Low affinity PEGylated hemoglobin from Trematomus bernacchii, a model for hemoglobin-based blood substitutes
© Coppola et al; licensee BioMed Central Ltd. 2011
Received: 26 July 2011
Accepted: 20 December 2011
Published: 20 December 2011
Conjugation of human and animal hemoglobins with polyethylene glycol has been widely explored as a means to develop blood substitutes, a novel pharmaceutical class to be used in surgery or emergency medicine. However, PEGylation of human hemoglobin led to products with significantly different oxygen binding properties with respect to the unmodified tetramer and high NO dioxygenase reactivity, known causes of toxicity. These recent findings call for the biotechnological development of stable, low-affinity PEGylated hemoglobins with low NO dioxygenase reactivity.
To investigate the effects of PEGylation on protein structure and function, we compared the PEGylation products of human hemoglobin and Trematomus bernacchii hemoglobin, a natural variant endowed with a remarkably low oxygen affinity and high tetramer stability. We show that extension arm facilitated PEGylation chemistry based on the reaction of T. bernacchii hemoglobin with 2-iminothiolane and maleimido-functionalyzed polyethylene glycol (MW 5000 Da) leads to a tetraPEGylated product, more homogeneous than the corresponding derivative of human hemoglobin. PEGylated T. bernacchii hemoglobin largely retains the low affinity of the unmodified tetramer, with a p50 50 times higher than PEGylated human hemoglobin. Moreover, it is still sensitive to protons and the allosteric effector ATP, indicating the retention of allosteric regulation. It is also 10-fold less reactive towards nitrogen monoxide than PEGylated human hemoglobin.
These results indicate that PEGylated hemoglobins, provided that a suitable starting hemoglobin variant is chosen, can cover a wide range of oxygen-binding properties, potentially meeting the functional requirements of blood substitutes in terms of oxygen affinity, tetramer stability and NO dioxygenase reactivity.
Hemoglobin-based oxygen carriers (HBOCs) are a novel therapeutic class consisting of hemoglobin (Hb) derivatives administered intravenously as substitutes for blood transfusions. Modifications of the natural tetramer are required to reduce toxicity, as unmodified, cell-free Hb, once dissociated into dimers, is easily filtered by the kidneys and causes severe nephrotoxicity. Moreover, Hb extravasates through the endothelium, where it scavenges the vasoactive mediator nitrogen monoxide (NO), causing a range of toxic effects that include vasoconstriction and blood pressure increase. The strategies so far explored to avoid such effects mainly aim at increasing the molecular size of the natural Hb tetramers, thus limiting the size-dependent vessel extravasation and renal ultrafiltration. Beside some attempts at designing recombinant Hbs with higher molecular weight or lower dimer-tetramer dissociation constants [1–5], most products proposed for clinical use consist of Hb purified from whole blood and chemically modified to achieve either intramolecular cross-linking or conjugation with polyethylene glycol (PEG) . PEG derivatization usually consists in the reaction between maleimido-functionalized PEG (MAL-PEG) molecules with either solvent-exposed cysteyl residues or thiol groups introduced through the reaction of lysyl side chains with 2-iminothiolane (IMT) under either aerobic  or anaerobic  conditions. PEG-decorated human Hb (HbA) derivatives have been evaluated in several clinical trials . Adverse effects have so far prevented their application as a replacement of red blood cells .
One of the limits of HBOCs lies in the large differences between their oxygen binding properties with those of red blood cells. As a matter of fact, cell-free HbA cannot bind the intra-erythrocyte allosteric effector 2,3-bisphosphoglycerate, which increases the P50 (the oxygen partial pressure required to achieve half saturation) from 10 Torr to around 26 Torr at 37°C, pH 7.4. Moreover, free Hb in the plasma is usually at concentrations low enough to significantly dissociate into dimers, which do not show cooperativity and exhibit a P50 close to that of R-state Hb. PEGylation itself destabilizes the Hb tetramer and shifts the tetramer-dimer equilibrium towards the latter, with loss of cooperativity and a further increase in affinity . Particularly, the reaction of PEG with Cys β93, conserved in 90% of vertebrates , was associated with tetramer dissociation and increased affinity [11, 13]. As a matter of fact, both PEGylation of HbA in the T quaternary state, where Cys β93 is not reactive  and the reversible protection of Cys β93 HbA prior to conjugation  result in higher tetramer stability and lower affinity. However, based on experiments on HbA mutants, an increase in oxygen affinity seems to be at least partially independent of the derivatization of Cys β93 , suggesting that PEGylation induces changes in the hydration shell of hemoglobin, shifting the conformational equilibrium towards the more hydrated R state, regardless of the PEGylation sites.
