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

Methaemalbumin formation in sickle cell disease: effect on oxidative protein modification and HO-1 induction

Normally, cell free haemoglobin is bound by haptoglobin and efficiently cleared. However, the chronic haemolysis in sickle cell disease (SCD) overwhelms haptoglobin binding capacity and protein turnover, resulting in elevated cell free haemoglobin. Cell free haemoglobin acts as both a scavenger of vasoactive nitric oxide and a pro-oxidant. In addition, methaemoglobin (metHb) releases the haem moiety, which can bind to albumin to form methaemalbumin (metHSA). This study used electron paramagnetic resonance to detect metHSA in SCD plasma and demonstrated that haptoglobin prevents haem transfer from metHb to HSA. MetHSA may either provide a second line of defence against haemoglobin/haem-mediated oxidation or contribute to the pro-oxidant environment of SCD plasma. We demonstrated that HSA inhibited oxidative protein modification induced by metHb. Additionally, we showed that while metHb induced haem oxygenase 1 (HO-1), an indicator of oxidative stress, HSA attenuated metHb induction of this enzyme, thereby limiting the potential benefits of HO-1. Furthermore, HO-1 induction by metHSA was less than HO-1 induction by equimolar metHb not bound to albumin. Our findings confirm the presence of metHSA in SCD and suggest that haem transfer from metHb to HSA reduces the oxidative effects of free haemoglobin/haem on endothelium with both beneficial (reduced protein oxidation) and potentially harmful (reduced HO-1 induction) outcomes.

Methylene blue covalently loaded polyacrylamide nanoparticles for enhanced tumor-targeted photodynamic therapy

The use of targeted nanoparticles (NPs) as a platform for loading photosensitizers enables selective accumulation of the photosensitizers in the tumor area, while maintaining their photodynamic therapy (PDT) effectiveness. Here two novel kinds of methylene blue (MB)-conjugated polyacrylamide (PAA) nanoparticles, MBI-PAA NPs and MBII-PAA NPs, based on two separate MB derivatives, are developed for PDT. This covalent conjugation with the NPs (i) improves the loading of MB, (ii) prevents any leaching of MB from the NPs and (iii) protects the MB from the effects of enzymes in the biological environment. The loading of MB into these two kinds of NPs was controlled by the input amount, resulting in concentrations with optimal singlet oxygen production. For each of the MB-NPs, the highest singlet oxygen production was found for an MB loading of around 11 nmol mg(-1). After attachment of F3 peptide groups, for targeting, each of these NPs was taken up, selectively, by MDA-MB-435 tumor cells, in vitro. PDT tests demonstrated that both kinds of targeted NPs resulted in effective tumor cell kill, following illumination, while not causing dark toxicity.

Global gene expression profiling of endothelium exposed to heme reveals an organ-specific induction of cytoprotective enzymes in sickle cell disease

BACKGROUND

Sickle cell disease (SCD) is characterized by hemolysis, vaso-occlusion and ischemia reperfusion injury. These events cause endothelial dysfunction and vasculopathies in multiple systems. However, the lack of atherosclerotic lesions has led to the idea that there are adaptive mechanisms that protect the endothelium from major vascular insults in SCD patients. The molecular bases for this phenomenon are poorly defined. This study was designed to identify the global profile of genes induced by heme in the endothelium, and assess expression of the heme-inducible cytoprotective enzymes in major organs impacted by SCD.

METHODS AND FINDINGS

Total RNA isolated from heme-treated endothelial monolayers was screened with the Affymetrix U133 Plus 2.0 chip, and the microarray data analyzed using multiple bioinformatics software. Hierarchical cluster analysis of significantly differentially expressed genes successfully segregated heme and vehicle-treated endothelium. Validation studies showed that the induction of cytoprotective enzymes by heme was influenced by the origin of endothelial cells, the duration of treatment, as well as the magnitude of induction of individual enzymes. In agreement with these heterogeneities, we found that induction of two major Nrf2-regulated cytoprotective enzymes, heme oxygenase-1 and NAD(P)H:quinone oxidoreductase-1 is organ-specific in two transgenic mouse models of SCD. This data was confirmed in the endothelium of post-mortem lung tissues of SCD patients.

CONCLUSIONS

Individual organ systems induce unique profiles of cytoprotective enzymes to neutralize heme in SCD. Understanding this heterogeneity may help to develop effective therapies to manage vasculopathies of individual systems.

