Enhanced Molecular Volume of Conservatively Pegylated Hb: (SP-PEG5K)6-HbA is Non-Hypertensive

Lyophilized annelid mega-hemoglobin retains its' quaternary structure and oxygen equilibrium properties after room temperature storage for over 6 months

The long-term storage stability and portability of hemoglobin (Hb)-based oxygen carriers are important design criteria in the development of these therapeutics. Lyophilization or storing proteins in a freeze-dried form is known to increase storage lifetime and reduce overall weight. In this study, we lyophilized the extracellular mega-hemoglobin of the annelid Lumbricus terrestris and tested the storage stability at different temperatures and oxygenation conditions. Storage in refrigerated conditions for over 6 months in the presence of N2 reduced oxidation by 50% while storage at room temperature in the presence of N2 reduced oxidation by 60%, all while maintaining the structural stability of the mega-hemoglobin. We also demonstrated a reliable strategy to freeze dry Hbs in the presence of a minimally non-reducing disaccharide sugar that could be easily re-solubilized in deionized water. Overall, this study made significant advances towards long term storage stability of oxygen therapeutics for direct applications in transfusion medicine.

Structural Stability and Biophysical Properties of the Mega-Protein Erythrocruorin Are Regulated by Polyethylene Glycol Surface Coverage

A wide variety of hemoglobin-based oxygen carriers (HBOCs) have been designed for use as red blood cell (RBC) substitutes in transfusion medicine, ex vivo organ perfusion, oxygen delivery to hypoxic tissues, and a myriad of other applications. However, hemoglobin (Hb) derived from annelids (erythrocruorins [Ecs]) comprise a natural class of HBOC, since they are larger in size (30 nm in diameter) and contain more heme groups per molecule (144 heme groups) compared to human Hb (hHb; 5 nm in diameter and 4 heme groups). The larger size of Ec compared to hHb reduces tissue extravasation from the vascular space, thus, reducing vasoconstriction, systemic hypertension, and tissue oxidative injury when used as an RBC substitute. In addition, prior research has shown that Ecs possess slower auto-oxidation rates than hHb at physiological temperature, thus, making them attractive candidates for use as RBC substitutes. Unfortunately, it was also observed that Ecs have a much lower circulatory half-life in vivo compared to other HBOCs. Hence, conjugating polyethylene glycol (PEG) to the surface of Ec was proposed as a simple strategy to increase Ec circulatory half-life. Therefore, in order to inform future in vivo studies with PEGylated Ec, we decided to investigate the structural stability and biophysical properties of variable PEG surface coverage on Ec compared to native Ec. We observed an increase in PEG-Ec diameter and molecular weight (MW) and changes to the quaternary structure, secondary structure, and surface hydrophobicity after PEGylation. There was also an increase in oxygen binding affinity, reduction in oxygen offloading rate, and increase in auto-oxidation rate for increasing PEGylation ratios. Weak dissociation of Ec was also observed after dense PEGylation caused by steric repulsion of the conjugated PEG chains. Hence, we determined an optimum Ec PEGylation ratio that resulted in a substantial size and MW increase along with preservation of oxygen binding properties. In future studies, these materials will be tested in animal models to evaluate pharmacodynamics, pharmacokinetics, tissue oxygenation, microcirculatory responses, and overall safety.

Purification of Lumbricus terrestris Mega-Hemoglobin for Diverse Oxygen Therapeutic Applications

Oxygen therapeutics are being developed for a variety of applications in transfusion medicine. In order to reduce the side-effects (vasoconstriction, systemic hypertension, and oxidative tissue injury) associated with previous generations of oxygen therapeutics, new strategies are focused on increasing the molecular diameter of hemoglobin obtained from mammalian sources via polymerization and encapsulation. Another approach towards oxygen therapeutic design has centered on using naturally occurring large molecular diameter hemoglobins (i.e. erythrocruorins) derived from annelid sources. Therefore, the goal of this study was to purify erythrocruorin from the terrestrial worm Lumbricus terrestris for diverse oxygen therapeutic applications. Tangential flow filtration (TFF) was used as a scalable protein purification platform to obtain a >99% pure LtEc product, which was confirmed by size exclusion high performance liquid chromatography and SDS-PAGE analysis. In vitro characterization concluded that the ultra-pure LtEc product had oxygen equilibrium properties similar to human red blood cells, and a lower rate of auto-oxidation compared to human hemoglobin, both of which should enable efficient oxygen transport under physiological conditions. In vivo evaluation concluded that the ultra-pure product had positive effects on the microcirculation sustaining functional capillary density compared to a less pure product (~86% purity). In summary, we purified an LtEc product with favorable biophysical properties that performed well in an animal model using a reliable and scalable purification platform to eliminate undesirable proteins.

