The reactions of myoglobin, normal adult hemoglobin, sickle cell hemoglobin and hemin with hydroxyurea

Clinical methemoglobinemia secondary to administration of hydroxyurea at therapeutic doses in a dog

Methemoglobinemia secondary to administration of hydroxyurea is only reported in veterinary medicine as a result of accidental ingestion of high doses, and once at therapeutic dose in human medicine. A 2.5-year-old female spayed mixed breed dog was presented for acute signs of neurologic disease and diagnosed with severe erythrocytosis without an identified underlying cause, leading to suspicion of polycythemia vera. The dog was managed with phlebotomies, supportive care, and administration of hydroxyurea. Within 2 h of administration of hydroxyurea (37 mg/kg) administration, respiratory distress with cyanosis, and methemoglobinemia developed. Signs resolved within 24 h but recurred after a second administration of lower dosage of hydroxyurea (17 mg/kg) 20 days later. The dog remained asymptomatic except for mild cyanosis but was humanely euthanized for lack of relevant improvement of signs of neurologic disease. This case report documents the repeated occurrence of methemoglobinemia in a dog after administration of hydroxyurea at therapeutic doses.

Heme changes HIF-α, eNOS and nitrite production in HUVECs after simvastatin, HU, and ascorbic acid therapies

The sickle cell disease (SCD) is a hemolytic genetic anemia characterized by free heme and hemoglobin release into intravascular spaces, with endothelial activation. Heme is a proinflammatory molecule able to directly activate vascular endothelium, thus, endothelial dysfunction and vascular disease are major chronic events described in SCD. The aim of this study was to evaluate the production of endothelial nitric oxide synthase (eNOS), nitrite and hypoxia inducible factor alpha (HIF-α) in HUVECs (human umbilical vein endothelial cells) activated by heme in response to simvastatin, hydroxyurea (HU), and ascorbic acid therapies. eNOS and HIF-α production were evaluated by ELISA and nitrite was measured by the Griess technique. The production of HIF-α increased when the cells were stimulated by heme (p<0.01), while treatment with HU and simvastatin reduced the production (p<0.01), and treatment with ascorbic acid increased HIF-1a production by the cells (p<0.01). Heme increased eNOS production, (p<0.01) but showed a heterogeneous pattern, and the lowest concentrations of all the treatments reduced the enzyme production (p<0.01). The nitrite production by HUVECs was enhanced by stimulation with heme (p<0.001) and was reduced by treatment with HU (p<0.001), ascorbic acid (p<0.001) and simvastatin (p<0.01). In summary, our results suggest that the hemolytic vascular microenvironment in SCD requires different therapeutic approaches to promote clinical improvement, and that a combination of therapies may be a viable strategy for treating patients.

Toxicity of Hydroxyurea in Rats and Dogs

The toxicity of hydroxyurea, a treatment for specific neoplasms, sickle-cell disease, polycythemia, and thrombocytosis that kills cells in mitosis, was assessed in repeat-dose, oral gavage studies in rats and dogs and a cardiovascular study in telemetered dogs. Hydroxyurea produced hematopoietic, lymphoid, cardiovascular, and gastrointestinal toxicity with steep dose response curves. In rats dosed for 10 days, 50 mg/kg/day was tolerated; 500 mg/kg/day produced decreased body weight gain; decreased circulating leukocytes, erythrocytes, and platelets; decreased cellularity of thymus, lymph nodes, and bone marrow; and epithelial degeneration and/or dysplasia of the stomach and small intestine; 1,500 mg/kg/day resulted in deaths on day 5. In dogs, a single dose at ≥250 mg/kg caused prostration leading to unscheduled euthanasia. Dogs administered 50 mg/kg/day for 1 month had decreased circulating leukocytes, erythrocytes, and platelets; increased bone marrow cellularity with decreased maturing granulocytes; increased creatinine kinase activity; and increased iron pigment in bone marrow and hepatic sinusoidal cells. In telemetered dogs, doses ≥15 mg/kg decreased systolic blood pressure (BP); 50 mg/kg increased diastolic BP, heart rate, and change in blood pressure over time (+d P/d t), and decreased QT and PR intervals and maximum left ventricular systolic and end diastolic pressures with measures returning to control levels within 24 hr.