In the light of the recent setbacks suffered by PEGylated Hb in clinical trials , a deeper investigation of the relationship between the oxygen-binding properties and PEGylation in Hbs was undertaken. One of the possible strategies focused on the use of non-human PEGylated Hbs, taking advantage of the low immunogenicity of PEGylated proteins in general . Non-human Hbs might greatly differ in terms of PEGylation pattern, oxygen-binding properties and sensitivity to allosteric effectors. A product consisting of bovine Hb decorated with 10-12 units of 5000 Da-MW PEG was investigated as a possible blood substitute and showed a P50 of 10.2 Torr at 37°C , higher than that of PEGylated human Hb but still far from that of human blood (around 26 Torr). TetraPEGylated canine Hb  similarly showed a P50 of 10 Torr under the same conditions. In view of investigating the relationship between the oxygen affinity of animal Hbs and that of their PEGylation products, Hbs from Notothenioidei, the dominant suborder of teleosts in Antarctica, are particularly interesting, as they show peculiar features that make them potentially less sensitive to the undesirable effects of PEGylation. The oxygen affinity of these Hbs is exceptionally low , an evolutionary consequence of the high oxygen concentration in the cold Antarctic waters. Moreover, unlike HbA, fish Hbs show little or no dissociation of the tetramer into dimers, even in the ligated form . Finally, Cys β93, present in the great majority of vertebrate Hbs and known to greatly perturb the properties of PEGylated Hbs and to scavenge NO , is missing in Hbs of almost all teleosts. The remaining cysteyl residues are all buried inside the protein matrix , suggesting that PEG conjugation can be carried out regardless of the quaternary or ligation state. We therefore decorated Hb from Trematomus bernacchii (Tb Hb) with PEG and characterized the reactivity with oxygen and NO. The results were compared with those obtained for PEGylated HbA.
2-iminothiolane (IMT), HEPES buffer, ethylendiaminotetraacetic acid (EDTA), phosphate buffered saline solution (PBS), sodium ascorbate, catalase and the reagents for the Hayashi enzymatic reducing system  were purchased from Sigma Chemical Co. (St. Louis, MO, U.S.A.) and maleimido polyethylene glycol (MAL-PEG) (5600 Da-MW) from Nektar Molecule Engineering (Nektar Therapeutics, San Carlos, CA, U.S.A.). All other reagents were of the best available commercial quality.
Collection of specimens and Hbs purification
Specimens of Tb Hb were collected by gill nets or hook-and-line in the vicinity of Terra Nova Bay "Mario Zucchelli" Station (74°42'S, 164°07'E), Ross Sea, Antarctica, and kept in aquaria supplied with running, aerated sea water. Blood was withdrawn with heparinised syringes from the caudal vein. Hemolysates were prepared as described previously . Saline-washed erythrocytes were frozen at -80°C until use. Purification of Tb Hb at 98% was carried out as described previously . HbA was purified as described elsewhere .
Preliminarily to PEGylation experiments, the reactivity of the cysteyl residues of Tb Hb was evaluated in both the deoxy- (T) and carboxy- (R) states using 4,4'dithiodipyridine (4-PDS) . For the titration under anerobic conditions, the protein solution was incubated under helium flux until the absorption spectrum shifted to the deoxy-Hb form. A separately deoxygenated stock solution of 4-PDS was anaerobically added.