Influence of high dose tumescent local anaesthesia with prilocaine on systemic interleukin (IL)-6, IL-8 and tumour necrosis factor-α

BACKGROUND AND OBJECTIVE

Tumescent local anaesthesia (TLA) with high prilocaine doses leads to formation of methemoglobin (MHb) which is known to be a potent activator of pro-inflammatory endothelial cell response in vitro. As TLA is widely used for large dermatological resections, the aim of this study was to investigate the effects of high prilocaine doses on the systemic inflammatory response in vivo and its clinical relevance.

METHODS

This prospective study examines the influence of MHb on serum interleukin (IL)-6, IL-8 and tumour necrosis tumour necrosis (TNF)-α levels up to 72 h after application of TLA with prilocaine in doses higher than 600 mg.

RESULTS

A total of 30 patients received prilocaine in a median dose of 1500 mg (range: 880-4160 mg) for large resections. Peak prilocaine serum concentration was reached 4 h (0.72 ± 0.07 μg/mL), the maximum concentration of MHb (7.43 ± 0.87%) and IL-6 (28.4 ± 4.1 U/L) 12 h after TLA application. TNF-α and IL-8 release were not found significantly increased. Three patients developed MHb concentrations >15%.

CONCLUSIONS

This clinical study shows for the first time that a high prilocaine serum concentration leads in vivo to elevated systemic levels of IL-6 but not of IL-8 and TNF-α because of initial high MHb levels. Because of possible and unpredictable high MHb concentrations, TLA should only be performed with prilocaine in doses of 2.5 mg/kg. In general, new solutions of TLA are necessary to achieve adequate anaesthesia for large dermatological resections to decrease the risk of methemoglobinaemia.

Regulation of hemocytes in Drosophila requires dappled cytochrome b5

A major category of mutant hematopoietic phenotypes in Drosophila is melanotic tumors or nodules, which consist of abnormal and overproliferated blood cells, similar to granulomas. Our analyses of the melanotic mutant dappled have revealed a novel type of gene involved in blood cell regulation. The dappled gene is an essential gene that encodes cytochrome b5, a conserved hemoprotein that participates in electron transfer in multiple biochemical reactions and pathways. Viable mutations of dappled cause melanotic nodules and hemocyte misregulation during both hematopoietic waves of development. The sexes are similarly affected, but hemocyte number is different in females and males of both mutants and wild type. Additionally, initial tests show that curcumin enhances the dappled melanotic phenotype and establish screening of endogenous and xenobiotic compounds as a route for analysis of cytochrome b5 function. Overall, dappled provides a tractable genetic model for cytochrome b5, which has been difficult to study in higher organisms.

Dose response of sodium nitrite on vasoactivity associated with HBOC-201 in a swine model of controlled hemorrhage

Sodium nitrite (NaNO(2)) was evaluated in a 55% EBV hemorrhage swine model to mitigate the increased blood pressure due to HBOC-201. Animals were resuscitated by three 10 ml/kg infusions of either HBOC-201 or Hextend with and without NaNO(2). All vital signs, coagulation and blood chemistry were measured for 2 hr. HBOC-201-vasoconstriction was attenuated only after the first 10.8 μmol/kg NaNO(2) infusion. Complete abolition was obtained with the highest 3 NaNO(2) dose, but side effects were observed. There was no reduction in platelet function due to NaNO(2). NaNO(2) ability to reduce HBOC-201 vasoactivity was transient and 10.8 μmol/kg NaNO(2) seems an acceptable dose for further investigation.

Interactions of anionic surfactants with methemoglobin

Interactions of two anionic surfactants, sodium dodecyl sulphate (SDS) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at concentrations below and above critical micelle concentration with methemoglobin (metHb) have been investigated by conventional as well as by stopped-flow absorption and fluorescence spectroscopy. The absorption spectra of metHb in AOT reverse micelles have been also analyzed. Both surfactants in their monomeric form convert metHb to reversible hemichrome. This is connected with a diminution of peroxidase-like activity of metHb and with an increase of the susceptibility of heme for a damage by H(2)O(2). In micellar solutions of AOT and SDS as well as in AOT reverse micelles pentacoordinated ferric species seems to be the predominant form of this protein. It has been concluded, basing on a kinetic analysis, that conformational changes in the heme environment of metHb as induced by both surfactants occur independently of the alterations in the tertiary structure of this protein.

