Interaction of nitrogen monoxide with hemoglobin and the artefactual production of S-nitroso-hemoglobin

How Nitric Oxide Hindered the Search for Hemoglobin-Based Oxygen Carriers as Human Blood Substitutes

The search for a clinically affordable substitute of human blood for transfusion is still an unmet need of modern society. More than 50 years of research on acellular hemoglobin (Hb)-based oxygen carriers (HBOC) have not yet produced a single formulation able to carry oxygen to hemorrhage-challenged tissues without compromising the body's functions. Of the several bottlenecks encountered, the high reactivity of acellular Hb with circulating nitric oxide (NO) is particularly arduous to overcome because of the NO-scavenging effect, which causes life-threatening side effects as vasoconstriction, inflammation, coagulopathies, and redox imbalance. The purpose of this manuscript is not to add a review of candidate HBOC formulations but to focus on the biochemical and physiological events that underly NO scavenging by acellular Hb. To this purpose, we examine the differential chemistry of the reaction of NO with erythrocyte and acellular Hb, the NO signaling paths in physiological and HBOC-challenged situations, and the protein engineering tools that are predicted to modulate the NO-scavenging effect. A better understanding of two mechanisms linked to the NO reactivity of acellular Hb, the nitrosylated Hb and the nitrite reductase hypotheses, may become essential to focus HBOC research toward clinical targets.

EPR characterisation of the ferrous nitrosyl complex formed within the oxygenase domain of NO synthase

Nitric oxide is produced in mammals by a class of enzymes called NO synthases (NOSs). It plays a central role in cellular signalling but also has deleterious effects, as it leads to the production of reactive oxygen and nitrogen species. NO forms a relatively stable adduct with ferrous haem proteins, which, in the case of NOS, is also a key catalytic intermediate. Despite extensive studies on the ferrous nitrosyl complex of other haem proteins (in particular myoglobin), little characterisation has been performed in the case of NOS. We report here a temperature-dependent EPR study of the ferrous nitrosyl complex of the inducible mammalian NOS and the bacterial NOS-like protein from Bacillus subtilis. The results show that the overall behaviours are similar to those observed for other haem proteins, but with distinct ratios between axial and rhombic forms in the case of the two NOS proteins. The distal environment appears to control the existence of the axial form and the evolution of the rhombic form.

Relative roles of heme-irons and globin-thiols in the genesis of acellular hemoglobin mediated vasoconstriction

In addition to heme-irons, reactive (beta-globin thiols (betaCys93s) of hemoglobin (Hb) also have been shown to interact with endogenous nitric oxide (NO) thereby contributing to vascular tone regulation. What relative roles do these NO binding sites contribute to the overall Hb-mediated vasoactivity? Several test Hbs with either or both the NO binding sites preliganded or blocked were prepared and tested in a rat thoracic aortic ring model. Hbs tested were: NEM-Hb (ferrous Hb with masked thiols), HbNO (ferrous Hb preliganded with NO), Hb(+)CN (ferric Hb liganded with CN(-)), NEM-HbNO and NEM-Hb(+)CN (Hbs with both heme-iron and cysteine sites preliganded or blocked). Typically, >0.2 microM control Hb significantly increased isometric tension in agonist stimulated vessel rings (58.1 +/-7.0% over baseline). At comparable concentrations, NEM-Hb also caused a significant contraction (50.7+/-9.5%) while HbNO and Hb(+)CN did not (-5.5+/-6.0% and -3.7+/-4.6%, respectively). For these Hbs, masking thiols as well did not significantly alter respective vascular effects. Ferrous sperm whale myoglobin (Mb), which has no reactive thiol, elicited a significant contraction (55.1+/-13.2%) while metMb did not (-0.8+/-3.2%), suggesting the relative importance of heme-iron ligand and oxidation state in Hb vasoactivity. Additionally, ferrous or ferric equine heart cytochrome-C, a heme protein with no readily available heme-iron and cysteine binding sites, did not elicit notable contraction. Human Hb variants in which (betaCys93s are deleted or substituted with non-cysteine residues did not reveal any documented significant hemodynamic abnormalities. These results indicate that reactive globin-thiols do not appear to play a prominent role relative to heme-irons in Hb-mediated vasoconstriction.