Determination of oxygen-binding curves
Oxygen-binding curves of HbA, Tb Hb and their PEGylated derivatives, called PEG-Hboxy and PEGTb Hb, respectively, were measured with a modified tonometer . Before titration, the stock solutions of the proteins were diluted in a solution containing 100 mM HEPES, 1 mM sodium EDTA, at either pH 7.0 or 8.0, to a final concentration of 30 μM. Sodium ascorbate and catalase were added to final concentrations of 5 mM and 103 U/ml, respectively, to prevent significant autoxidation during the measurement. For Tb Hb and PEGTb Hb, experiments were carried out in the presence and absence of the allosteric effector ATP at a final concentration of 3 mM. For the experiments on Tb Hb and PEGTb Hb stored in the carbomonoxy form, CO was removed by exposure to pure oxygen for 2 hours prior to titration, taking advantage of the relatively low affinity of Tb Hb for CO (data not shown). The samples where then exposed to oxygen partial pressures ranging from 0 to 760 Torr generated using an Environics 4000 (Environics inc, Tolland, CT, U.S.A.) gas mixer and pre-mixed helium/oxygen bottles, at 10°C. Spectra were collected in the 350-700 nm range using a Cary 4000 (Agilent Technologies, Lexington, MA, U.S.A.) spectrophotometer. The oxygen saturation at each partial oxygen pressure was determined by deconvoluting the spectra in the 450-700 nm range to a linear combination of the reference spectra of deoxy-, oxy- and met-Hb, plus a baseline. The deoxy reference spectra were obtained for HbA and Tb Hb in the presence of sodium dithionite, whereas reference spectra for the oxy forms were obtained in pure oxygen in the presence of the Hayashi reducing system . The Hill's coefficient (n) and P50 were calculated by linear regression of the Hill's plots in the saturation range 20-80%.
Flash photolysis experiments
The experimental set up has been described previously [28, 29]. Flash photolysis measurements were performed using the circularly polarized second harmonic of a Q-switched Nd:YAG laser (Surelite II Continuum) and a cw Xe arc lamp as a monitoring beam. The transient absorbance signals were measured at 436 nm with a 5-stage photomultiplier.
NO dioxygenase activity
The rates of the NO dioxygenase reactivity at a single NO concentration were determined for HbA, Tb Hb, PEG-Hboxy and PEGTb Hb by rapid mixing using a stopped-flow apparatus (SX.18MV, Applied Photophysics). The NO solutions were generated by equilibrating a previously deoxygenated PBS solution at pH 7.4 with a gas mixture of NO in nitrogen. The exact concentration of NO was then measured by titration of the solution with deoxygenated HbA under anaerobic conditions and determined to be 12 μM. The protein concentration was 3 μM. The reaction was monitored at 405 nm. Between 5 and 10 traces were collected and averaged. All measurements were carried out under strict anaerobic conditions at 20°C.
Results and Discussion
Sulfhydryl reactivity towards 4-PDS of carbomonoxy- and deoxy- Tb Hb was very slow (data not shown) with the fastest-reacting cysteine completing the reaction in more than 24 hours. The slow reactivity of Tb Hb confirmed the structural data , which indicated the absence of exposed cysteyl residues. 2-iminothiolane-generated SH groups are therefore predicted to be the only reactive sites towards MAL-PEG (Figure 1). For comparison, HbA reacts with a twice equimolar amount 4-PDS within 10 minutes (data not shown) due to the exposed Cys β93. It is therefore expected that Tb Hb, unlike HbA, would not react directly with MAL-PEG.
Oxygen binding parameters of human and T. bernacchii Hbs and their PEGylated derivatives
Flash photolysis experiments
NO dioxygenase activity
The functional characterization of the PEG-conjugated derivative of the highly stable Hb tetramer of T. bernacchii confirms the non-specific effects of PEGylation already observed in human, bovine and canine Hbs, including an increase in oxygen affinity, a decrease in cooperativity and a reduction of the R- to T-quaternary switching upon flash photolysis. However, these non-specific effects are accompanied by the partial retention of the remarkably low affinity for oxygen, the sensitivity to allosteric effectors and the low NO dioxygenase reactivity. These results indicate that PEGylated Hbs, provided that a suitable starting Hb variant is chosen, can cover a wide range of oxygen-binding properties, potentially meeting the functional requirements of blood substitutes.
hemoglobin-based oxygen carrier
maleimido polyethylene glycol
phosphate buffered saline
dodecylsulfate/polyacrylamide gel electrophoresis
- Tb Hb Trematomus bernacchii:
This study is financially supported by the Italian National Programme for Antarctic Research (PNRA) and Fondazione Cariparma (to AM), and is in the framework of the SCAR programme Evolution and Biodiversity in the Antarctic (EBA) and the project CAREX (Coordination Action for Research Activities on Life in Extreme Environments), European Commission FP7 call ENV.2007.2.2.1.6.
This project was supported by the Italian Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR), Direzione Generale per l'Internazionalizzazione della Ricerca, Progetti di Grande Rilevanza
Italia-Argentina. We are grateful to Prof. Lelio Mazzarella for invaluable discussion, ideas and enthusiastic support in addressing these themes.
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