O-raffinose crosslinked hemoglobin lacks site-specific chemistry in the central cavity: Structural and functional consequences of β93Cys modification

Entropy-Driven Supramolecular Ring-Opening Polymerization of a Cyclic Hemoglobin Monomer for Constructing a Hemoglobin-PEG Alternating Polymer with Structural Regularity

Our earlier report described that a cyclic hemoglobin (Hb) monomer with two β subunits of a Hb molecule (α₂β₂) bound through a flexible polyethylene glycol (PEG) chain undergoes reversible supramolecular ring-opening polymerization (S-ROP) to produce a supramolecular Hb polymer with a Hb-PEG alternating structure. In this work, we polymerized cyclic Hb monomers with different ring sizes (2, 5, 10, or 20 kDa PEG) to evaluate the thermodynamics of S-ROP equilibrium. Quantification of the produced supramolecular Hb polymers and the remaining cyclic Hb monomers in the equilibrium state revealed a negligibly small enthalpy change in S-ROP (ΔH_p ≤ 1 kJ·mol^-1) and a markedly positive entropy change increasing with the ring size (ΔS_p = 26.8-33.2 J·mol^-1·K^-1). The results suggest an entropy-driven mechanism in S-ROP: a cyclic Hb monomer with the larger ring size prefers to form a supramolecular Hb polymer. The S-ROP used for this study has the potential to construct submicrometer-sized Hb-PEG alternating polymers having structural regularity.

βCysteine 93 in human hemoglobin: a gateway to oxidative stability in health and disease

βcysteine 93 residue plays a key role in oxygen (O2)-linked conformational changes in the hemoglobin (Hb) molecule. This solvent accessible residue is also a target for binding of thiol reagents that can remotely alter O2 affinity, cooperativity, and Hb's sensitivity to changes in pH. In recent years, βCys93 was assigned a new physiological role in the transport of nitric oxide (NO) through a process of S-nitrosylation as red blood cells (RBCs) travel from lungs to tissues. βCys93 is readily and irreversibly oxidized in the presence of a mild oxidant to cysteic acid, which causes destabilization of Hb resulting in improper protein folding and the loss of heme. Under these oxidative conditions, ferryl heme (HbFe4+), a higher oxidation state of Hb is formed together with its protein radical (.HbFe4+). This radical migrates to βCys93 and interacts with other "hotspot" amino acids that are highly susceptible to oxidative modifications. Oxidized βCys93 may therefore be used as a biomarker of oxidative stress, reflecting the deterioration of Hb within RBCs intended for transfusion or RBCs from patients with hemoglobinopathies. Site specific mutation of a redox active amino acid(s) to reduce the ferryl heme or direct chemical modifications that can shield βCys93 have been proposed to improve oxidative resistance of Hb and may offer a protective therapeutic strategy.

Mechanisms of Toxicity and Modulation of Hemoglobin-based Oxygen Carriers

Several adverse events have been associated with the infusion of hemoglobin-based oxygen carriers (HBOCs), including transient hypertension, gastrointestinal, pancreatic/liver enzyme elevation, and cardiac/renal injury in humans. Although several mechanisms have been suggested, the basis of HBOC toxicity is still poorly understood. Scavenging of vascular endothelial nitric oxide (NO) and heme-mediated oxidative side reactions are thought to be the major causes of toxicity. However, based on more recent preclinical studies, oxidative pathways (driven by the heme prosthetic group) seem to play a more prominent role in the overall toxicity of free Hb or HBOCs. HBOCs display a diversity of physicochemical properties, including molecular size/cross-linking characteristics leading to differences in oxygen affinity, allosteric, redox properties, and even oxidative inactivation by protein/heme clearing mechanisms. These diverse characteristics can therefore be manipulated independently, leaving open the possibility of engineering a safe and effective HBOC. To date, several antioxidative strategies have been proposed to counteract the redox side reactions of current generation HBOCs.