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.

Polyethylene Glycol Camouflaged Earthworm Hemoglobin

Nearly 21 million components of blood and whole blood and transfused annually in the United States, while on average only 13.6 million units of blood are donated. As the demand for Red Blood Cells (RBCs) continues to increase due to the aging population, this deficit will be more significant. Despite decades of research to develop hemoglobin (Hb) based oxygen (O2) carriers (HBOCs) as RBC substitutes, there are no products approved for clinical use. Lumbricus terrestris erythrocruorin (LtEc) is the large acellular O2 carrying protein complex found in the earthworm Lumbricus terrestris. LtEc is an extremely stable protein complex, resistant to autoxidation, and capable of transporting O2 to tissue when transfused into mammals. These characteristics render LtEc a promising candidate for the development of the next generation HBOCs. LtEc has a short half-life in circulation, limiting its application as a bridge over days, until blood became available. Conjugation with polyethylene glycol (PEG-LtEc) can extend LtEc circulation time. This study explores PEG-LtEc pharmacokinetics and pharmacodynamics. To study PEG-LtEc pharmacokinetics, hamsters instrumented with the dorsal window chamber were subjected to a 40% exchange transfusion with 10 g/dL PEG-LtEc or LtEc and followed for 48 hours. To study the vascular response of PEG-LtEc, hamsters instrumented with the dorsal window chamber received multiple infusions of 10 g/dL PEG-LtEc or LtEc solution to increase plasma LtEc concentration to 0.5, then 1.0, and 1.5 g/dL, while monitoring the animals' systemic and microcirculatory parameters. Results confirm that PEGylation of LtEc increases its circulation time, extending the half-life to 70 hours, 4 times longer than that of unPEGylated LtEc. However, PEGylation increased the rate of LtEc oxidation in vivo. Vascular analysis verified that PEG-LtEc showed the absence of microvascular vasoconstriction or systemic hypertension. The molecular size of PEG-LtEc did not change the colloid osmotic pressure or blood volume expansion capacity compared to LtEc, due to LtEc's already large molecular size. Taken together, these results further encourage the development of PEG-LtEc as an O2 carrying therapeutic.

Enhanced Nitrite Reductase Activity and Its Correlation with Oxygen Affinity in Hemoglobin Bis-Tetramers

The vasoactivity of circulating cross-linked hemoglobin is consistent with the acellular protein penetrating the endothelial lining of blood vessels where hemoglobin can bind nitric oxide, the signal for relaxation of the muscles that surround blood vessels. In an important contrast, derivatives of bis-tetramers that are produced from hemoglobin by chemical coupling do not cause vasoconstriction in animal models. Presumably, they are unable to enter the endothelia where hemoglobin tetramers bind to nitric oxide. In addition, hemoglobin bis-tetramers can produce nitric oxide in circulation through their intrinsic nitrite reductase activity. Examination of this activity for hemoglobin-derived bis-tetramers that are acetylated at lysyl amino groups in their α subunits reveals enhanced activity (k = 2.21 M(-1) s(-1)) compared to that of nonacetylated bis-tetramers (k = 0.70 M(-1) s(-1)). Plots of nitrite reductase activities as a function of the corresponding oxygen affinities of certain allosteric-state-stabilized derivatives reveal a significant correlation, providing a basis for interpretation of the correlated functions.

Sanguinate's effect on pial arterioles in healthy rats and cerebral oxygen tension after controlled cortical impact