How does catalase release nitric oxide? A computational structure-activity relationship study

Hydroxyurea (HU) is the only FDA approved medication for treating sickle cell disease in adults. The primary mechanism of action is pharmacological elevation of nitric oxide (NO) levels which induces propagation of fetal hemoglobin. HU is known to undergo redox reactions with heme based enzymes like hemoglobin and catalase to produce NO. However, specific details about the HU based NO release remain unknown. Experimental studies indicate that interaction of HU with human catalase compound I produces NO. Presently, we combine flexible receptor-flexible substrate induced fit docking (IFD) with energy decomposition analyses to examine the atomic level details of a possible key step in the clinical conversion of HU to NO. Substrate binding modes of nine HU analogs with catalase compound I were investigated to determine the essential properties necessary for effective NO release. Three major binding orientations were found that provide insight into the possible reaction mechanisms for producing NO. Further results show that anion/radical intermediates produced as part of these mechanisms would be stabilized by hydrogen bonding interactions from distal residues His75, Asn148, Gln168, and oxoferryl-heme. These details will ideally contribute to both a clearer mechanistic picture and provide insights for future structure based drug design efforts.

Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(III) porphyrin: formation of the nitrosylated Mn(II) porphyrin as an intermediate

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn(III)TTEG-2-PyP(5+), a synthetic water soluble Mn(III) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of Mn(III)TTEG-2-PyP(5+), which is followed by a ligand-to-metal electron transfer to get Mn(II)TTEG-2-PyP(4+) and the free radical derived from HU (HU˙). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of Mn(II)TTEG-2-PyP(4+), is the reaction of Mn(II)TTEG-2-PyP(4+) with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO(2)(-) and the initial Mn(III)TTEG-2-PyP(5+). The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.

Sickle cell disease vasculopathy: a state of nitric oxide resistance

Sickle cell disease (SCD) is a hereditary hemoglobinopathy characterized by microvascular vaso-occlusion with erythrocytes containing polymerized sickle (S) hemoglobin, erythrocyte hemolysis, vasculopathy, and both acute and chronic multiorgan injury. It is associated with steady state increases in plasma cell-free hemoglobin and overproduction of reactive oxygen species (ROS). Hereditary and acquired hemolytic conditions release into plasma hemoglobin and other erythrocyte components that scavenge endothelium-derived NO and metabolize its precursor arginine, impairing NO homeostasis. Overproduction of ROS, such as superoxide, by enzymatic (xanthine oxidase, NADPH oxidase, uncoupled eNOS) and nonenzymatic pathways (Fenton chemistry), promotes intravascular oxidant stress that can likewise disrupt NO homeostasis. The synergistic bioinactivation of NO by dioxygenation and oxidation reactions with cell-free plasma hemoglobin and ROS, respectively, is discussed as a mechanism for NO resistance in SCD vasculopathy. Human physiological and transgenic animal studies provide experimental evidence of cardiovascular and pulmonary resistance to NO donors and reduced NO bioavailability that is associated with vasoconstriction, decreased blood flow, platelet activation, increased endothelin-1 expression, and end-organ injury. Emerging epidemiological data now suggest that chronic intravascular hemolysis is associated with certain clinical complications: pulmonary hypertension, cutaneous leg ulcerations, priapism, and possibly stroke. New therapeutic strategies to limit intravascular hemolysis and ROS generation and increase NO bioavailability are discussed.

The interaction between hemoglobin and two surfactants with different charges

The interactions of hemoglobin (Hb) with sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (DTAB) are investigated by several methods. We observed the formation of hemichrome below the critical micelle concentration (cmc) of surfactant and the release of heme from Hb above the cmc. When pH value of Hb/surfactant system is lower than isoelectric point (pI) of Hb, the interaction of SDS with Hb is both electrostatic and hydrophobic, while the interaction of DTAB with Hb is hydrophobic mainly. On the contrary, when pH>pI, the interaction of SDS with Hb is hydrophobic mainly, while the interaction of DTAB with Hb is both electrostatic and hydrophobic. In the case where both the electrostatic interaction and hydrophobic interaction exist, the electrostatic interaction plays a more important role. Thus, SDS tends to interact with Hb more obviously than DTAB does when pH<pI and the interaction between DTAB and Hb is stronger when pH>pI.