Artificial Blood: The History and Current Perspectives of Blood Substitutes

Blood transfusions are one of the most common procedures performed in hospitalized patients. Yet, despite all of the measures taken to ensure the safety of the blood supply, there are known risks associated with transfusions, including infectious and noninfectious complications. Meanwhile, issues with blood product availability, the need for compatibility testing, and the storage and transport requirements of blood products, have presented challenges for the administration of blood transfusions. Additionally, there are individuals who do not accept blood transfusions (e.g., Jehovah's Witnesses). Therefore, there is a need to develop alternative agents that can reliably and safely replace blood. However, although there have been many attempts to develop blood substitutes over the years, there are currently no such products available that have been approved by the United States Food and Drug Administration (FDA). However, a more-recently developed hemoglobin-based oxygen carrier has shown promise in early clinical trials and has achieved the status of "Orphan Drug" under the FDA.

Core-shell protein clusters comprising haemoglobin and recombinant feline serum albumin as an artificial O2 carrier for cats

This report describes the synthesis and structure of core-shell protein clusters comprising haemoglobin (Hb) at the centre and recombinant feline serum albumin (rFSA) at the exterior, named as haemoglobin-albumin clusters (Hb-rFSA₃). Specifically, we highlight their capability as an artificial O₂ carrier that can be used as a red blood cell (RBC) substitute for cats, the most populous pet animal in the world. First, rFSA was expressed by genetic engineering using Pichia yeast. The proteins show identical features to the native FSA derived from feline plasma. Single crystals of rFSA were prepared under a microgravity environment on the international space station (ISS), from which the structure was first revealed at 3.4 Å resolution. Subsequently, bovine Hb was wrapped covalently by rFSA using an α-succinimidyl-ε-maleimide crosslinker, yielding Hb-rFSA₃ clusters. Three rFSA entities enfolded the Hb nuclei satisfactorily, giving the protein clusters a negative surface net charge (pI = 4.7) and preventing an immunological response against anti-Hb antibodies. The O₂ affinity was higher (P₅₀ = 9 Torr) than that of the native Hb. The Hb-rFSA₃ clusters are anticipated for use as an alternative material for RBC transfusion, and as an O₂ therapeutic reagent that can be exploited in various veterinary medicine scenarios.

Mixtures of tense and relaxed state polymerized human hemoglobin regulate oxygen affinity and tissue construct oxygenation

Pure tense (T) and relaxed (R) quaternary state polymerized human hemoglobins (PolyhHbs) were synthesized and their biophysical properties characterized, along with mixtures of T- and R-state PolyhHbs. It was observed that the oxygen affinity of PolyhHb mixtures varied linearly with T-state mole fraction. Computational analysis of PolyhHb facilitated oxygenation of a single fiber in a hepatic hollow fiber (HF) bioreactor was performed to evaluate the oxygenation potential of T- and R-state PolyhHb mixtures. PolyhHb mixtures with T-state mole fractions greater than 50% resulted in hypoxic and hyperoxic zones occupying less than 5% of the total extra capillary space (ECS). Under these conditions, the ratio of the pericentral volume to the perivenous volume in the ECS doubled as the T-state mole fraction increased from 50 to 100%. These results show the effect of varying the T/R-state PolyhHb mole fraction on oxygenation of tissue-engineered constructs and their potential to oxygenate tissues.

Hemoglobin-Based Blood Substitutes and the Treatment of Sickle Cell Disease: More Harm than Help?

Intense efforts have been made by both industry and academia over the last three decades to produce viable hemoglobin (Hb)-based oxygen carriers (HBOCs), also known as “blood substitutes”. Human trials conducted so far by several manufactures in a variety of clinical indications, including trauma, and elective surgeries have failed and no product has gained the Food and Drug Administration approval for human use. Safety concerns due to frequent incidences of hemodynamic, cardiac events, and even death led to the termination of some of these trials. Several second generation HBOC products that have been chemically and/or genetically modified (or in some cases ligated with carbon monoxide (CO)) found a new clinical application in conditions as complex as sickle cell disease (SCD). By virtue of higher oxygen affinity (P₅₀) (R-state), and smaller size, HBOCs may be able to reach the microvasculature unload of oxygen to reverse the cycles of sickling/unsickling of the deoxy-sickle cell Hb (HbS) (T-state), thus preventing vaso-occlusion, a central event in SCD pathophysiology. However, biochemically, it is thought that outside the red blood cell (due to frequent hemolysis), free HbS or infused HBOCs are capable of interfering with a number of oxidative and signaling pathways and may, thus, negate any benefit that HBOCs may provide. This review discusses the advantages and disadvantages of using HBOCs in SCD.

