Oxygen binding and oxidation reactions of human hemoglobin conjugated to carboxylate dextran

Next-generation polymerized human hemoglobins in hepatic bioreactor simulations

Hepatic hollow fiber (HF) bioreactors can be used to provide temporary support to patients experiencing liver failure. Before being connected to the patient's circulation, cells in the bioreactor must be exposed to a range of physiological O₂ concentrations as observed in the liver sinusoid to ensure proper performance. This zonation in cellular oxygenation promotes differences in hepatocyte phenotype and may better approximate the performance of a real liver within the bioreactor. Polymerized human hemoglobin (PolyhHb) locked in the tense quaternary state (T-state) has the potential to both supply and regulate O₂ transport to cultured hepatocytes in the bioreactor due to its low O₂ affinity. In this study, T-state PolyhHb production and purification processes were optimized to minimize the concentration of low-molecular-weight PolyhHb species in solution. Deconvolution of size-exclusion chromatography spectra was performed to calculate the distribution of polymeric Hb species in the final product. Fluid flow and mass transport within a single fiber of a hepatic HF bioreactor was computationally modeled with finite element methods to simulate the effects of employing T-state PolyhHb to facilitate O₂ transport in a hepatic bioreactor system. Optimal bioreactor performance was defined as having a combined hypoxic and hyperoxic volume fraction in the extracapillary space of less than 0.05 where multiple zones were observed. The Damköhler number and Sherwood number had strong inverse relationships at each cell density and fiber thickness combination. These results suggest that targeting a specific Damköhler number may be beneficial for optimal hepatic HF bioreactor operation.

Recent and prominent examples of nano- and microarchitectures as hemoglobin-based oxygen carriers

Blood transfusions, which usually consist in the administration of isolated red blood cells (RBCs), are crucial in traumatic injuries, pre-surgical conditions and anemias. Although RBCs transfusion from donors is a safe procedure, donor RBCs can only be stored for a maximum of 42 days under refrigerated conditions and, therefore, stockpiles of RBCs for use in acute disasters do not exist. With a worldwide shortage of donor blood that is expected to increase over time, the creation of oxygen-carriers with long storage life and compatibility without typing and cross-matching, persists as one of the foremost important challenges in biomedicine. However, research has so far failed to produce FDA approved RBCs substitutes (RBCSs) for human usage. As such, due to unacceptable toxicities, the first generation of oxygen-carriers has been withdrawn from the market. Being hemoglobin (Hb) the main component of RBCs, a lot of effort is being devoted in assembling semi-synthetic RBCS utilizing Hb as the oxygen-carrier component, the so-called Hb-based oxygen carriers (HBOCs). However, a native RBC also contains a multi-enzyme system to prevent the conversion of Hb into non-functional methemoglobin (metHb). Thus, the challenge for the fabrication of next-generation HBOCs relies in creating a system that takes advantage of the excellent oxygen-carrying capabilities of Hb, while preserving the redox environment of native RBCs that prevents or reverts the conversion of Hb into metHb. In this review, we feature the most recent advances in the assembly of the new generation of HBOCs with emphasis in two main approaches: the chemical modification of Hb either by cross-linking strategies or by conjugation to other polymers, and the Hb encapsulation strategies, usually in the form of lipidic or polymeric capsules. The applications of the aforementioned HBOCs as blood substitutes or for oxygen-delivery in tissue engineering are highlighted, followed by a discussion of successes, challenges and future trends in this field.

Site-directed mutagenesis of cysteine residues alters oxidative stability of fetal hemoglobin

Redox active cysteine residues including βCys93 are part of hemoglobin's "oxidation hotspot". Irreversible oxidation of βCys93 ultimately leads to the collapse of the hemoglobin structure and release of heme. Human fetal hemoglobin (HbF), similarly to the adult hemoglobin (HbA), carries redox active γCys93 in the vicinity of the heme pocket. Site-directed mutagenesis has been used in this study to examine the impact of removal and/or addition of cysteine residues in HbF. The redox activities of the recombinant mutants were examined by determining the spontaneous autoxidation rate, the hydrogen peroxide induced ferric to ferryl oxidation rate, and irreversible oxidation of cysteine by quantitative mass spectrometry. We found that substitution of γCys93Ala resulted in oxidative instability characterized by increased oxidation rates. Moreover, the addition of a cysteine residue at α19 on the exposed surface of the α-chain altered the regular electron transfer pathway within the protein by forming an alternative oxidative site. This may also create an accessible site for di-sulfide bonding between Hb subunits. Engineering of cysteine residues at suitable locations may be useful as a tool for managing oxidation in a protein, and for Hb, a way to stave off oxidation reactions resulting in a protein structural collapse.

