxygen affinity.

Reactive nitrogen intermediates and antimicrobial activity: role of nitrite

The reactive nitrogen intermediate (RNI) nitric oxide (NO.) is formed from L-arginine by an NO. synthase and, following secondary reactions yielding additional toxic intermediates, nitrite (NO2-) and nitrate are formed. Nitrite, however, also has toxic properties. At acid pH, nitrous acid (HNO2) is bactericidal to Escherichia coli, in association with the loss of HNO2/NO2- and the uptake of oxygen, an effect which is increased by H2O2. Under conditions in which HNO2/NO2- +/- H2O2 were ineffective, the further addition of peroxidase (myeloperoxidase [MPO], eosinophil peroxidase, lactoperoxidase) or catalase resulted in bactericidal activity and the disappearance of HNO2/NO2-. Paradoxically, HNO2/NO2- also inhibited the bactericidal activity of MPO by the formation of a complex with MPO with a shift in the absorption spectrum, and by reaction with hypochlorous acid (HOCl) (the product of the chloride-supplemented MPO-H2O2 system), with loss of the bactericidal activity of HOCl and the disappearance of both HOCl and HNO2/NO2- from the reaction mixture. Thus, HNO2/NO2-, rather than being solely an end product of RNI formation, may influence antimicrobial activity either by acting alone, with H2O2, or with H2O2 and peroxidase as a source of toxic agents, or by inhibiting the peroxidase-mediated antimicrobial systems.

Synergistic stimulation of nitric oxide hemoglobin production in rats by recombinant interleukin 1 and tumor necrosis factor

Nitric oxide (NO) is formed from arginine in Escherichia coli lipopolysaccharide (LPS) treated rat; however, none of specific cytokine inducing NO generation is yet determined. We studied the effect of interleukin 1 (IL-1) and tumor necrosis factor (TNF) on NO production in rats by detecting NO-hemoglobin in their blood, using electron spin resonance. Either IL-1 or TNF alone stimulated NO-hemoglobin formation. Combined administration of IL-1 and TNF markedly enhanced NO-hemoglobin generation, demonstrating the synergistic character of both stimuli on NO production. Further, LPS and TNF in combination were more potent stimulator of NO-hemoglobin production in rats than each alone.

Detection of nitric oxide production in lipopolysaccharide-treated rats by ESR using carbon monoxide hemoglobin

Release of nitric oxide (NO), from macrophages activated with E. coli lipopolysaccharide (LPS) and endothelial cells, has been proposed using chemiluminescence and spectrophotometry. However these methods can not distinguish NO from NO2-. The present study was aimed to prove in vivo production of NO, by ESR using CO-hemoglobin (HbCO) as a trapping agent of NO in the peritoneal cavity of rats treated with LPS. We detected a broad signal in the recovered HbCO solution. Inositol hexaphosphate induced a three-line hyperfine structure, characteristic of NO-hemoglobin (HbNO). In the arterial blood, ESR signal of HbNO with faint hyperfine structure was detected. NG-Monomethyl-L-arginine inhibited the formation of HbNO. HbNO was not detected in the peritoneal cavity of the LPS-untreated rat given i.p. both NO2- and HbCO. HbNO was, therefore, derived from NO, not from NO2-. These results show that free NO is produced in vivo by the stimulation of LPS.

Histamine H1-receptor-mediated cyclic GMP production in guinea-pig lung tissue is an L-arginine-dependent process

Histamine produces a rapid and massive increase of the c-GMP level of guinea-pig lung tissue. The EC50 value for this in vitro response is found to be 27 microM and the c-GMP level is maximally 9-fold elevated by 100 microM histamine. The response is stereoselectively inhibited by the enantiomers of chlorpheniramine, indicating H1-receptor involvement. Preincubation of lung tissue with 200 microM NCDC, a phospholipase C inhibitor, reduces the histamine (100 microM) responses to 16 +/- 3% (N = 6) of the control c-GMP production. Inhibition of protein kinase C by 50 microM H-7 does not significantly attenuate the H1-receptor response, whereas omittance of extracellular Ca2+ results in almost complete inhibition of the c-GMP production. The histamine-induced c-GMP response is inhibited by hemoglobin, methylene blue and the antioxidants butylated hydroxytoluene and nordihydroguaretic acid, indicating the involvement of a nitric oxide-dependent activation of soluble guanylate cyclase. This suggestion is supported by the concentration-dependent inhibition of the c-GMP production by NG-monomethyl-L-arginine (NMA). At a concentration of 20 microM NMA the histamine (100 microM) response is inhibited to 34 +/- 8% (N = 6) of the control response. This inhibition is reversed to 127 +/- 20% (N = 6) by the exogenous addition of 1 mM L-arginine. These findings show that after an initial H1-receptor-mediated, phospholipase C-dependent, Ca(2+)-mobilization the enzymatic conversion of L-arginine to nitric oxide is stimulated. This nitric oxide production is finally responsible for the activation of soluble guanylate cyclase, leading to the production of c-GMP.

Degradation of uric acid during autocatalytic oxidation of oxyhemoglobin induced by sodium nitrite

The oxidation of oxyhemoglobin produced by sodium nitrite occurs in two stages: 1) an initial slow phase followed by 2) a rapid autocatalytic phase that carries the reaction to completion. The length of the slow phase is extended when uric acid is added to the reaction mixture. As the concentration of uric acid increases, the length of the slow phase increases until a concentration is reached at which the rate of methemoglobin formation is nearly linear until the reaction is complete. Further increases in the concentration of uric acid do not affect the rate of the reaction in the slow phase. At low concentrations of uric acid, where an autocatalytic phase is reached, uric acid is degraded during the reaction. At concentrations of uric acid that keep the reaction in the linear phase, the uric acid is not degraded. It is concluded that uric acid may protect oxyhemoglobin by reacting with HbO2H to yield [HbOH]+ and the urate radical. The urate radical may react with a second molecule of HbO2H and become oxidized. At higher concentrations, the radical may undergo electron transfer with oxyhemoglobin to regenerate the uric acid and form methemoglobin.