Methaemoglobinaemia due to mephedrone ('snow')

Acquired methaemoglobinaemia is a serious complication caused by many oxidising drugs. It presents as cyanosis unresponsive to oxygen therapy. The case of 33-year-old male patient who presented in our department after noticing blue lips and fingers is presented. He had sniffed 1 g of 'snow' after buying it from a head shop. His oxygen saturation by pulse oximeter on room air at presentation was 90%, which did not improve with supplemental oxygen. Arterial blood gas analyses showed partial pressure of oxygen 37 kPa while on supplemental oxygen and a methaemoglobin concentration greater than 25%. The patient denied using any other recreational drugs and was not on regular treatment. Therefore, a diagnosis of methaemoglobinaemia due to mephedrone, which is the active ingredient of 'snow', was made. Treatment is with intravenous methylene blue. Our patient started to improve so methylene blue was not used and he was discharged after 8 h.

Shr of group A streptococcus is a new type of composite NEAT protein involved in sequestering haem from methaemoglobin

A growing body of evidence suggests that surface or secreted proteins with NEAr Transporter (NEAT) domains play a central role in haem acquisition and trafficking across the cell envelope of Gram-positive bacteria. Group A streptococcus (GAS), a β-haemolytic human pathogen, expresses a NEAT protein, Shr, which binds several haemoproteins and extracellular matrix (ECM) components. Shr is a complex, membrane-anchored protein, with a unique N-terminal domain (NTD) and two NEAT domains separated by a central leucine-rich repeat region. In this study we have carried out an analysis of the functional domains in Shr. We show that Shr obtains haem in solution and furthermore reduces the haem iron; this is the first report of haem reduction by a NEAT protein. More specifically, we demonstrate that both of the constituent NEAT domains of Shr are responsible for binding haem, although they are missing a critical tyrosine residue found in the ligand-binding pocket of other haem-binding NEAT domains. Further investigations show that a previously undescribed region within the Shr NTD interacts with methaemoglobin. Shr NEAT domains, however, do not contribute significantly to the binding of methaemoglobin but mediate binding to the ECM components fibronectin and laminin. A protein fragment containing the NTD plus the first NEAT domain was found to be sufficient to sequester haem directly from methaemoglobin. Correlating these in vitro findings to in vivo biological function, mutants analysis establishes the role of Shr in GAS growth with methaemoglobin as a sole source of iron, and indicates that at least one NEAT domain is necessary for the utilization of methaemoglobin. We suggest that Shr is the prototype of a new group of NEAT composite proteins involved in haem uptake found in pyogenic streptococci and Clostridium novyi.

Inorganic nitrite and chronic tissue ischaemia: a novel therapeutic modality for peripheral vascular diseases

Ischaemic tissue damage represents the ultimate form of tissue pathophysiology due to cardiovascular disease, which is the leading cause of morbidity and mortality across the globe. A significant amount of basic research and clinical investigation has been focused on identifying cellular and molecular pathways to alleviate tissue damage and dysfunction due to ischaemia and subsequent reperfusion. Over many years, the gaseous molecule nitric oxide (NO) has emerged as an important regulator of cardiovascular health as well as protector against tissue ischaemia and reperfusion injury. However, clinical translation of NO therapy for these pathophysiological conditions has not been realized for various reasons. Work from our laboratory and several others suggests that a new form of NO-associated therapy may be possible through the use of nitrite anion (sodium nitrite), a prodrug which can be reduced to NO in ischaemic tissues. In this manner, nitrite anion serves as a highly selective NO donor in ischaemic tissues without substantially altering otherwise normal tissue. This surprising and novel discovery has reinvigorated hopes for effectively restoring NO bioavailability in vulnerable tissues while continuing to reveal the complexity of NO biology and metabolism within the cardiovascular system. However, some concerns may exist regarding the effect of nitrite on carcinogenesis. This review highlights the emergence of nitrite anion as a selective NO prodrug for ischaemic tissue disorders and discusses the potential therapeutic utility of this agent for peripheral vascular disease.

Acquired methemoglobinemia revisited

Dentistry has two medications in its pain management armamentarium that may cause the potentially life-threatening disorder methemoglobinemia. The first medications are the topical local anesthetics benzocaine and prilocaine. The second medication is the injectable local anesthetic prilocaine. Acquired methemoglobinemia remains a source of morbidity and mortality in dental and medical patients despite the fact that it is better understood now than it was even a decade ago. It is in the interest of all dental patients that their treating dentists review this disorder. The safety of dental patients mandates professional awareness.