Simple method for preparing poly(ethylene glycol)-surface-conjugated liposome-encapsulated hemoglobins: physicochemical properties, long-term storage stability, and their reactions with O2, CO, and NO

During the last few decades, liposome-encapsulated hemoglobin (LEH) dispersions have been investigated for use as red blood cell (RBC) substitutes. However, the process for formulating LEHs is cumbersome, and the composition of the lipid mixture is often complex. This work investigates a simple approach to formulating LEHs from a simple lipid mixture composed of high-phase-transition lipid distearoylphosphatidylcholine (DSPC) and cholesterol. To improve the circulation half-life and colloidal state of LEHs, the surfaces of unmodified LEHs were conjugated with poly(ethylene glycol) (PEG-LEHs). The results of this work show that PEG-LEH dispersions exhibited average diameters ranging from 166 to 195 nm that were colloidally stable for 4 to 5 months, hemoglobin (Hb) concentrations ranging from 9.6 to 14 g/dL, methemoglobin levels of less than 1%, oxygen affinities (i.e., P(50) values) ranging from 20 to 23 mm Hg, and cooperativity coefficients ranging from 1.4 to 2.2. The reactions of PEG-LEHs with physiologically important ligands, such as oxygen (O(2)), carbon monoxide (CO), and nitric oxide (NO), were also measured. It was observed that PEG-LEHs and RBCs exhibited retarded gaseous ligand binding/release kinetics compared to that of acellular Hb's. This result provides important insight into the pivotal role that the intracellular diffusion barrier plays in the transport of gases into and out of these structures. Collectively, our results demonstrate that the PEG-LEH dispersions prepared in this study show good potential as an RBC substitute.

Synthesis, biophysical properties, and oxygenation potential of variable molecular weight glutaraldehyde-polymerized bovine hemoglobins with low and high oxygen affinity

In a recent study, ultrahigh molecular weight (Mw ) glutaraldehyde-polymerized bovine hemoglobins (PolybHbs) were synthesized with low O2 affinity and exhibited no vasoactivity and a slight degree of hypertension in a 10% top-load model.(1) In this work, we systematically investigated the effect of varying the glutaraldehyde to hemoglobin (G:Hb) molar ratio on the biophysical properties of PolybHb polymerized in either the low or high O2 affinity state. Our results showed that the Mw of the resulting PolybHbs increased with increasing G:Hb molar ratio. For low O2 affinity PolybHbs, increasing the G:Hb molar ratio reduced the O2 affinity and CO association rate constants in comparison to bovine hemoglobin (bHb). In contrast for high O2 affinity PolybHbs, increasing the G:Hb molar ratio led to increased O2 affinity and significantly increased the CO association rate constants compared to unmodified bHb and low O2 affinity PolybHbs. The methemoglobin level and NO dioxygenation rate constants were insensitive to the G:Hb molar ratio. However, all PolybHbs displayed higher viscosities compared to unmodified bHb and whole blood, which also increased with increasing G:Hb molar ratio. In contrast, the colloid osmotic pressure of PolybHbs decreased with increasing G:Hb molar ratio. To preliminarily evaluate the ability of low and high O2 affinity PolybHbs to potentially oxygenate tissues in vivo, an O2 transport model was used to simulate O2 transport in a hepatic hollow fiber (HF) bioreactor. It was observed that low O2 affinity PolybHbs oxygenated the bioreactor better than high O2 affinity PolybHbs. This result points to the suitability of low O2 affinity PolybHbs for use in tissue engineering and transfusion medicine. Taken together, our results show the quantitative effect of varying the oxygen saturation of bHb and G:Hb molar ratio on the biophysical properties of PolybHbs and their ability to oxygenate a hepatic HF bioreactor. We suggest that the information gained from this study can be used to guide the design of the next generation of hemoglobin-based oxygen carriers (HBOCs) for use in tissue engineering and transfusion medicine applications.