Differential effect of materials for surface hemostasis on red blood cell morphology

The design of devices for surface (topical) hemostasis has been based on maximizing activation of platelets and accelerating coagulation pathways. The studies reported herein examine another aspect of blood contact with topical hemostasis materials, i.e., surface binding of red blood cells (RBCs) and related alterations in RBC morphology. Whole blood was allowed to contact poly-N-acetyl glucosamine (pGlcNAc) containing materials: pGlcNAc nanofibers with parallel polymer alignment (beta-pGlcNAc), chitin, and chitosan. The effect on RBC morphology and function via contact with the artificial surfaces on the cell's morphology was examined with scanning and transmission electron microscopy (TEM). beta-pGlcNAc was found to densely bind RBCs and induce a stomatocytic-like morphology. Chitin and chitosan also bound RBCs, but with approximately 10-fold lower levels and with less distinct general morphologies. beta-pGlcNAc is thus unique in the nature of its interaction with RBCs. These studies indicate that the differential ability of various materials to bind and alter the morphology of RBCs at the artificial surface interface with blood is an important consideration in the design of devices for surface hemostasis.

The structure and function of plant hemoglobins

Plants, like humans, contain hemoglobin. Three distinct types of hemoglobin exist in plants: symbiotic, non-symbiotic, and truncated hemoglobins. Crystal structures and other structural and biophysical techniques have revealed important knowledge about ligand binding and conformational stabilization in all three types. In symbiotic hemoglobins (leghemoglobins), ligand binding regulatory mechanisms have been shown to differ dramatically from myoglobin and red blood cell hemoglobin. In the non-symbiotic hemoglobins found in all plants, crystal structures and vibrational spectroscopy have revealed the nature of the structural transition between the hexacoordinate and ligand-bound states. In truncated hemoglobins, the abbreviated globin is porous, providing tunnels that may assist in ligand binding, and the bound ligand is stabilized by more than one distal pocket residue. Research has implicated these plant hemoglobins in a number of possible functions differing among hemoglobin types, and possibly between plant species.

Modulation of synaptic and channel activities in the respiratory network of the mice by NO/cGMP signalling pathways

We examined signalling pathways which can involve NO as a second messenger in the respiratory network. In the functional slice preparation, NO donors depressed the respiratory motor output and enhanced its depression after brief episodes of hypoxia. In the inspiratory neurons, NO donors suppressed spontaneous excitatory and inhibitory synaptic currents, activated single K(ATP) channels and inhibited L-type Ca2+ channels. NO scavengers, PTIO and hemoglobin, and the blocker of NO synthase, N-monomethyl-L-arginine, induced effects opposite to those of NO donors and indicated the role of endogenously generated NO in the modulation of the respiratory activity. Using fluorescent dyes DAF-2 and DCF, we imaged NO and reactive oxygen species (ROS). Concentrations of NO and ROS increased during brief episodes of hypoxia and they both contributed to the activation of K(ATP) channels due to oxygen withdrawal. The oxidizing agent t-butyl-hydroperoxide acted similarly to NO donors but it did not interfere with the effects of NO. Increase in cGMP levels with 8-Br-cGMP reproduced the actions of NO donors and occluded the effects of their subsequent applications. We propose that in the respiratory neurons, a constitutive production of NO is responsible for a tonic activation of cGMP-coupled signalling pathways and changes in NO levels modulate the respiratory motor output by altering the activity of K(ATP) and L-type Ca2+ channels.

Oxyleghemoglobin scavenges nitrogen monoxide and peroxynitrite: a possible role in functioning nodules?