Quantum chemical design of hydroxyurea derivatives for the treatment of sickle-cell anemia

Treatment of sickle-cell anemia by hydroxyurea has been shown to decrease patient mortality by 40%. In a rate-limiting step, hydroxyurea reacts with hemoglobin to form the nitroxide radical, which then decomposes to yield nitric oxide (NO). In this paper, we examine derivatives of hydroxyurea and their radicals by quantum chemical methods to identify derivatives that generate NO-producing radicals at a faster rate than hydroxyurea. The molecules are treated with Hartree-Fock theory, correlated wave function methods such as perturbation theory and coupled-cluster methods, and density functional theory. We observe that the inclusion of the correlation energy is important for an accurate comparison of the energy changes associated with modifications of the hydroxyurea molecule and its radical. The computational results are compared with available experimental data. All 19 derivatives of hydroxyurea, including a new medication for asthma Zileuton, manifest changes in their electronic energies that mark them as candidates for a faster formation of NO-producing radicals.

Tunable Delocalization of Unpaired Electrons of Nitroxide Radicals for Sickle-Cell Disease Drug Improvements

Hydroxyurea is a drug recently approved to treat sickle cell diseases. Hydroxyurea benefits the patients by increasing the level of fetal hemoglobin via a nitroxide radical pathway. Here, we report an unpaired-electron-delocalization approach to tune the stability of nitroxide radicals. In this approach, the substitution by an unsaturated alkyl group containing conjugated C=C double bonds for the hydrogen on the nitrogen atom attached to the hydroxyl of hydroxyurea can significantly increase its ability to generate nitroxide radical. Furthermore, the increase can be remarkably enhanced by increasing the number of conjugated C=C double bonds. For a hydroxyurea derivative that contains two conjugated C=C double bonds, the reaction rate to generate its radical is 118 times faster than that of hydroxyurea, and for a hydroxyurea derivative containing 20 conjugated C=C double bonds, the reaction rate to form its radical is 238 times faster than that of hydroxyurea. For this reason, hydroxyurea derivatives with conjugated C=C double bonds may constitute new potential drugs for the treatment of sickle-cell diseases.

The role of beta93 Cys in the inhibition of Hb S fiber formation

Recent studies have suggested that nitric oxide (NO) binding to hemoglobin (Hb) may lead to the inhibition of sickle cell fiber formation and the dissolution of sickle cell fibers. NO can react with Hb in at least 3 ways: 1) formation of Hb(II)NO, 2) formation of methemoglobin, and 3) formation of S-nitrosohemoglobin, through nitrosylation of the beta93 Cys residue. In this study, the role of beta93 Cys in the mechanism of sickle cell fiber inhibition is investigated through chemical modification with N-ethylmaleimide. UV resonance Raman, FT-IR and electrospray ionization mass spectroscopic methods in conjunction with equilibrium solubility and kinetic studies are used to characterize the effect of beta93 Cys modification on Hb S fiber formation. Both FT-IR spectroscopy and electrospray mass spectrometry results demonstrate that modification can occur at both the beta93 and alpha104 Cys residues under relatively mild reaction conditions. Equilibrium solubility measurements reveal that singly-modified Hb at the beta93 position leads to increased amounts of fiber formation relative to unmodified or doubly-modified Hb S. Kinetic studies confirm that modification of only the beta93 residue leads to a faster onset of polymerization. UV resonance Raman results indicate that modification of the alpha104 residue in addition to the beta93 residue significantly perturbs the alpha(1)beta(2) interface, while modification of only beta93 does not. These results in conjunction with the equilibrium solubility and kinetic measurements are suggestive that modification of the alpha104 Cys residue and not the beta93 Cys residue leads to T-state destabilization and inhibition of fiber formation. These findings have implications for understanding the mechanism of NO binding to Hb and NO inhibition of Hb S fiber formation.