Sanguinate, a polyethylene glycol-conjugated carboxyhemoglobin, was investigated for cerebral vasoactivity in healthy male Sprague-Dawley rats (Study 1) and for its ability to increase brain tissue oxygen pressure (PbtO2) after controlled cortical impact (CCI) - traumatic brain injury (TBI) (Study 2). In both studies ketamine-acepromazine anesthetized rats were ventilated with 40% O2. In Study 1, a cranial window was used to measure the diameters of medium - (50-100μm) and small-sized (<50μm) pial arterioles before and after four serial infusions of Sanguinate (8mL/kg/h, cumulative 16mL/kg IV), volume-matched Hextend, or normal saline. In Study 2, PbtO2 was measured using a phosphorescence quenching method before TBI, 15min after TBI (T15) and then every 10min thereafter for 155min. At T15, rats received either 8mL/kg IV Sanguinate (40mL/kg/h) or no treatment (saline, 4mL/kg/h). Results showed: 1) in healthy rats, percentage changes in pial arteriole diameter were the same among the groups, 2) in TBI rats, PbtO2 decreased from 36.5±3.9mmHg to 19.8±3.0mmHg at T15 in both groups after TBI and did not recover in either group for the rest of the study, and 3) MAP increased 16±4mmHg and 36±5mmHg after Sanguinate in healthy and TBI rats, respectively, while MAP was unchanged in control groups. In conclusion, Sanguinate did not cause vasoconstriction in the cerebral pial arterioles of healthy rats but it also did not acutely increase PbtO2 when administered after TBI. Sanguinate was associated with an increase in MAP in both studies.

Effects of the Oxygen-Carrying Solution OxyVita C on the Cerebral Microcirculation and Systemic Blood Pressures in Healthy Rats

The use of hemoglobin-based oxygen carriers (HBOC) as oxygen delivering therapies during hypoxic states has been hindered by vasoconstrictive side effects caused by depletion of nitric oxide (NO). OxyVita C is a promising oxygen-carrying solution that consists of a zero-linked hemoglobin polymer with a high molecular weight (~17 MDa). The large molecular weight is believed to prevent extravasation and limit NO scavenging and vasoconstriction. The aim of this study was to assess vasoactive effects of OxyVita C on systemic blood pressures and cerebral pial arteriole diameters. Anesthetized healthy rats received four intravenous (IV) infusions of an increasing dose of OxyVita C (2, 25, 50, 100 mg/kg) and hemodynamic parameters and pial arteriolar diameters were measured pre- and post-infusion. Normal saline was used as a volume-matched control. Systemic blood pressures increased (P ≤ 0.05) with increasing doses of OxyVita C, but not with saline. There was no vasoconstriction in small (<50 µm) and medium-sized (50–100 µm) pial arterioles in the OxyVita C group. In contrast, small and medium-sized pial arterioles vasoconstricted in the control group. Compared to saline, OxyVita C showed no cerebral vasoconstriction after any of the four doses evaluated in this rat model despite increases in blood pressure.

PEGylation of αα-Hb using succinimidyl propionic acid PEG 5K: Conjugation chemistry and PEG shell structure dictate respectively the oxygen affinity and resuscitation fluid like properties of PEG αα-Hbs

PEGylation of intramolecularly crosslinked Hb has been studied here to overcome the limitation of dissociation of Hb tetramers. New hexa and deca PEGylated low oxygen affinity PEG-ααHbs have been generated. Influence of PEG conjugation chemistry and the PEG shell structure on the functional properties as well as PEGylation induced plasma expander like properties of the protein has been delineated. The results have established that in the design of PEG-Hbs as oxygen therapeutics, the influence of conjugation chemistry and the PEG shell structure on the oxygen affinity of Hb needs to be optimized independently besides optimizing the PEG shell structure for inducing resuscitation fluid like properties.

Packing density of the PEG-shell in PEG-albumins: PEGylation induced viscosity and COP are inverse correlate of packing density

PEG-Alb represents a new class of low viscogenic plasma expanders that achieve super perfusion in vivo by mimicking the vasodilatory influence of high viscogenic plasma expanders. PEGylation-engineered structure of PEG albumin can be envisaged as a deformable molecular domain around the rigid central protein core. The correlation between the structure of PEG-shell in terms of packing of the PEG inside the PEG shell and PEGylation induced plasma expander (PE)-like properties of albumin has been investigated as a function of the number and length of the PEG-chain. The increase in molecular radius of albumin on PEGylation is non-linear as a function of the number of PEG chains conjugated. The packing density of PEG within the PEG-shell is an inverse correlate of PEG-chain size; i.e. the shorter chains pack more compactly than the longer ones. The PEGylation induced increase in the viscosity and COP of albumin is an exponential correlation of the number of ethylene oxide units (-CH(2)-CH(2)-O-) conjugated and is also a function of the PEG-chain length. At equivalence of PEG mass conjugated, the viscosity and COP of PEG-albumin adducts correlate inversely with packing density of PEG. All PEGylated albumins are not equivalent on the basis of total PEG mass conjugated. Accordingly, the structure of PEG albumin and its solution properties can be engineered to optimize a given total PEG mass for the application of PEG albumin as a resuscitation fluid. The extension arms minimize the influence of PEG shell on the structure of the protein core. We speculate that EAF-PEGylation is a preferable platform for PEGylation of protein therapeutics and is expected to generate products with better therapeutic efficacy.