It has been demonstrated that the NO* produced by nitric oxide synthase or by the reduction of nitrite by nitrate reductase plays an important role in plants' defense against microbial pathogens. The detection of nitrosyl Lb in nodules strongly suggests that NO* is also formed in functional nodules. Moreover, NO* may react with superoxide (which has been shown to be produced in nodules by various processes), leading to the formation of peroxynitrite. We have determined the second-order rate constants of the reactions of soybean oxyleghemoglobin with nitrogen monoxide and peroxynitrite. At pH 7.3 and 20 degrees C, the values are on the order of 10(8) and 10(4) M-1 s-1, respectively. In the presence of physiological amounts of CO2 (1.2 mM), the second-order rate constant of the reaction of oxyleghemoglobin peroxynitrite is even larger (10(5) M-1 s-1). The results presented here clearly show that oxyleghemoglobin is able to scavenge any NO* and peroxynitrite formed in functional nodules. This may help to stop NO* triggering a plant defense reaction.

The solid-state molecular structure of the S-nitroso derivative of L-cysteine ethyl ester hydrochloride

Nitrosation of protein sulfhydryl groups to form thionitrites (S-nitrosothiols) has been reported to be important in the biochemistry of nitric oxide. Such S-nitrosation of protein thiol residues has been shown to alter the function of some proteins. In this brief communication, we report the X-ray crystal structure of S-nitroso-L-cysteine ethyl ester hydrochloride. Two rotamers with respect to the NCCS moiety are present in the crystal: the major rotamer is in the gauche+ conformation, and the minor rotamer is in the rare anti (trans, antiperiplanar) conformation for a cysteinyl compound. Importantly, the CSNO groups for both rotamers are in the syn (cis, synperiplanar) form. To the best of our knowledge, this is the first reported high-resolution solid-state structure of an S-nitroso derivative of a cysteine or cysteinyl-containing compound.

Kinetics and Mechanism of•NO2Reacting with Various Oxidation States of Myoglobin

Nitrogen dioxide ((*)NO(2)) participates in a variety of biological reactions. Of great interest are the reactions of (*)NO(2) with oxymyoglobin and oxyhemoglobin, which are the predominant hemeproteins in biological systems. Although these reactions occur rapidly during the nitrite-catalyzed autoxidation of hemeproteins, their roles in systems producing (*)NO(2) in the presence of these hemeproteins have been greatly underestimated. In the present study, we employed pulse radiolysis to study directly the kinetics and mechanism of the reaction of oxymyoglobin (MbFe(II)O(2)) with (*)NO(2). The rate constant of this reaction was determined to be (4.5 +/- 0.3) x 10(7) M(-1)s(-1), and is among the highest rate constants measured for (*)NO(2) with any biomolecule at pH 7.4. The interconversion among the various oxidation states of myoglobin that is prompted by nitrogen oxide species is remarkable. The reaction of MbFe(II)O(2) with (*)NO(2) forms MbFe(III)OONO(2), which undergoes rapid heterolysis along the O-O bond to yield MbFe(V)=O and NO(3-). The perferryl-myoglobin (MbFe(V)=O) transforms rapidly into the ferryl species that has a radical site on the globin ((*)MbFe(IV)=O). The latter oxidizes another oxymyoglobin (10(4) M(-1)s(-1) < k(17) < 10(7) M(-1)s(-1)) and generates equal amounts of ferrylmyoglobin and metmyoglobin. At much longer times, the ferrylmyoglobin disappears through a relatively slow comproportionation with oxymyoglobin (k(18) = 21.3 +/- 5.3 M(-1)s(-1)). Eventually, each (*)NO(2) radical converts three oxymyoglobin molecules into metmyoglobin. The same intermediate, namely MbFe(III)OONO(2), is also formed via the reaction peroxynitrate (O(2)NOO(-)/O(2)NOOH) with metmyoglobin (k(19) = (4.6 +/- 0.3) x 10(4) M(-1)s(-1)). The reaction of (*)NO(2) with ferrylmyoglobin (k(20) = (1.2 +/- 0.2) x 10(7) M(-1)s(-1)) yields MbFe(III)ONO(2), which in turn dissociates (k(21) = 190 +/- 20 s(-1)) into metmyoglobin and NO(3-). This rate constant was found to be the same as that measured for the decay of the intermediate formed in the reaction of MbFe(II)O(2) with (*)NO, which suggests that MbFe(III)ONO(2) is the intermediate observed in both processes. This conclusion is supported by thermokinetic arguments. The present results suggest that hemeproteins may detoxify (*)NO(2) and thus preempt deleterious processes, such as nitration of proteins. Such a possibility is substantiated by the observation that the reactions of (*)NO(2) with the various oxidation states of myoglobin lead to the formation of metmyoglobin, which, though not functional in the gas transport, is nevertheless nontoxic at physiological pH.