The interaction of hemoglobin with hexadecyltrimethylammonium bromide

The interaction of hemoglobin (Hb) with hexadecyltrimethylammonium bromide (CTAB) is investigated by UV-vis absorption spectra and fluorescence spectra method. CTAB monomer can convert methemoglobin (metHb) to hemichrome, and CTAB molecular assemblies, such as micelle, microemulsion and lamellar liquid crystal, can induce heme monomer to leave the hydrophobic cavity of Hb. TEM results show that Hb maintains the spherical structure in CTAB microemulsions while it is unfolded in CTAB lamellar liquid crystals. The existence of proton in the above systems can increase the stability of metHb.

Nitric oxide production from hydroxyurea

Hydroxyurea is a relatively new treatment for sickle cell disease. A portion of hydroxyurea's beneficial effects may be mediated by nitric oxide, which has also drawn considerable interest as a sickle cell disease treatment. Patients taking hydroxyurea show a significant increase in iron nitrosyl hemoglobin and plasma nitrite and nitrate within 2 h of ingestion, providing evidence for the in vivo conversion of hydroxyurea to nitric oxide. Hydroxyurea reacts with hemoglobin to produce iron nitrosyl hemoglobin, nitrite, and nitrate, but these reactions do not occur fast enough to account for the observed increases in these species in patients taking hydroxyurea. This report reviews recent in vitro studies directed at better understanding the in vivo nitric oxide release from hydroxyurea in patients. Specifically, this report covers: (1) peroxidase-mediated formation of nitric oxide from hydroxyurea; (2) nitric oxide production after hydrolysis of hydroxyurea to hydroxylamine; and (3) the nitric oxide-producing structure-activity relationships of hydroxyurea. Results from these studies should provide a better understanding of the nitric oxide donor properties of hydroxyurea and guide the development of new hydroxyurea-derived nitric oxide donors as potential sickle cell disease therapies.

Catalase-Mediated Nitric Oxide Formation from Hydroxyurea

Hydroxyurea reduces the incidence of painful crises in patients with sickle cell disease and has recently been approved for the treatment of this condition. A number of in vitro studies show that the oxidation of hydroxyurea results in the formation of nitric oxide, which also has drawn considerable interest as a sickle cell disease therapy. While patients on hydroxyurea demonstrate elevated levels of nitric oxide-derived metabolites, little information regarding the site or mechanism of the in vivo conversion of hydroxyurea to nitric oxide exists. Chemiluminescence detection experiments show the ability of catalase to catalyze the formation of nitrite and nitrate from hydroxyurea. Spectroscopic studies show that the reaction of hydroxyurea and catalase in the presence of a hydrogen peroxide generating system produces a ferrous-NO catalase complex. Trapping studies indicate the intermediacy of a nitroso species during this reaction. The proposed mechanism for this conversion includes initial hydrogen peroxide-dependent oxidation of hydroxyurea by catalase to form the nitroso species, hydrolysis of this nitroso species to produce nitroxyl, and reductive nitrosylation of the ferric heme of catalase by nitroxyl to yield the ferrous-NO catalase complex. Addition of Angeli's salt, a nitroxyl donor, to ferric catalase also produces the ferrous-NO catalase complex. Spectroscopic studies show that the ferrous-NO catalase complex releases nitric oxide as judged by the oxyhemoglobin assay and an NO specific EPR specific trap. These results demonstrate nitric oxide production from the ferric catalase oxidation of nitroxyl and identify a catalase-mediated pathway as a potential source of nitric oxide production from hydroxyurea.