Sub-chronic effects of nitrate in drinking water on red-legged partridge (Alectoris rufa): oxidative stress and T-cell mediated immune function

In order to evaluate the effects of nitrates on birds, we have exposed captive red-legged partridges to nitrates concentrations of 0 (control), 100 (dwell water in farming areas) or 500 mg/l (fertirrigation level). The cellular immune response, plasma biochemistry, methemoglobin concentration (metHb), and oxidative stress biomarkers in blood and tissues were studied after two weeks of exposure. Several blood parameters such as aspartate aminotransferase, creatinine phosphokinase and lactate dehydrogenase activities and magnesium level decreased with nitrate exposure, whereas alkaline phosphatase activity and creatinine level increased. The oxidant effect of nitrates was evidenced by the increase in blood metHb, accompanied by the lipid peroxidation of red blood cells, the increased levels of oxidized glutathione (GSH) in liver, and the generation of oxidative DNA damage in plasma lymphocytes. GSH in erythrocytes was negatively correlated with blood metHb. The cellular immune function was slightly lower at partridges exposed to nitrates. These results suggest that adverse effects of nitrates on birds occur at concentrations potentially present in the field.

Deletion of microsomal cytochrome b5 profoundly affects hepatic and extrahepatic drug metabolism

We demonstrated recently that cytochrome b(5) plays an important in vivo role in hepatic cytochrome P450 (P450) function [J Biol Chem 283:31385-31393, 2008]. We have now generated a model in which cytochrome b(5) has been deleted in all tissues [cytochrome b(5) complete null (BCN)], which surprisingly results in a viable mouse despite the putative in vivo roles of this protein in lipid and steroid hormone metabolism and the reduction of methemoglobin. In contrast to the liver-specific deletion, complete deletion of cytochrome b(5) leads to a neonatal increase in the expression of many hepatic P450s at both the protein and mRNA level. In extrahepatic tissues, some changes in P450 expression were also observed that were isoform-dependent. In vitro cytochrome P450 activities in liver, kidney, lung, and small intestine of BCN mice were determined for a range of model substrates and probe drugs; a profound reduction in the metabolism of some substrates, particularly in lung, kidney, and small intestine, was observed. In vivo, the metabolism of metoprolol was significantly altered in BCN mice, in contrast to the previous finding in the liver-specific cytochrome b(5) deletion, suggesting that extrahepatic cytochrome b(5) plays a significant role in its disposition. Testicular Cyp17 hydroxylase and lyase activities were also significantly reduced by cytochrome b(5) deletion, leading to significantly lower levels of testicular testosterone. The BCN mouse provides an additional model system with which to further investigate the functions of cytochrome b(5), particularly in extrahepatic tissues.

Heme oxygenase-1 gene delivery by Sleeping Beauty inhibits vascular stasis in a murine model of sickle cell disease

Increases in heme oxygenase-1 (HO-1) and administration of heme degradation products CO and biliverdin inhibit vascular inflammation and vasoocclusion in mouse models of sickle cell disease (SCD). In this study, an albumin (alb) promoter-driven Sleeping Beauty (SB) transposase plasmid with a wild-type rat hmox-1 (wt-HO-1) transposable element was delivered by hydrodynamic tail vein injections to SCD mice. Eight weeks after injection, SCD mice had three- to five-fold increases in HO-1 activity and protein expression in liver, similar to hemin-treated mice. Immunohistochemistry demonstrated increased perinuclear HO-1 staining in hepatocytes. Messenger RNA transcription of the hmox-1 transgene in liver was confirmed by quantitative real-time polymerase chain reaction restriction fragment length polymorphism (qRT-PCR RFLP) with no detectible transgene expression in other organs. The livers of all HO-1 overexpressing mice had activation of nuclear phospho-p38 mitogen-activated protein kinase (MAPK) and phospho-Akt, decreased nuclear expression of nuclear factor-kappa B (NF-kappaB) p65, and decreased soluble vascular cell adhesion molecule-1 (sVCAM-1) in serum. Hypoxia-induced stasis, a characteristic of SCD, but not normal mice, was inhibited in dorsal skin fold chambers in wt-HO-1 SCD mice despite the absence of hmox-1 transgene expression in the skin suggesting distal effects of HO activity on the vasculature. No protective effects were seen in SCD mice injected with nonsense (ns-) rat hmox-1 that encodes carboxy-truncated HO-1 with little or no enzyme activity. We speculate that HO-1 gene delivery to the liver is beneficial in SCD mice by degrading pro-oxidative heme, releasing anti-inflammatory heme degradation products CO and biliverdin/bilirubin into circulation, activating cytoprotective pathways and inhibiting vascular stasis at sites distal to transgene expression.