A facile way to prepare functionalized dextran nanogels for conjugation of hemoglobin

Nanogels with several special advantages have been widely applied in protein delivery. However, biocompatible and biodegradable nanogels used for hemoglobin (Hb) delivery are far less explored. Herein, we developed a facile method to prepare functionalized dextran nanogels for conjugation of Hb. In situ cross-linked and aldehyde group functionalized nanogels (FNGs) were prepared from dextran-g-succinic anhydride-g-dopamine conjugate (Dex-SA-DA) assembly by simple pH adjustion and oxidization in water. Hb was further conjugated into the swelling FNGs by Schiff base reaction under mild condition. The obtained hemoglobin-loaded nanogels (HbNGs) exhibited high stability, oxygen affinity and good hemo-compatibility, suggesting the potential for oxygen carriers. We expected that the designed functionalized nanogels with high stability and loading capacity could bring a new opportunity for protein delivery.

Artificial Blood Substitutes: First Steps on the Long Route to Clinical Utility

The 21st century is challenging for human beings. Increased population growth, population aging, generation of new infectious agents, and natural disasters are some threatening factors for the current state of blood transfusion. However, it seems that science and technology not only could overcome these challenges but also would turn many human dreams to reality in this regard. Scientists believe that one of the future evolutionary innovations could be artificial blood substitutes that might pave the way to a new era in transfusion medicine. In this review, recent status and progresses in artificial blood substitutes, focusing on red blood cells substitutes, are summarized. In addition, steps taken toward the development of artificial blood technology and some of their promises and hurdles will be highlighted. However, it must be noted that artificial blood is still at the preliminary stages of development, and to fulfill this dream, ie, to routinely transfuse artificial blood into human vessels, we still have to strengthen our knowledge and be patient.

Oxygen delivery using engineered microparticles

A continuous supply of oxygen to tissues is vital to life and interruptions in its delivery are poorly tolerated. The treatment of low-blood oxygen tensions requires restoration of functional airways and lungs. Unfortunately, severe oxygen deprivation carries a high mortality rate and can make otherwise-survivable illnesses unsurvivable. Thus, an effective and rapid treatment for hypoxemia would be revolutionary. The i.v. injection of oxygen bubbles has recently emerged as a potential strategy to rapidly raise arterial oxygen tensions. In this report, we describe the fabrication of a polymer-based intravascular oxygen delivery agent. Polymer hollow microparticles (PHMs) are thin-walled, hollow polymer microcapsules with tunable nanoporous shells. We show that PHMs are easily charged with oxygen gas and that they release their oxygen payload only when exposed to desaturated blood. We demonstrate that oxygen release from PHMs is diffusion-controlled, that they deliver approximately five times more oxygen gas than human red blood cells (per gram), and that they are safe and effective when injected in vivo. Finally, we show that PHMs can be stored at room temperature under dry ambient conditions for at least 2 mo without any effect on particle size distribution or gas carrying capacity.

Post-translational transformation of methionine to aspartate is catalyzed by heme iron and driven by peroxide: a novel subunit-specific mechanism in hemoglobin