Pre-clinical studies using OxyVita hemoglobin, a zero-linked polymeric hemoglobin: a review

Hemoglobin-based oxygen carriers (HBOC) are being developed to provide the oxygen necessary in clinical situations when whole blood is not available. The safety and effectiveness of each HBOC must be determined before clinical approval. In the past several years animal studies have been conducted with zero-linked polymers to evaluate their effectiveness at delivering oxygen in vivo. Studies have addressed issues associated with interstitial extravasation, cerebral ischemia and blood flow, resuscitation, and coagulation interactions. Several of the investigations reviewed are based on early preparations of zero-linked polymerized bovine hemoglobins (ZL-HbBv), which contained a wide range of high-molecular-weight polymers. Recent studies using the Oxyvita product OxyVita Hb, which contains a more homogenous population (97%) of large-molecular-weight species (~17 MDa), are also included in this review.

Enhancing nitrite reductase activity of modified hemoglobin: bis-tetramers and their PEGylated derivatives

The clinical evaluation of stabilized tetrameric hemoglobin as alternatives to red cells revealed that the materials caused significant increases in blood pressure and related problems and this was attributed to the scavenging of nitric oxide and extravasation. The search for materials with reduced vasoactivity led to the report that conjugates of hemoglobin tetramers and polyethylene glycol (PEG) chains did not elicit these pressor effects. However, this material does not deliver oxygen efficiently due to its lack of cooperativity and high oxygen affinity, making it unsuitable as an oxygen carrier. It has been recently reported that PEG-conjugated hemoglobin converts nitrite to nitric oxide at a faster rate than does the native protein, which may compensate for the scavenging of nitric oxide. It is therefore important to alter hemoglobin in order to enhance nitrite reductase activity while retaining its ability to deliver oxygen. If the beneficial effect of PEG is associated with the increased size reducing extravasation, this can also be achieved by coupling cross-linked tetramers to one another, giving materials with appropriate oxygen affinity and cooperativity for use as circulating oxygen carriers. In the present study it is shown that cross-linked bis-tetramers with good oxygen delivery potential have enhanced nitrite reductase activity with k(obs) = 0.70 M(-1) s(-1) (24 degrees C), compared to native protein and cross-linked tetramers, k(obs) = 0.25 M(-1) s(-1) and k(obs) = 0.52 M(-1) s(-1), respectively, but are less active in reduction of nitrite than Hb-PEG5K(2) (k(obs) = 2.5 M(-1) s(-1)). However, conjugation of four PEG chains to the bis-tetramer (at each beta-Cys-93) produces a material with greatly increased nitrite reductase activity (k(obs) = 1.8 M(-1) s(-1)) while retaining cooperativity (P(50) = 4.1, n(50) = 2.4). Thus, PEGylated bis-tetramers combine increased size and enhanced nitrite reductase activity expected for decreased vasoactivity with characteristics of an acceptable HBOC.

Hexa-thiocarbamoyl phenyl PEG5K Hb: vasoactivity and structure: influence of rigidity of the conjugation linkage on the PEGylation induced plasma expander-like solution properties of PEG-Hb adducts

A new hexaPEGylated hemoglobin, (TCP-PEG5K)(6)-Hb (TCP, thiocarbamoyl phenyl) has been developed using PEG-phenyl-isothiocyanate and its vasoactivity and structure has been investigated. Of the six PEG5K chains of (TCP-PEG5K)(6)-Hb, 4 are conjugated to the alpha-amino groups of Hb, and the other 2 chains are distributed on epsilon-amino groups, identified as Lys-40(alpha) (approximately 45%), Lys-56(alpha) (approximately 25%), and Lys-8(beta) (approximately 24%). The studies with hamster infused with a bolus of a 4 gm % solution of (TCP-PEG5K)(6)-Hb equivalent to 10% of their blood volume have established that this new hexaPEGylated Hb is vasoinactive. The viscosity and the colloidal osmotic pressure of (TCP-PEG5K)(6)-Hb at 4% is 1.9 cP and 69.7 mmHg, respectively. The molecular radius of (TCP-PEG5K)(6)-Hb is about 4.6 nm and is significantly smaller than hexaPEGylated Hbs developed using other direct and extension arm facilitated PEGylation platform. The presence of an outside the central cavity intramolecular crosslink, succinimidophenyl-PEG2K between Cys-93(beta, beta') in (TCP-PEG5K)(6)-betabeta-Hb strongly impacts its solution properties. These patterns of influence suggest that the inter-dimeric interactions in (TCP-PEG5K)(6)-Hb is weakened just as with other direct PEGylation platforms, and (SP-PEG5K)(6)-Hb generated by EAF-PEGylation is unique in not inducing this effect. A comparison of the properties of hexaPEGylated Hbs establishes that rigidity of the conjugation linkage between PEG and Hb plays a significant influence on the resultant dictating solution properties and/structure/conformation of PEG-Hb adduct.