Critical redox and allosteric aspects of nitric oxide interactions with hemoglobin

Nitric oxide (NO) is an important signaling molecule. Relatively long-lived NO adducts at the heme and SH groups of hemoglobin (Hb) could enable NO to carry out long-range signaling functions. In spite of significant advances, there remain as yet unresolved issues regarding the possible role of Hb in moderating NO-signaling events that affect blood pressure regulation. In this review, we summarize recent reports concerning the redox and allosteric aspects of NO/Hb interactions that have advanced our understanding of the physiological significance of NO binding to heme groups (forming NO-Hb) and of reactions promoting formation of S-nitrosated Hb (SNO-Hb). Allosteric mechanisms modify the bioactivity of NO/Hb complexes by altering the lifetime of NO-Hb and the properties of SNO-Hb. Redox reactions are significant because of the complex chemistry possible for NO and its oxidation products. Reactions at ferrous and ferric heme sites have differing consequences and affinities for interactions with NO. Moreover, redox changes at heme groups affect reactivity of SH groups and vice versa. In spite of low levels of NO-Hb and SNO-Hb found in vivo, recent findings do not rule out participation of NO-Hb or SNO-Hb in NO-dependent signaling reactions.

Red blood cell pH, the Bohr effect, and other oxygenation-linked phenomena in blood O2 and CO2 transport

The discovery of the S-shaped O2 equilibrium curve and the Bohr effect in 1904 stimulated a fertile and continued research into respiratory functions of blood and allosteric mechanisms in haemoglobin (Hb). The Bohr effect (influence of pH/CO2 on Hb O2 affinity) and the reciprocal Haldane effect (influence of HbO2 saturation on H+/CO2 binding) originate in the Hb oxy-deoxy conformational change and allosteric interactions between O2 and H+/CO2 binding sites. In steady state, H+ is passively distributed across the vertebrate red blood cell (RBC) membrane, and intracellular pH (pHi) changes are related to changes in extracellular pH, Hb-O2 saturation and RBC organic phosphate content. As the Hb molecule shifts between the oxy and deoxy conformation in arterial-venous gas transport, it delivers O2 and takes up CO2 and H+ in tissue capillaries (elegantly aided by the Bohr effect). Concomitantly, the RBC may sense local O2 demand via the degree of Hb deoxygenation and release vasodilatory agents to match local blood flow with requirements. Three recent hypotheses suggest (1) release of NO from S-nitroso-Hb upon deoxygenation, (2) reduction of nitrite to vasoactive NO by deoxy haems, and (3) release of ATP. Inside RBCs, carbonic anhydrase (CA) provides fast hydration of metabolic CO2 and ensures that the Bohr shift occurs during capillary transit. The formed H+ is bound to Hb (Haldane effect) while HCO3- is shifted to plasma via the anion exchanger (AE1). The magnitude of the oxylabile H+ binding shows characteristic differences among vertebrates. Alternative strategies for CO2 transport include direct HCO3- binding to deoxyHb in crocodilians, and high intracellular free [HCO3-] (due to high pHi) in lampreys. At the RBC membrane, CA, AE1 and other proteins may associate into what appears to be an integrated gas exchange metabolon. Oxygenation-linked binding of Hb to the membrane may regulate glycolysis in mammals and perhaps also oxygen-sensitive ion transport involved in RBC volume and pHi regulation. Blood O2 transport shows several adaptive changes during exposure to environmental hypoxia. The Bohr effect is involved via the respiratory alkalosis induced by hyperventilation, and also via the pHi change that results from modulation of RBC organic phosphate content. In teleost fish, beta-adrenergic activation of Na+/H+ exchange rapidly elevates pHi and O2 affinity, particularly under low O2 conditions.