Lipid status, anti-oxidant enzyme defence and haemoglobin content in the blood of long-term clozapine-treated schizophrenic patients

OBJECTIVE

Despite clozapine's unique effectiveness in patients with schizophrenia, a number of adverse effects have been recognised including abnormalities in lipid and glucose metabolisms. A high clozapine level in red blood cells (RBCs) and disturbed anti-oxidant enzyme activities in blood from schizophrenic patients prompted us to investigate lipid status and anti-oxidant enzyme defence in the blood of chronic schizophrenic patients on long-term clozapine therapy.

METHODS

Plasma lipids, RBC anti-oxidant enzyme activities and haemoglobin (Hb) content were measured using established procedures in a group of eighteen chronically-medicated (average 630 days of therapy) schizophrenic patients receiving clozapine (average dose of 295 mg/day) and data were compared with those from a group of eighteen well-matched normal controls.

RESULTS

Significantly higher levels of plasma triglycerides (by 47%, p<0.01) and total cholesterol and phospholipids (by 8% and 11%, respectively p<0.05) in patients were found. CuZn-superoxide dismutase (SOD1) activity was markedly higher (by 35%, p<0.001) while selenium-dependent glutathione peroxidase (GSH-Px1) activity was markedly lower (by 41%, p<0.001) in patients. In addition, metHb and HbA1c levels in patients were significantly higher (by 58% and 25%, respectively p<0.001). SOD1 activity was negatively correlated (p<0.001) to GSH-Px1 activity in patients.

CONCLUSIONS

The findings support the view that ongoing oxidative stress may be a mechanism by which clozapine induces some adverse effects that increase the risk of diabetes and metabolic syndrome. If valid, this would indicate that in parallel with long-term clozapine treatment, schizophrenic patients could be encouraged to make some lifestyle changes to limit the detrimental effects of the medication.

Heme degradation and vascular injury

Heme is an essential molecule in aerobic organisms. Heme consists of protoporphyrin IX and a ferrous (Fe(2+)) iron atom, which has high affinity for oxygen (O(2)). Hemoglobin, the major oxygen-carrying protein in blood, is the most abundant heme-protein in animals and humans. Hemoglobin consists of four globin subunits (alpha(2)beta(2)), with each subunit carrying a heme group. Ferrous (Fe(2+)) hemoglobin is easily oxidized in circulation to ferric (Fe(3+)) hemoglobin, which readily releases free hemin. Hemin is hydrophobic and intercalates into cell membranes. Hydrogen peroxide can split the heme ring and release "free" redox-active iron, which catalytically amplifies the production of reactive oxygen species. These oxidants can oxidize lipids, proteins, and DNA; activate cell-signaling pathways and oxidant-sensitive, proinflammatory transcription factors; alter protein expression; perturb membrane channels; and induce apoptosis and cell death. Heme-derived oxidants induce recruitment of leukocytes, platelets, and red blood cells to the vessel wall; oxidize low-density lipoproteins; and consume nitric oxide. Heme metabolism, extracellular and intracellular defenses against heme, and cellular cytoprotective adaptations are emphasized. Sickle cell disease, an archetypal example of hemolysis, heme-induced oxidative stress, and cytoprotective adaptation, is reviewed.

Methemoglobinemia due to topical pharyngeal anesthesia during endoscopic procedures

Acquired methemoglobinemia is a known complication of topical anesthesia during endoscopic procedures. Benzocaine has been implicated in a majority of cases and has been removed from some hospital formularies. However, a lot of uncertainty exists about the risk factors for this uncommon complication. This review attempts to summarize the current knowledge of this condition and describe known risk factors. Anesthetic and patient-specific risk factors are described along with some hypothetical procedure-related factors. Clinicians should identify patients who are at high risk and apply appropriate postprocedure screening. Prospective studies are needed to further our knowledge and improve patient safety.

Imidazoquines as antimalarial and antipneumocystis agents

Peptidomimetic imidazolidin-4-one derivatives of primaquine (imidazoquines) recently displayed in vitro activity against blood schizonts of a chloroquine-resistant strain of Plasmodium falciparum. Preliminary studies with a subset of such imidazoquines showed them to both block transmission of P. berghei malaria from mouse to mosquito and be highly stable toward hydrolysis at physiological conditions. This prompted us to have deeper insight into the activity of imidazoquines against both Plasmodia and Pneumocystis carinii, on which primaquine is also active. Full assessment of the in vivo transmission-blocking activity of imidazoquines, in vitro tissue-schizontocidal activity on P. berghei-infected hepatocytes, and in vitro anti-P. carinii activity is now reported. All compounds were active in these biological assays, with generally lower activity than the parent drug. However, imidazoquines' stability against both oxidative deamination and proteolytic degradation suggest that they will probably have higher oral bioavailability and lower hematotoxicity than primaquine, which might translate into higher therapeutic indexes.