Molecular Aspects of the High Oxygen Afinity of Non-Hypertensive Hexa Pegylated Hemoglobin, [(SP-PEG5K)6-Hb]

The development of hexaPEGylated Hb, (SP-PEG5K)(6)-Hb, using the newly designed thiolation-mediated maleimide chemistry based PEGylation, has validated the concept that engineering 'plasma volume expander' -like properties to Hb neutralizes its vasoactivity. The high O(2) affinity of hexaPEGylated Hb has been attributed to the two PEG-5K chains on its two Cys-93(beta) residues. In an attempt to map the influence of the additional four PEG-5K chains of HexaPEGylated Hb on the O(2) affinity, we have now investigated the influence of PEGylation of the surface amino groups alone on the subunit interface interactions and O(2) affinity of Hb using rHb(betaC93A). The molecular radius of PEGylated rHb(betaC93A) was slightly smaller than that of (SP-PEG5K)(6)-Hb, and the overall site-selectivity of PEGylation in the PEGylated rHb(betaC93A) at Lys-residues was comparable to that of (SP-PEG5K)(6)-Hb. Proton NMR studies have shown that the conjugation of the protein with PEG-5K does not have any significant influence on its subunit interface interactions. Surprisingly, the influence of PEGylation on the O(2) affinity and Bohr effect of HbA and rHb(betaC93A) is also nearly the same. Apparently, conjugation of PEG-chains to Lys residues of Hb by the thiolation mediated PEGylation induces unique changes in the structure of the hydration shell of Hb (layer of tightly bound water molecules), which, in turn, induces constraints in its R to T conformational transition to favor the more hydrated R-state.

Role of extension arm in PEG-Hb conjugates on the stability of the tetramer: non-conservative EAF maleimide thio-PEG mediated PEGylation

PEGylation induced increase in colloidal osmotic pressure (COP) of Hb, one of the unique molecular properties of PEG-Hb conjugates, facilitates the neutralization of the vasoconstrictive activity of acellular Hb is a function of chemistry of conjugation of PEG-chains. The dependence of COP with the chemistry is a consequence of PEGylation induced weakening of interdimeric interactions. The conservative Extension Arm Facilitated (EAF) PEGylation exerts least influence on lowering the tetramer stability as compared to direct PEGylation. We have now designed a new, non-conservative EAF PEGylation that uses acylation chemistry to introduce the extension arm onto proteins to delineate the role of the extension arm and of the charge at the site of attachment in PEG-Hb conjugate on tetramer stability. The non-conservative EAF PEGylation does not lower the tetramer stability just as the non-conservative direct PEGylation of Hb. The impact of the extension arm in PEGylated Hb in terms of their structure and potential significance of higher tetramer stability of PEG-Hb conjugates generated by EAF-PEGylation in the in vivo toxicity and in the design of oxygen carrying plasma expander has been discussed.

Non-hypertensive tetraPEGylated canine haemoglobin: correlation between PEGylation, O2 affinity and tissue oxygenation