A pathogenic V67M mutation occurs at the E11 helical position within the heme pockets of variant human fetal and adult hemoglobins (Hb). Subsequent post-translational modification of Met to Asp was reported in γ subunits of human fetal Hb Toms River (γ67(E11)Val → Met) and β subunits of adult Hb (HbA) Bristol-Alesha (β67(E11)Val → Met) that were associated with hemolytic anemia. Using kinetic, proteomic, and crystal structural analysis, we were able to show that the Met → Asp transformation involves heme cycling through its oxoferryl state in the recombinant versions of both proteins. The conversion to Met and Asp enhanced the spontaneous autoxidation of the mutants relative to wild-type HbA and human fetal Hb, and the levels of Asp were elevated with increasing levels of hydrogen peroxide (H2O2). Using H2(18)O2, we verified incorporation of (18)O into the Asp carboxyl side chain confirming the role of H2O2 in the oxidation of the Met side chain. Under similar experimental conditions, there was no conversion to Asp at the αMet(E11) position in the corresponding HbA Evans (α62(E11)Val → Met). The crystal structures of the three recombinant Met(E11) mutants revealed similar thioether side chain orientations. However, as in the solution experiments, autoxidation of the Hb mutant crystals leads to electron density maps indicative of Asp(E11) formation in β subunits but not in α subunits. This novel post-translational modification highlights the nonequivalence of human Hb α, β, and γ subunits with respect to redox reactivity and may have direct implications to α/β hemoglobinopathies and design of oxidatively stable Hb-based oxygen therapeutics.

Haptoglobin preferentially binds β but not α subunits cross-linked hemoglobin tetramers with minimal effects on ligand and redox reactions

Human hemoglobin (Hb) and haptoglobin (Hp) exhibit an extremely high affinity for each other, and the dissociation of Hb tetramers into dimers is generally believed to be a prerequisite for complex formation. We have investigated Hp interactions with native Hb, αα, and ββ cross-linked Hb (ααXLHb and ββXLHb, respectively), and rapid kinetics of Hb ligand binding as well as the redox reactivity in the presence of and absence of Hp. The quaternary conformation of ββ subunit cross-linking results in a higher binding affinity than that of αα subunit cross-linked Hb. However, ββ cross-linked Hb exhibits a four fold slower association rate constant than the reaction rate of unmodified Hb with Hp. The Hp contact regions in the Hb dimer interfaces appear to be more readily exposed in ββXLHb than ααXLHb. In addition, apart from the functional changes caused by chemical modifications, Hp binding does not induce appreciable effects on the ligand binding and redox reactions of ββXLHb. Our findings may therefore be relevant to the design of safer Hb-based oxygen therapeutics by utilizing this preferential binding of ββXLHb to Hp. This may ultimately provide a safe oxidative inactivation and clearance pathway for chemically modified Hbs in circulation.

Enhanced nitrite reductase activity associated with the haptoglobin complexed hemoglobin dimer: functional and antioxidative implications

The presence of acellular hemoglobin (Hb) within the circulation is generally viewed as a pathological state that can result in toxic consequences. Haptoglobin (Hp), a globular protein found in the plasma, binds with high avidity the αβ dimers derived from the dissociation of Hb tetramer and thus helps clear free Hb. More recently there have been compelling indications that the redox properties of the Hp bound dimer (Hb-Hp) may play a more active role in controlling toxicity by limiting the potential tissue damage caused by propagation of the free-radicals generated within the heme containing globin chains. The present study further examines the potential protective effect of Hp through its impact on the production of nitric oxide (NO) from nitrite through nitrite reductase activity of the Hp bound αβ Hb dimer. The presented results show that the Hb dimer in the Hb-Hp complex has oxygen binding, CO recombination and spectroscopic properties consistent with an Hb species having properties similar to but not exactly the same as the R quaternary state of the Hb tetramer. Consistent with these observations is the finding that the initial nitrite reductase rate for Hb-Hp is approximately ten times that of HbA under the same conditions. These results in conjunction with the earlier redox properties of the Hb-Hp are discussed in terms of limiting the pathophysiological consequences of acellular Hb in the circulation.

Isolated Hb Providence β82Asn and β82Asp fractions are more stable than native HbA(0) under oxidative stress conditions