Hydroxyurea analogues as kinetic and mechanistic probes of the nitric oxide producing reactions of hydroxyurea and oxyhemoglobin

Derivatives of N-hydroxyurea that contain an N-hydroxy group react with oxyhemoglobin to form methemoglobin and variable amounts of nitrite/nitrate. Compounds with an unsubstituted -NHOH group produce the most nitrite/nitrate, which provides evidence for nitric oxide formation. The rate of reaction of these N-hydroxyurea derivatives with oxyhemoglobin correlates well with that compound's oxidation potential. Aromatic N-hydroxyureas react 25-80-fold faster with oxyhemoglobin than with N-hydroxyurea, suggesting other N-hydroxyurea analogues may be superior nitric oxide donors. Electron paramagnetic resonance spectroscopy shows that the formation of a low-spin methemoglobin-hydroxyurea complex is critical for iron nitrosyl hemoglobin formation. These results show that iron nitrosyl hemoglobin formation from the reaction of hydroxyureas and hemoglobin requires an unsubstituted -NHOH group and that the nitrogen atom of the non-N-hydroxy group must contain at least a single hydrogen atom. These results should guide the development of new hydroxyurea-based nitric oxide donors and sickle cell disease therapies.

An emerging role for nitric oxide in sickle cell disease vascular homeostasis and therapy

Nitric oxide participates in the compensatory response to chronic vascular injury in patients with sickle cell disease. The authors have found reductions of basal and stimulated nitric oxide production and responses to exogenous nitric oxide in male patients with sickle cell disease. Gender differences in nitric oxide bioavailability are probably caused in part by the protective effects of ovarian estrogen on nitric oxide synthase expression and activity in women. Further, in men, and likely all patients during vaso-occlusive crisis and the acute chest syndrome, nitric oxide is destroyed by increased circulating plasma hemoglobin and superoxide. The combined effects of inhaled nitric oxide gas of improving pulmonary ventilation to perfusion matching and hemodynamics, reducing alveolar and systemic inflammation, and inhibiting circulating plasma hemoglobin (and thus restoring peripheral nitric oxide bioavailability) may modulate the course of the disease, including the frequency and severity of vaso-occlusive crises and acute chest syndrome episodes. Possible effects of chronic nitric oxide-based therapies on erythrocyte density, pulmonary artery pressures, and fetal hemoglobin induction deserve study.

Horseradish peroxidase catalyzed nitric oxide formation from hydroxyurea

Hydroxyurea represents an approved treatment for sickle cell anemia and a number of cancers. Chemiluminescence and electron paramagnetic resonance spectroscopic studies show horseradish peroxidase catalyzes the formation of nitric oxide from hydroxyurea in the presence of hydrogen peroxide. Gas chromatographic headspace analysis and infrared spectroscopy also reveal the production of nitrous oxide in this reaction, which provides evidence for nitroxyl, the one-electron reduced form of nitric oxide. These reactions also generate carbon dioxide, ammonia, nitrite, and nitrate. None of these products form within 1 h in the absence of hydrogen peroxide or horseradish peroxidase. Electron paramagnetic resonance spectroscopy and trapping studies show the intermediacy of a nitroxide radical and a C-nitroso species during this reaction. Absorption spectroscopy indicates that both compounds I and II of horseradish peroxidase act as one-electron oxidants of hydroxyurea. Nitroxyl, generated from Angeli's salt, reacts with ferric horseradish peroxidase to produce a ferrous horseradish peroxidase-nitric oxide complex. Electron paramagnetic resonance experiments with a nitric oxide specific trap reveal that horseradish peroxidase is capable of oxidizing nitroxyl to nitric oxide. A mechanistic model that includes the observed nitroxide radical and C-nitroso compound intermediates has been forwarded to explain the observed product distribution. These studies suggest that direct nitric oxide producing reactions of hydroxyurea and peroxidases may contribute to the overall pharmacological properties of this drug.