Biopreservation of red blood cells--the struggle with hemoglobin oxidation

One of the least recognized causes of cellular damage during ex vivo preservation of red blood cells is oxidative injury to the hemoglobin. The latter has been associated with hemolysis through the release of toxic substances and oxidation of vital cell components. This review delineates some of the major pathways that link hemoglobin oxidation and cellular damage, and summarizes the incidence of red blood cell oxidative injury during hypothermic storage, cryopreservation and desiccation stress. Red blood cell hypothermic storage, despite its success, is not exempt from oxidative injury. Growing evidence portrays a time-dependant oxidative assault including formation of reactive oxygen species, attachment of denatured hemoglobin to membrane phospholipids and the release of hemoglobin-containing membrane microvesicles throughout storage. Similar symptoms have been observed in attempts to stabilize red blood cells in the dried state, in which methemoglobin levels of reconstituted red blood cells reached 50%. Factors affecting the rate of hemoglobin oxidation during red blood cell ex vivo storage include compromised antioxidant activity, high concentrations of glucose in the storage media and the presence of molecular oxygen. Hemoglobin oxidation largely dictates our ability to effectively preserve red blood cells. Understanding its origins along with investigating methods to minimize it can significantly improve the quality of our future blood products.

Infection and the risk of topical anesthetic induced clinically significant methemoglobinemia after transesophageal echocardiography

BACKGROUND

Methemoglobinemia is a recognized complication of topical anesthesia with benzocaine during transesophageal echocardiography (TEE). Though several risk factors have been described, the importance of individual factors is not known. We performed a retrospective study to identify determinants of the risk of methemoglobinemia.

MATERIALS AND METHODS

All patients who underwent TEE with benzocaine topical anesthesia between June 2005 and June 2007 were included in this retrospective study.

RESULTS

Of the 886 patients who were included in the study, 140 had active infection (15.8%). The incidence of methemoglobinemia in this group was 2.9% (vs. 0%, P < 0.001). Compared to those without infection, patients in the active infection group were more likely to have a lower hemoglobin (P < 0.001), serum albumin level (P < 0.001), glomerular filtration rate less than 60 ml/min per 1.73 m(2) (P < 0.001), higher rates of dialysis (P < 0.001), a higher incidence of malignancy (P = 0.01), and increased use of acetaminophen and sulfa drugs (P < 0.001). However, multivariate logistic regression analysis did not identify any statistically significant covariates.

CONCLUSION

In conclusion, patients with an active systemic infection who undergo TEE are at a higher risk of methemoglobinemia. However, none of the risk factors for methemoglobinemia including active infection reached statistical significance in the regression analysis which has to be interpreted with caution in view of the low event rate.

[Methemoglobin intoxication by prilocaine in EMLA. Accidental intoxication of an infant with scald injuries]

The case of an infant who had received EMLA(R) for local pain therapy after scalding to 5% of the body surface with boiling water is reported. Due to the application of EMLA(R) on the injured skin and exceeding the recommended doses of prilocaine and lidocaine the child developed symptomatic methemoglobinemia. During surgical wound dressing the boy showed cyanosis, decreased peripheral oxygen saturation and potentially suffered a general seizure. With a symptomatic therapy including mechanical ventilation and anticonvulsive drugs the methemoglobinemia normalized within 9 h. The child recovered without any neurological impairment after wound treatment was completed.