Polymer/hemoglobin assemblies: biodegradable oxygen carriers for artificial red blood cells

In routine clinical procedures, blood transfusion is now suffering from the defects of the blood products, like cross-matching, short storage time and virus infection. Various blood substitutes have been designed by researchers through continual efforts. With recent progress in nanotechnology, new types of artificial red blood cells with cellular structure are available. This article aims to describe some artificial red blood cells which encapsulate or conjugate hemoglobin molecules through various approaches, especially the nanoscale self-assembly technique, to mitigate the adverse effects of free hemoglobin molecules. These types of artificial red blood cell systems, which make use of biodegradable polymers as matrix materials, show advantages over the traditional types.

Biophysical properties and oxygenation potential of high-molecular-weight glutaraldehyde-polymerized human hemoglobins maintained in the tense and relaxed quaternary states

Recent clinical evaluation of commercial glutaraldehyde-polymerized hemoglobins (PolyHbs) as transfusion solutions has demonstrated several adverse side effects. Chief among these is the hypertensive effect. Fortunately, previous studies have shown that the hypertensive effect can be attenuated by removing free hemoglobin (Hb) and low-molecular-weight (low-MW) PolyHbs from the PolyHb mixture. In this work, polymerized human Hb (PolyhHb) solutions were synthesized in two distinct quaternary states with high MW and subjected to extensive diafiltration to remove free Hb and low-MW PolyhHb components (<500 kDa). The resultant PolyhHb solutions possessed high MW, distinct quaternary state, distinct reactivities with O(2) and CO, similar NO deoxygenating rate constants, distinct autoxidation rate constants, high viscosity, and low colloid osmotic pressure. To preliminarily assess the ability of PolyhHb solutions to oxygenate surrounding tissues fed by a blood vessel, we evaluated the ability of PolyhHbs to transport O(2) to cultured hepatocytes in a mathematical model of a hollow fiber bioreactor. The structure of individual hollow fibers in the bioreactor is similar to that of a blood vessel and provides an easy way to assess the oxygenation potential of PolyhHbs without the need for expensive and time-consuming animal studies. It was observed that PolyhHbs with low O(2) affinities were more effective in oxygenating cultured hepatocytes inside the bioreactor than high O(2) affinity PolyhHbs. Taken together, our results show that it is possible to synthesize high-MW PolyhHbs with no free Hb and low-MW PolyhHb components that are capable of transporting O(2) to cultured cells/tissues.

The redox activity of hemoglobins: from physiologic functions to pathologic mechanisms

Pentacoordinate respiratory hemoproteins such as hemoglobin and myoglobin have evolved to supply cells with oxygen. However, these respiratory heme proteins are also known to function as redox enzymes, reacting with compounds such as nitric oxide and peroxides. The recent discoveries of hexacoordinate hemoglobins in vertebrates and nonsymbiotic plants suggest that the redox activity of globins is inherent to the molecule. The uncontrolled formation of radical species resulting from such redox chemistry on respiratory hemoproteins can lead to oxidative damage and cellular toxicity. In this review, we examine the functions of various globins and the mechanisms by which these globins act as redox enzymes under physiologic conditions. Evidence that redox reactions also occur under disease conditions, leading to pathologic complications, also is examined, focusing on recent discoveries showing that the ferryl oxidation state of these hemoproteins is present in these disease states in vivo. In addition, we review the latest advances in the understanding of globin redox mechanisms and how they might affect cellular signaling pathways and how they might be controlled therapeutically or, in the case of hemoglobin-based blood substitutes, through rational design.