We have previously shown that hydrogen peroxide (H(2)O(2)) triggers irreversible oxidation of amino acids exclusive to the β-chains of purified human hemoglobin (HbAo). However, it is not clear, whether α- or β-subunit Hb variants exhibit different oxidative resistance to H(2)O(2) when compared to their native HbAo. Hb Providence contains two β-subunit variants with single amino acid mutations at βLys82→Asp (βK82D) and at βLys82→Asn (βK82N) positions and binds oxygen at lower affinity than wild type HbA. We have separated Hb Providence into its 3 component fractions, and contrasted oxidative reactions of its β-mutant fractions with HbAo. Relative to HbAo, both βK82N and βK82D fractions showed similar autoxidation kinetics and similar initial oxidation reaction rates with H(2)O(2). However, a more profound pattern of changes was seen in HbAo than in the two Providence fractions. The structural changes in HbAo include a collapse of β-subunits, and α-α dimer formation in the presence of excess H(2)O(2). Mass spectrometric and amino acid analysis revealed that βCys93 and βCys112 were oxidized in the HbAo fraction, consistent with oxidative pathways driven by a ferrylHb and its protein radical. These amino acids were oxidized at a lesser extent in βK82D fraction. While the 3 isolated components of Hb Providence exhibited similar ligand binding and oxidation reaction kinetics, the variant fractions were more effective in consuming H(2)O(2) and safely internalizing radicals through the ferric/ferryl pseudoperoxidase cycle.

Functional comparison of hemoglobin purified by different methods and their biophysical implications

Hemoglobin (Hb) that is purified from red blood cells (RBCs) is commonly subjected to harsh processing conditions, such as high temperatures and extensive column separation, which may damage the Hb by altering the heme prosthetic group and/or the Hb protein structure. In this study, bovine and human Hb purified by tangential flow filtration (TFF) was compared to commercial preparations of human Hb (Hemosol, Inc., Toronto, Canada) and bovine Hb (Biopure, Inc., Cambridge, MA). Purified Hbs were characterized by measuring their overall purity (SDS-PAGE, SEC, and ESI-MS), susceptibility to oxidation (k(ox)), responses to physiological conditions (pH, [Cl(-)], [IHP], and T), and ligand binding kinetics (O(2), NO, and CO). All Hbs evaluated possessed comparable biophysical properties, however, a small amount of catalase was detected in the TFF-purified Hbs that reduced the rate of autoxidation. Mass changes observed by mass spectrometry suggest that structural alterations may be introduced into Hbs that are purified by extensive chromatographic separations. These results demonstrate that TFF is a suitable process for the purification of Hb from RBCs with a quality equivalent to that of commercial Hb preparations that employ more extensive purification strategies. This work also shows that TFF can yield highly pure Hb which can be used for Hb-based O(2) carrier synthesis.

A direct and simultaneous detection of zinc protoporphyrin IX, free protoporphyrin IX, and fluorescent heme degradation product in red blood cell hemolysates

Fluorescence emission of free protoporphyrin IX (PPIX, em. approximately 626 nm), zinc protoporphyrin IX (ZPP, em. approximately 594 nm) and fluorescent heme degradation product (FHDP, em. approximately 466 nm) are identified and simultaneously detected in mouse and human red cell hemolysates, when excited at 365 nm. A novel method is established for comparing relative FHDP, PPIX and ZPP levels in hemolysates without performing red cell porphyrin extractions. The ZPP fluorescence directly measured in hemolysates (F(365/594)) correlates with the ZPP fluorescence obtained from acetone/water extraction (R(2) = 0.9515, P < 0.0001). The relative total porphyrin (ZPP and PPIX) fluorescence obtained from direct hemolysate fluorescence measurements also correlates with red blood cell total porphyrins determined by ethyl acetate extraction (Piomelli extraction, R(2) = 0.88, P < 0.0001). These fluorescent species serves as biomarkers for alterations in Hb synthesis and Hb stability.

Autoxidation of the site-specifically PEGylated hemoglobins: role of the PEG chains and the sites of PEGylation in the autoxidation

The PEGylated hemoglobin (Hb) has been evaluated as a potential blood substitute. In an attempt to understand the autoxidation of the PEGylated Hb, we have studied the autoxidation of the PEGylated Hb site-specifically modified at Cys-93(beta) or at Val-1(beta). PEGylation of Hb at Cys-93(beta) perturbed the heme environment and increased the autoxidation rate of Hb, which is at a higher level than that caused by PEGylation at Val-1(beta). The perturbation of the heme environment of Hb is attributed to the maleimide modification at Cys-93(beta) and not due to conjugation of the PEG chains. However, the PEG chains enhance the autoxidation and the H 2O 2 mediated oxidation of Hb. Accordingly, the PEG chains are assumed to increase the water molecules in the hydration layer of Hb and enhance the autoxidation by promoting the nucleophilic attack of heme. The autoxidation rate of the PEGylated Hb does not show an inverse correlation with the oxygen affinity. The H 2O 2 mediated structural loss and the heme loss of Hb are increased by maleimide modification at Cys-93(beta) and further decreased by conjugation of the PEG chains. The autoxidation of the PEGylated Hbs is attenuated significantly in the plasma, possibly due to the presence of the antioxidant species in the plasma. This result is consistent with the recent suggestion that there is no direct correlation between the in vitro and in vivo autoxidation of the PEGylated Hb. Therefore, the pattern of PEGylation can be manipulated for the design of the PEGylated Hb with minimal autoxidation.