Structural analysis of fish versus mammalian hemoglobins: effect of the heme pocket environment on autooxidation and hemin loss

The underlying stereochemical mechanisms for the dramatic differences in autooxidation and hemin loss rates of fish versus mammalian hemoglobins (Hb) have been examined by determining the crystal structures of perch, trout IV, and bovine Hb at high and low pH. The fish Hbs autooxidize and release hemin approximately 50- to 100-fold more rapidly than bovine Hb. Five specific amino acid replacements in the CD corner and along the E helix appear to cause the increased susceptibility of fish Hbs to oxidative degradation compared with mammalian Hbs. Ile is present at the E11 helical position in most fish Hb chains whereas a smaller Val residue is present in all mammalian alpha and beta chains. The larger IleE11 side chain sterically hinders bound O(2) and facilitates dissociation of the neutral superoxide radical, enhancing autooxidation. Lys(E10) is found in most mammalian Hb and forms favorable electrostatic and hydrogen bonding interactions with the heme-7-propionate. In contrast, Thr(E10) is present in most fish Hbs and is too short to stabilize bound heme, and causes increased rates of hemin dissociation. Especially high rates of hemin loss in perch Hb are also due to a lack of electrostatic interaction between His(CE3) and the heme-6 propionate in alpha subunits whereas this interaction does occur in trout IV and bovine Hb. There is also a larger gap for solvent entry into the heme crevice near beta CD3 in the perch Hb (approximately 8 A) compared with trout IV Hb (approximately 6 A) which in turn is significantly higher than that in bovine Hb (approximately 4 A) at low pH. The amino acids at CD4 and E14 differ between bovine and the fish Hbs and have the potential to modulate oxidative degradation by altering the orientation of the distal histidine and the stability of the E-helix. Generally rapid rates of lipid oxidation in fish muscle can be partly attributed to the fact that fish Hbs are highly susceptible to oxidative degradation.

Subcellular localization of the Staphylococcus aureus heme iron transport components IsdA and IsdB

Staphylococcus aureus is a human pathogen that represents a tremendous threat to global public health. An important aspect of S. aureus pathogenicity is the ability to acquire iron from its host during infection. In vertebrates, iron is sequestered predominantly within heme, the majority of which is bound by hemoglobin. To acquire iron, S. aureus binds hemoglobin, removes heme, and transports it into the cytoplasm, where heme is degraded. This process is carried out by the iron-regulated surface determinant system (Isd); however, the mechanism by which hemoglobin recognition occurs is not completely understood. Here we report that the surface receptor components of the Isd system, IsdA and IsdB, physically interact with each other and are anchored to a discrete location within the cell wall. This organized localization pattern is dependent upon the iron status of the bacterium. Furthermore, we have found that hemoglobin colocalizes with IsdB at discrete sites within the cell wall. Virulence studies revealed that IsdB is required for the efficient colonization of the heart and that IsdB is differentially expressed within infected organs, suggesting that S. aureus experiences various degrees of iron starvation depending on the site of infection. These findings significantly expand our understanding of hemoglobin iron acquisition and demonstrate an orchestrated pattern of regulation and localization for the S. aureus heme iron acquisition system.

Methemoglobinemia related to local anesthetics: a summary of 242 episodes

BACKGROUND

The purpose of this article is to summarize all episodes of local anesthetic-related methemoglobinemia found in the medical literature.

METHODS

I performed a search of the American National Library of Medicine's PubMed with the following key words: "local anesthetic" and "methemoglobinemia."

RESULTS

Two-hundred-forty-two episodes (40.1% published in year 2000 or after) were found. Chocolate-colored blood suggests methemoglobinemia but other colors may be found. A discrepancy between the pulse oximeter saturation (< or = 90%) and the arterial oxygen partial pressure (> or = 70 mm Hg) was present in 91.8% of the episodes. The difference between oxygen saturation measured by pulse oximetry and co-oximetry varied from -6.2% to 44.7%. Plain prilocaine may induce clinically symptomatic methemoglobinemia in children older than 6 mo at doses exceeding 2.5 mg/kg. In adults, the dose of prilocaine should be kept lower than 5.0 mg/kg, which is reduced to 3.2 mg/kg in the presence of renal insufficiency and to 1.3 mg/kg if other oxidizing drugs are used concurrently. A single spray of benzocaine may induce methemoglobinemia. Clinical symptoms may be observed at relatively low methemoglobin values, including coma at 32.2 and 29.1% in children and adults, respectively. Rebound methemoglobinemia (benzocaine on mucous membranes) with methemoglobin values as high as 59.9% may occur up to 18 h after methylene blue administration. Complications of methemoglobinemia include hypoxic encephalopathy, myocardial infarction, and death.

CONCLUSION

Benzocaine should no longer be used. Prilocaine should not be used in children younger than 6-mo-old, in pregnant women, or in patients taking other oxidizing drugs. The dose should be limited to 2.5 mg/kg.