Synthesis and characterization of hemoglobin conjugates with antioxidant enzymes via poly(ethylene glycol) cross-linker (Hb-SOD-CAT) for protection from free radical stress

Hemoglobin (Hb) conjugated with the antioxidant enzymes (SOD and CAT), by employing dicarboxymethylated poly(ethylene glycol), was designed for protection of hemoglobin against free radicals. In this study, the conjugation process was confirmed by employing SDS-PAGE and SEC techniques. The average molecular weight of the conjugates was estimated to be around 1000 kDa. The enzymatic activities of the SOD and CAT in the conjugates (Hb-SOD-CAT) after conjugation were found to retain greater than 70% and 90% of the original bioactivity. Results show that antioxidant enzymes helped minimize methemoglobin (non-carrier of oxygen) formation during the conjugation process and also during storage at 4°C over a period of 1 month. In summary, the optimized (1:10 Hb/PEG) crosslinked conjugates with antioxidant enzymes showed protective properties from severe free radical stresses when incubated with hydrogen peroxide (0.1 and 1 mM) and xanthine (1 mM)/xanthine oxidase (10 and 20 mU/ml) system.

Setbacks in blood substitutes research and development: a biochemical perspective

Recent setbacks in using Hb-based technology to develop oxygen carriers or blood substitutes may spur new and fundamentally different approaches for the development of a new generation of hemoglobin-based oxygen carriers (HBOCs). This article briefly details some underlying mechanisms that may have been responsible for the adverse-event profile associated with HBOCs, with a focus on the contribution of the author's laboratory toward identifying some of these biochemical pathways and some ways and means to control them. It is hoped that this will aid in the development of a safe and effective second generation of HBOCs.

Hemoglobin can attenuate hydrogen peroxide-induced oxidative stress by acting as an antioxidative peroxidase

Hemoglobin is considered a potentially toxic molecule when released from erythrocytes during hemolysis, inflammation, or tissue injury. The mechanisms of toxicity involve reduced nitric oxide bioavailability and oxidative processes both occurring at the heme prosthetic groups. When the endogenous oxidant H(2)O(2) reacts with Hb, transient radicals are generated during the peroxidative consumption of H(2)O(2). If not neutralized, these radicals can lead to tissue toxicity. The net biologic effect of extracellular Hb in an H(2)O(2)-rich environment will therefore be determined by the balance of H(2)O(2) decomposition (potential protective effect) and radical generation (potential damaging effect). Here we show that Hb can protect different cell types from H(2)O(2)-mediated cell death and the associated depletion of intracellular glutathione and ATP. Importantly, Hb blunts the transcriptional oxidative-stress response induced by H(2)O(2) in human vascular smooth muscle cells (VSMCs). Based on spectrophotometric and quantitative mass spectrometry analysis, we suggested a novel mechanism in which Hb redox-cycles H(2)O(2) and simultaneously internalizes the radical burden, with irreversible structural globin changes starting with specific amino acid oxidation involving the heme proximate betaCys93 and ultimately ending with protein precipitation. Our results suggest that complex interactions determine whether extracellular Hb, under certain circumstances, acts a protective or a damaging factor during peroxidative stress conditions.

Modern cross-linking strategies for synthesizing acellular hemoglobin-based oxygen carriers

Unmodified cell-free hemoglobin (Hb) is structurally unstable when transfused into the blood stream (Valeri et al., Artif Cells Blood Substit Immobil Biotechnol. 2000;28:451-475; Chan et al., Toxicol Pathol. 2000;28:635-642; Eike, Dissertation, 2005; Eike and Palmer, Biotechnol Prog. 2004;20:946-952). This review examines some of the latest chemical strategies used over the last 5 years to intra- and intermolecularly cross-link Hb, thereby stabilizing its quaternary structure. Therefore, this work will address the following aspects: (1) site-specific chemical modifications of Hb and (2) non-site-specific chemical modifications of Hb, including, but not limited to, PolyHeme, Hemopure, Oxyglobin, and SOD-Hb. Current strategies for synthesizing PEGylated Hb is outside the scope of this review and will not be discussed herein. For a more thorough review of PEGylated Hb, the reader is directed to the following works: Cabrales and Friedman, Transfus Alternatives in Transfus Med. 2007;9:281-293 and Winslow, Biochim Biophys Acta, 2008;1784(10):1382-1386.