Effects of (-)-epigallocatechin gallate on the redox reactions of human hemoglobin

The toxicity of acellular hemoglobin (Hb)-based therapeutics has been attributed in part to the uncontrolled oxidative reactions. A variety of antioxidant strategies to ameliorate potential oxidative damage in vivo have been suggested. We have examined the effects of (-)-epigallocatechin gallate (EGCG), a green tea polyphenol compound widely regarded as a chain-breaking antioxidant, on the oxidative stability of diaspirin crosslinked Hb (DBBF) and its cytotoxic ferryl intermediate. DBBF (ferrous) was rapidly oxidized to the ferric form in the presence of EGCG relative to the normal spontaneous oxidation of this Hb. The fast elimination of ferrous Hb is probably due to the ability of EGCG to produce hydrogen peroxide (H2O2) as these reactions were almost completely reversed by the addition of catalase and superoxide dismutase to the reaction medium. EGCG, however, effectively reduced ferryl back to ferric Hb in a biphasic kinetic reaction at physiological pH. At acidic pH where the autoreduction of protonated ferryl Hb is enhanced, a monophasic reduction process of the ferryl heme is achieved. A balance between pro and antioxidant properties of EGCG should be taken into account if EGCG is used in combination therapy with redox active acellular Hbs.

Structural stabilization in tetrameric or polymeric hemoglobin determines its interaction with endogenous antioxidant scavenger pathways

Hemoglobin (Hb) released into the circulation during hemolysis and chemically modified Hb proposed for use as oxygen therapeutics exert toxic effects that are partially attributable to heme's oxidant activity. Native extracellular Hb is scavenged by haptoglobin (Hp) after alphabeta-subunit dimerization. In the absence of Hp, monocyte/macrophage cell-surface CD163 binds and clears Hb. We evaluated several chemically modified Hbs to establish the role of chemical cross-linking patterns and molecular sizes on binding and clearance by each pathway. We found that Hbs possessing beta-globin cross-linking, irrespective of polymerization, demonstrate increased Hp affinity compared with alpha-globin-stabilized Hbs. These data suggest that Hb alpha-subunit accessibility is critical for Hp binding in the absence of dimerization. beta-Globin chain cross-linked tetramers/polymers displayed strong polyvalent Hp binding with increased viscosity and formation of visible gel matrices. Modified Hb interaction with CD163 and cellular uptake demonstrated an inverse relation with molecular size, irrespective of alpha and beta cross-linking. These findings were confirmed by HO-1 induction and intracellular ferritin accumulation in CD163-expressing HEK293 cells. Based on these results, a rational and systematic approach to HBOC design may be used to optimize interaction with endogenous Hb clearance and detoxification pathways.

Pharmacological and physicochemical factors in the pressor effects of conjugated haemoglobin-based oxygen carriers in vivo

BACKGROUND

The hypertension induced by haemoglobin-based oxygen carriers could be a result of different pharmacological and physicochemical factors.

OBJECTIVE

To investigate whether production of superoxide anion (O2*-) and release of endothelin could be the factors responsible.

METHODS

We studied the variation in mean arterial pressure (MAP) in guinea pigs by carrying out a 50% isovolaemic exchange transfusion with conjugated oxyhaemoglobin (non-oxidized form) or conjugated methaemoglobin (fully oxidized form) in the presence or absence of BQ-788 (5 nmol/l), an endothelin receptor type B (ETR-B) antagonist. At key timepoints of variation in MAP, the plasma concentrations of O2*- were measured. The presence of conjugated oxyhaemoglobin and increases in ETR-B concentrations inside the vascular wall were investigated in different vessels, using western blotting.