Dapsone induced methemoglobinemia in a patient with glioblastoma

Primary brain tumor patients have multiple risk factors for Pneumocystis jiroveci and may require prophylaxis with TMP-SMZ or dapsone. Although dapsone is generally safe and efficacious, we present a case of a patient diagnosed with a brain stem glioblastoma who developed methemoglobinemia and haemolytic anemia after presenting with worsening confusion and cardiopulmonary system dysfunction. This case highlights one of the potentially severe complications associated with dapsone therapy. Although this illustrates an unusual toxicity of dapsone, a high index of suspicion should be given to high-risk patients due to ethnic heritage, anemia, or advanced age. Furthermore, given the toxicities of TMP-SMZ and dapsone, further work is needed to determine the threshold CD4(+) count at which empiric prophylaxis should be initiated.

Neonatal cyanosis due to a novel fetal hemoglobin: Hb F-Circleville [Ggamma63(E7)His-->Leu, CAT>CTT]

Neonatal cyanosis can result from a multitude of acquired and inherited causes. Cyanosis resulting from fetal M hemoglobin (Hb) variants is very rare. Only two (G)gamma variants causing methemoglobinemia and cyanosis in the newborn have been reported to date. Here we describe a novel fetal Hb variant, Hb F-Circleville [Ggamma63(E7)His-->Leu], associated with methemoglobinemia and cyanosis in the newborn. The patient's sister also had neonatal cyanosis at birth.

Functionally distinct NEAT (NEAr Transporter) domains within the Staphylococcus aureus IsdH/HarA protein extract heme from methemoglobin

The pathogen Staphylococcus aureus uses iron-regulated surface determinant (Isd) proteins to scavenge the essential nutrient iron from host hemoproteins. The IsdH protein (also known as HarA) is a receptor for hemoglobin (Hb), haptoglobin (Hp), and the Hb-Hp complex. It contains three NEAT (NEAr Transporter) domains: IsdH(N1), IsdH(N2), and IsdH(N3). Here we show that they have different functions; IsdH(N1) binds Hb and Hp, whereas IsdH(N3) captures heme that is released from Hb. The staphylococcal IsdB protein also functions as an Hb receptor. Primary sequence homology to IsdH indicates that it will also employ functionally distinct NEAT domains to bind heme and Hb. We have used site-directed mutagenesis and surface plasmon resonance methods to localize the Hp and Hb binding surface on IsdH(N1). High affinity binding to these structurally unrelated proteins requires residues located within a conserved aromatic motif that is positioned at the end of the beta-barrel structure. Interestingly, this site is quite malleable, as other NEAT domains use it to bind heme. We also demonstrate that the IsdC NEAT domain can capture heme directly from Hb, suggesting that there are multiple pathways for heme transfer across the cell wall.

Postoperative methemoglobinemia following infiltrative lidocaine administration for combined anesthesia in pediatric craniofacial surgery

BACKGROUND

Infiltrative anesthesia of the scalp with lidocaine was used in an attempt to reduce blood loss and anesthetic requirements during pediatric craniofacial surgery. Lidocaine, however, has the potential to cause methemoglobinemia. In this retrospective cohort-study we analyzed the incidence and effects of postoperative methemoglobinemia following subcutaneous lidocaine administration.

METHODS

During 1999-2006, 50 infants (age: 3-31 months) undergoing elective craniofacial surgery were analyzed. All infants received general anesthesia and routine monitoring, including invasive arterial blood pressure measurement. Prior to incision, the scalp was infiltrated with 6-15 ml lidocaine 1% (with epinephrine 1 : 200.000). Blood loss and blood transfusions were recorded. Methemoglobin (Met-Hb) levels were determined postoperatively using co-oximetry.

RESULTS

Twenty percent of the operated infants showed elevated Met-Hb levels (median of maximal levels: 6%; range: 2.2-18%) at admission on the PICU. In 80% of these methemoglobinemia resolved spontaneously within 12 h, only two children received methylene blue because of visible cyanosis. The intra- and postoperative course was otherwise uneventful in all the children despite significant total blood loss (median of blood loss as percentage from total estimated blood volume: 43%; range: 11-110%). Lidocaine was the only substance identified to have the potential to cause methemoglobinemia. However, the average administered dose of lidocaine was not significantly different between patients with or without methemoglobinemia (13 +/- 3.1 vs 12 +/- 3.5 mg.kg(-1); P = 0.37).

CONCLUSIONS

Even though we did not measure lidocaine plasma levels, lidocaine was the most likely cause of postoperative methemoglobinemia. Despite a high incidence, methemoglobinemia occurred sporadically and was without dangerous consequences.