Mixed S-nitrosylated polymerized bovine hemoglobin species moderate hemodynamic effects in acutely hypoxic rats

Hemoglobin (Hb)-based oxygen carriers (HBOCs) are being developed as a potential therapy for increasing tissue oxygenation, yet they have not reached their full potential because of unwanted hemodynamic side effects (vasoconstriction, low cardiac output, and oxygen delivery) due in part to nitric oxide (NO) scavenging by cell-free Hb. It may be possible to overcome the NO scavenging effect by coinfusing S-nitrosylated (SNO) HBOC along with unmodified HBOC. SNO-HBOC, like free Hb, may act as an NO donor in low-oxygen conditions. We hypothesized that an unaltered HBOC, polymerized bovine Hb (PBvHb), coinfused with an SNO-PBvHb, would improve hemodynamics and oxygen delivery during hypoxia. Vascular oxygen content and hemodynamics were determined after euvolemic rats were infused (3 ml) with lactated Ringer's solution, PBvHb, SNO-PBvHb, or PBvHb plus SNO-PBvHb (1:10) during normoxia or acute hypoxia (fraction of inspired oxygen = 10%, 120 min). Hemodynamic side effects resulting from PBvHb infusion (vasoconstriction, elevated pulmonary blood pressure, and reduced cardiac output) were offset by SNO-PBvHb in acute hypoxic, but not normoxic, conditions. These data support the potential use of HBOC mixed with SNO-HBOC for the treatment of conditions in which acute hypoxia is present, such as tumor oxygenation, wound healing, hemorrhagic trauma, and sickle cell and hemolytic anemia.

Site-selective glycosylation of hemoglobin on Cys beta93

In this work, we describe the synthesis and characterization of a novel glycosylated hemoglobin (Hb) with high oxygen affinity as a potential Hb-based oxygen carrier. Site-selective glycosylation of bovine Hb was achieved by conjugating a lactose derivative to Cys 93 on the beta subunit of Hb. LC-MS analysis indicates that the reaction was quantitative, with no unmodified Hb present in the reaction product. The glycosylation site was identified by chymotrypsin digestion of the glycosylated bovine Hb followed with LC-MS/MS and from the X-ray crystal structure of the glycosylated Hb. The chemical conjugation of the lactose derivative at Cys beta93 yields an oxygen carrier with a high oxygen affinity (P(50) of 4.94 mmHg) and low cooperativity coefficient (n) of 1.20. Asymmetric flow field-flow fractionation (AFFFF) coupled with multiangle static light scattering (MASLS) was used to measure the absolute molecular weight of the glycosylated Hb. AFFFF-MASLS analysis indicates that glycosylation of Hb significantly altered the alpha(2)beta(2)-alphabeta equilibrium compared to native Hb. Subsequent X-ray analysis of the glycosylated Hb crystal showed that the covalently linked lactose derivative is sandwiched between the beta(1) and alpha(2) (and hence by symmetry the beta(2) and alpha(1)) subunits of the tetramer, and the interaction between the saccharide and amino acid residues located at the interface is apparently stabilized by hydrogen bonding interactions. The resultant structural analysis of the glycosylated Hb helps to explain the shift in the alpha(2)beta(2)-alphabeta equilibrium in terms of the hydrogen bonding interactions at the beta(1)alpha(2)/beta(2)alpha(1) interface. Taken together, all of these results indicate that it is feasible to site-specifically glycosylate Hb. This work has great potential in developing an oxygen carrier with defined chemistry that can target oxygen delivery to low pO(2) tissues and organs.