RESULTS

We found that the administration of conjugated oxyhaemoglobin induced a significant increase in MAP, whereas conjugated methaemoglobin had no significant haemodynamic effect. Pretreatment with BQ-788 attenuated the increase in MAP induced by conjugated oxyhaemoglobin. This haemoglobin induced the production of high concentrations of O2*- that declined towards control values after 120 min and decreased in the presence of BQ-788. Western blot analysis showed that the presence of conjugated oxyhaemoglobin inside the vascular wall was time-dependent and correlated with increased ETR-B.

CONCLUSION

These results show that the release of O2*- during auto-oxidation of conjugated oxyhaemoglobin is associated with the observed increase in MAP, which may be a result of the vasoconstriction caused by an increase in activation of ETR-B. This activation may be caused by the massive release of endothelin induced by the production of O2*-.

Hydrogen-bonding conformations of tyrosine B10 tailor the hemeprotein reactivity of ferryl species

Ferryl compounds [Fe(IV)=O] in living organisms play an essential role in the radical catalytic cycle and degradation processes of hemeproteins. We studied the reactions between H2O2 and hemoglobin II (HbII) (GlnE7, TyrB10, PheCD1, PheE11), recombinant hemoglobin I (HbI) (GlnE7, PheB10, PheCD1, PheE11), and the HbI PheB10Tyr mutant of L. pectinata. We found that the tyrosine residue in the B10 position tailors, in two very distinct ways, the reactivity of the ferryl species, compounds I and II. First, increasing the reaction pH from 4.86 to 7.50, and then to 11.2, caused the the second-order rate constant for HbII to decrease from 141.60 to 77.78 M-1 s-1, and to 2.96 M-1 s-1, respectively. This pH dependence is associated with the disruption of the heme-tyrosine (603 nm) protein moiety, which controls the access of the H2O2 to the hemeprotein active center, thus regulating the formation of the ferryl species. Second, the presence of compound I was evident in the UV-vis spectra (648-nm band) in the reactions of HbI and recombinant HbI with H2O2, This band, however, is completely absent in the analogous reaction with HbII and the HbI PheB10Tyr mutant. Therefore, the existence of a hydrogen-bonding network between the heme pocket amino acids (i.e., TyrB10) and the ferryl compound I created a path much faster than 3.0x10(-2) s-1 for the decay of compound I to compound II. Furthermore, the decay of the heme ferryl compound I to compound II was independent of the proximal HisF8 trans-ligand strength. Thus, the pH dependence of the heme-tyrosine moiety complex determined the overall reaction rate of the oxidative reaction limiting the interaction with H2O2 at neutral pH. The hydrogen-bonding strength between the TyrB10 and the heme ferryl species suggests the presence of a cycle where the ferryl consumption by the ferric heme increases significantly the pseudoperoxidase activity of these hemeproteins.

Hemoglobin-Based Oxygen Carrier Distribution Inside Vascular Wall and Arterial Pressure Evolution: Is There a Relationship?

The hemoglobin-based oxygen carriers (HBOC), like dextran-benzene-tetracarboxylate-hemoglobin (Dex-BTC-Hb), which are present at high concentrations in plasma disturb arterial pressure and induce hypertension. To study if the increase of mean arterial pressure (MAP) is due to the presence of cell-free hemoglobin (Hb) inside abdominal aortic wall, we followed on a model of 50% isovolemic exchange transfusion (IET) in anesthetized guinea pigs, the kinetic of Dex-BTC-Hb distribution inside abdominal aortic wall and we investigated the relationship between arterial pressure modifications and modified Hb distribution. The administration of Dex-BTC-Hb induced instantaneously an increase of MAP that reached its maximum (53% of hypertension from baseline) at 17 min after the end of the IET and was maintained maximally up to 30 min. A significantly decrease of MAP (45% of hypertension from baseline) was observed after 60 min and the baseline level was recovered at 180 min. The investigation of tissue at 17 min by confocal microscopy showed the presence of free Hb in or upon endothelial cells (EC) in intima and in vasa vasorum. At 180 min, the free Hb was found in or upon EC and inside all abdominal aortic wall meanwhile MAP recovered its basal value. These results suggest for the first time that Hb in intima seems to induce the hypertension observed upon IET but can not sustain it even if Hb stayed present in intima and in abdominal aortic wall.