Iron nitrosyl hemoglobin formation from the reactions of hemoglobin and hydroxyurea

Hydroxyurea represents an approved treatment for sickle cell anemia and acts as a nitric oxide donor under oxidative conditions in vitro. Electron paramagnetic resonance spectroscopy shows that hydroxyurea reacts with oxy-, deoxy-, and methemoglobin to produce 2-6% of iron nitrosyl hemoglobin. No S-nitrosohemoglobin forms during these reactions. Cyanide and carbon monoxide trapping studies reveal that hydroxyurea oxidizes deoxyhemoglobin to methemoglobin and reduces methemoglobin to deoxyhemoglobin. Similar experiments reveal that iron nitrosyl hemoglobin formation specifically occurs during the reaction of hydroxyurea and methemoglobin. Experiments with hydroxyurea analogues indicate that nitric oxide transfer requires an unsubstituted acylhydroxylamine group and that the reactions of hydroxyurea and deoxy- and methemoglobin likely proceed by inner-sphere mechanisms. The formation of nitrate during the reaction of hydroxyurea and oxyhemoglobin and the lack of nitrous oxide production in these reactions suggest the intermediacy of nitric oxide as opposed to its redox form nitroxyl. A mechanistic model that includes a redox cycle between deoxyhemoglobin and methemoglobin has been forwarded to explain these results that define the reactivity of hydroxyurea and hemoglobin. These direct nitric oxide producing reactions of hydroxyurea and hemoglobin may contribute to the overall pathophysiological properties of this drug.

Nitric oxide donor properties of hydroxyurea in patients with sickle cell disease

Hydroxyurea therapy reduces the rates of vaso-occlusive crisis in patients with sickle cell anaemia and recent data suggest that hydroxyurea treatment can generate nitric oxide (NO). Nitric oxide has been proposed as a novel therapy for sickle cell disease via a number of pathways. We therefore sought to determine whether hydroxyurea has NO donor properties in patients with sickle cell anaemia and explore potential mechanisms by which NO production could be therapeutic. Venous blood was collected from 19 fasting sickle cell anaemia patients, on chronic hydroxyurea therapy, at baseline and 2 and 4 h after a single morning dose of hydroxyurea, as well as 10 patients not taking hydroxyurea. The plasma and red cell NO reaction products nitrate, nitrite and nitrosylated- haemoglobin were measured using ozone-based chemiluminescent assays (using vanadium, KI and I3- reductants respectively). Consistent with NO release from hydroxyurea, baseline levels of total nitrosylated haemoglobin increased from 300 nmol/l to 500 nmol/l (P = 0.01). Plasma nitrate and nitrite levels also significantly increased with peak levels observed at 2 h. Glutathionyl-haemoglobin levels were unchanged, while plasma secretory vascular cellular adhesion molecule-1 levels were reduced in patients taking hydroxyurea (419 +/- 40 ng/ml) compared with control patients with sickle cell anaemia (653 +/- 55 ng/ml; P = 0.003), and were inversely correlated with fetal haemoglobin levels (r = -0.72; P = 0.002). These results demonstrate that hydroxyurea therapy is associated with the intravascular and intraerythrocytic generation of NO. The role of NO in the induction of fetal haemoglobin and possible synergy between NO donor therapy and classic cytostatic and differentiating medications should be explored.

Nitric oxide therapy in sickle cell disease

Recent clinical and experimental data suggest that nitric oxide (NO) may play a role in the pathogenesis and therapy of sickle cell disease. NO, a soluble gas continuously synthesized in endothelial cells by the NO synthase (NOS) enzyme systems, regulates basal vascular tone and endothelial function, and maintains blood oxygenation via hypoxic pulmonary vasoconstriction and reduced shunt physiology. These vital homeostatic processes may be impaired in sickle cell disease and contribute to its pathogenesis. Therapeutic NO inhalation exerts significant direct effects on the pulmonary vasculature to reduce pulmonary pressures and increase oxygenation that may prove beneficial in acute chest syndrome and secondary pulmonary hypertension. Delivery of NO bound to hemoglobin or in plasma may improve blood flow and hemoglobin saturation, and thus reduce ischemia-reperfusion injury. Other NO-related effects on adhesion molecule expression and fetal hemoglobin induction are of interest. While direct evidence for a clinical benefit of NO therapy in sickle cell disease has not been reported, studies are underway to determine if inhaled NO will reduce the substantial morbidity and mortality suffered by these patients.