Haptoglobin preserves the CD163 hemoglobin scavenger pathway by shielding hemoglobin from peroxidative modification

Detoxification and clearance of extracellular hemoglobin (Hb) have been attributed to its removal by the CD163 scavenger receptor pathway. However, even low-level hydrogen peroxide (H(2)O(2)) exposure irreversibly modifies Hb and severely impairs Hb endocytosis by CD163. We show here that when Hb is bound to the high-affinity Hb scavenger protein haptoglobin (Hp), the complex protects Hb from structural modification by preventing alpha-globin cross-links and oxidations of amino acids in critical regions of the beta-globin chain (eg, Trp15, Cys93, and Cys112). As a result of this structural stabilization, H(2)O(2)-exposed Hb-Hp binds to CD163 with the same affinity as nonoxidized complex. Endocytosis and lysosomal translocation of oxidized Hb-Hp by CD163-expressing cells were found to be as efficient as with nonoxidized complex. Hp complex formation did not alter Hb's ability to consume added H(2)O(2) by redox cycling, suggesting that within the complex the oxidative radical burden is shifted to Hp. We provide structural and functional evidence that Hp protects Hb when oxidatively challenged with H(2)O(2) preserving CD163-mediated Hb clearance under oxidative stress conditions. In addition, our data provide in vivo evidence that unbound Hb is oxidatively modified within extravascular compartments consistent with our in vitro findings.

The reaction of hydrogen peroxide with hemoglobin induces extensive alpha-globin crosslinking and impairs the interaction of hemoglobin with endogenous scavenger pathways

Cell-free hemoglobin (Hb) enhances the oxidation-related toxicity associated with inflammation, ischemia, and hemolytic disorders. Hb is highly vulnerable to oxidative damage, and irreversible structural changes involving iron/heme oxidation, heme-adduct products, and amino acid oxidation have been reported. Specific structural features of Hb, such as unconstrained alpha-chains and molecular size, determine the efficiency of interactions between the endogenous Hb scavengers haptoglobin (Hp) and CD163. Using HPLC, mass spectrometry, and Western blotting, we show that H(2)O(2)-mediated Hb oxidation results in the formation of covalently stabilized globin multimers, with prominent intramolecular crosslinking between alpha-globin chains. These structural alterations are associated with reduced Hp binding, reduced CD163 interaction, and severely impaired endocytosis of oxidized Hb by the Hp-CD163 pathway. As a result, when exposed to oxidized Hb, CD163-positive HEK293 cells and human macrophages do not increase hemeoxygenase-1 (HO-1) expression, the physiological anti-oxidative macrophage response to Hb exposure. Failed Hb clearance, inadequate HO-1 expression, and the subsequent accumulation of oxidatively damaged Hb species might thus contribute to pathologies related to oxidative stress.

First-generation blood substitutes: what have we learned? Biochemical and physiological perspectives

Chemically modified or recombinant hemoglobin (Hb)-based oxygen carriers (HBOCs) have been developed as oxygen therapeutics or 'blood substitutes' for use in a variety of clinical settings. Oxidative and nitrosative reactions of acellular Hb can limit the effectiveness and compromise the safety of HBOCs. The reactions between Hb and biologically relevant redox active molecules may also perturb redox sensitive signaling pathways. In recent years, systematic in vitro and in vivo structural and functional evaluation of several HBOCs has been carried out and, in some cases, delineated the 'structural' origin of their toxicity. This enables potential protective strategies against Hb-mediated side reactions to be rationally suggested. Here the authors provide an overview of their research experiences, novel insights into the molecular basis of toxicities of these products and some lessons learned.

Structural basis of peroxide-mediated changes in human hemoglobin: a novel oxidative pathway

Hydrogen peroxide (H(2)O(2)) triggers a redox cycle between ferric and ferryl hemoglobin (Hb) leading to the formation of a transient protein radical and a covalent hemeprotein cross-link. Addition of H(2)O(2) to highly purified human hemoglobin (HbA(0)) induced structural changes that primarily resided within beta subunits followed by the internalization of the heme moiety within alpha subunits. These modifications were observed when an equal molar concentration of H(2)O(2) was added to HbA(0) yet became more abundant with greater concentrations of H(2)O(2). Mass spectrometric and amino acid analysis revealed for the first time that betaCys-93 and betaCys-112 were oxidized extensively and irreversibly to cysteic acid when HbA(0) was treated with H(2)O(2). Oxidation of further amino acids in HbA(0) exclusive to the beta-globin chain included modification of betaTrp-15 to oxyindolyl and kynureninyl products as well as betaMet-55 to methionine sulfoxide. These findings may therefore explain the premature collapse of the beta subunits as a result of the H(2)O(2) attack. Analysis of a tryptic digest of the main reversed phase-high pressure liquid chromatography fraction revealed two alpha-peptide fragments (alpha128-alpha139) and a heme moiety with the loss of iron, cross-linked between alphaSer-138 and the porphyrin ring. The novel oxidative pathway of HbA(0) modification detailed here may explain the diverse oxidative, toxic, and potentially immunogenic effects associated with the release of hemoglobin from red blood cells during hemolytic diseases and/or when cell-free Hb is used as a blood substitute.

Effects of S-nitrosation on hemoglobin-induced microvascular damage

Blood substitutes, such as diaspirin cross-linked hemoglobin (Hb), cause microvascular leakiness to macromolecules. Because of the potentially stabilizing effects of nitric acid (NO) on endothelium, experiments were performed to determine whether S-nitrosohemoglobin (SNO-Hb), a potential NO-donor Hb-based blood substitute, would not cause microvascular damage. Release of NO, or its metabolites, from the SNO-Hb was facilitated by addition of glutathione, which aids in the decomposition of S-nitrosothiols. In anesthetized rats, the mesenteric microvasculature was perfused with SNO-Hb with glutathione (six rats), SNO-Hb alone (six rats), or saline (eight rats) for 10 min, followed by fluorescein isothiocyanate (FITC)-albumin for 1 min, and finally fixed for epifluorescence microscopic examination. When comparing the SNO-Hb group with saline, both the numbers and areas of leaks were significantly increased [0.019 +/- 0.003 (SEM) microm vs. 0.0030 +/- 0.0004 and 7.36 +/- 1.50 vs. 0.156 +/- 0.035 (p < 0.005)]. With the addition of glutathione, leakage was still high (0.005 +/- 0.00005 microm and 5.086 +/- 0.064 microm) but decreased compared with SNO-Hb alone (p < 0.005). In conclusion, NO, or a related vasodilator, when released from SNO-Hb, significantly reduces but does not eliminate microvascular damage. Further improvements may result by S-nitrosating a more stable form of modified hemoglobin.

Chemical Characterization of Diaspirin Cross-Linked Hemoglobin Polymerized with Poly(ethylene glycol)

A lack of specificity associated with chemical modification methods used in the preparation of certain hemoglobin (Hb)-based oxygen carriers (HBOCs) may alter Hb structure and function, as amino acids located in critical regions (e.g., alpha-beta interfaces and the 2,3-DPG binding pocket) may unintentionally be targeted. Hb protein surface modifications with various poly(ethylene glycol) (PEG) derivatives have been used as conjugating and polymerizing agents with the intent of improving reaction site specificity/reproducibility and ultimately reducing the untoward hypertensive response due to nitric oxide scavenging by smaller molecular size tetrameric species (i.e., 64 kDa) in HBOC solutions. Previous experiments performed in our laboratory have evaluated the influence of polymerization of diaspirin alpha-alpha cross-linked Hb (alphaalpha-DBBF-Hb) with a bifunctional modified PEG, bis(maleoylglycylamide) PEG (BMAA-PEG), in terms of oxygen carrying capacity, redox properties, hypertensive response, and renal clearance in rats. The data presented in this paper specifically evaluate the influence of BMAA-PEG on alphaalpha-DBBF-Hb (Poly-alphaalpha-DBBF-Hb) to identify molecular weight distribution, protein conformation, and site-specific modification, as well as to provide insight into the previously determined in vitro and in vivo functional and vasoactive characteristics of this HBOC. Chemical analysis performed herein reveals nonspecific modifications induced by BMAA-PEG that result in the full modification of alphaalpha-DBBF-Hb leaving no tetrameric cross-linked starting material in solution. These data are inconsistent with the continuing assumption that molecular size (i.e., 64 kDa) has a direct influence on HBOC-mediated vasoactivity and that other protective strategies should be considered to control blood pressure imbalances.