TetraPEGylated canine Hb, [SP (succinimidophenyl)-PEG5K]4-canine-Hb, with PEGylation at its four reactive cysteine residues (a111 and b93) has been prepared and characterized. The hydrodynamic volume and the molecular radius of (SP-PEG5K)4-canine-Hb are intermediate to those of di- and hexaPEGylated human Hb as expected. However, the COP (colloidal osmotic pressure) of tetraPEGylated canine Hb is closer to that of hexaPEGylated human Hb than to that of diPEGylated human Hb. The O2 affinity of tetraPEGylated canine Hb is higher than that of canine Hb and comparable with that of hexaPEGylated Hb. The O2 affinity of tetraPEGylated canine Hb is not responsive to the presence of DPG (diphosphoglycerate) or chloride, but it retains almost full response to L-35, an allosteric effector that interacts at the aa-end of the central cavity. The tetraPEGylated canine Hb is vasoinactive in hamster in 10% top load infusion studies. It is also essentially non-hypertensive in an extreme exchange haemodilution protocol in hamster just as di- and hexaPEGylated human Hb. The O2 delivery by tetraPEGylated canine Hb is comparable with that of hexaPEGylated Hb but not as efficient as diPEGylated Hb. These results demonstrate that PEGylation-induced solution properties of PEG [poly(ethylene glycol)]-Hb conjugates are dictated by the level and chemistry of PEGylation and the interplay of these plays a critical role in tissue oxygenation. The studies imply the need to establish the right level (and/or pattern) of PEGylation and O2 affinity of Hb-PEG adducts in designing O2-carrying plasma volume expanders, and this remains the primary challenge in the design of PEGylated Hb as blood substitutes.

Combining the influence of two low O2 affinity-inducing chemical modifications of the central cavity of hemoglobin

HexaPEGylated hemoglobin (Hb), a non-hypertensive Hb, exhibits high O2 affinity, which makes it difficult for it to deliver the desired levels of oxygen to tissues. The PEGylation of very low O2 affinity Hbs is now contemplated as the strategy to generate PEGylated Hbs with intermediate levels of O2 affinity. Toward this goal, a doubly modified Hb with very low O2 affinity has been generated. The amino terminal of the beta-chain of HbA is modified by 2-hydroxy, 3-phospho propylation first to generate a low oxygen affinity Hb, HPPr-HbA. The oxygen affinity of this Hb is insensitive to DPG and IHP. Molecular modeling studies indicated potential interactions between the covalently linked phosphate group and Lys-82 of the trans beta-chain. To further modulate the oxygen affinity of Hb, the alpha alpha-fumaryl cross-bridge has been introduced into HPPr-HbA in the mid central cavity. The doubly modified HbA (alpha alpha-fumaryl-HPPr-HbA) exhibits an O2 affinity lower than that of either of the singly modified Hbs, with a partial additivity of the two modifications. The geminate recombination and the visible resonance Raman spectra of the photoproduct of alpha alpha-fumaryl-HPPr-HbA also reflect a degree of additive influence of each of these modifications. The two modifications induced a synergistic influence on the chemical reactivity of Cys-93(beta). It is suggested that the doubly modified Hb has accessed the low affinity T-state that is non-responsive to effectors. The doubly modified Hb is considered as a potential candidate for generating PEGylated Hbs with an O2 affinity comparable to that of erythrocytes for developing blood substitutes.

Influence of intramolecular cross-links on the molecular, structural and functional properties of PEGylated haemoglobin

The influence of intramolecular cross-links on the molecular, structural and functional properties of PEGylated {PEG [poly(ethylene glycol)]-conjugated} haemoglobin has been investigated. The sites and the extent of PEGylation of haemoglobin by reductive alkylation are not influenced by the presence of an alphaalpha-fumaryl cross-link at Lys-99(alpha). The propylated hexaPEGylated cross-linked haemoglobin, (propyl-PEG5K)(6)-alphaalpha-Hb, exhibits a larger molecular radius and lower colloidal osmotic pressure than propylated hexaPEGylated non-cross-linked haemoglobin, (propyl-PEG5K)(6)-Hb. Perturbation of the haem microenvironment and the alpha1beta2 interface by PEGylation of haemoglobin is reduced by intramolecular cross-linking. Sedimentation velocity analysis established that PEGylation destabilizes the tetrameric structure of haemoglobin. (Propyl-PEG5K)(6)-Hb and (propyl-PEG5K)(6)-alphaalpha-Hb sediment as stable dimeric and tetrameric molecules, respectively. The betabeta-succinimidophenyl PEG-2000 cross-link at Cys-93(beta) outside the central cavity also influences the molecular properties of haemoglobin, comparable to that by the alphaalpha-fumaryl cross-link within the central cavity. However, the influence of the two cross-links on the oxygen affinity of PEGylated haemoglobin are very distinct, indicating that the high oxygen affinity of PEGylated haemoglobin is not a direct consequence of the dissociation of the haemoglobin tetramers into dimers. alphaalpha-Fumaryl cross-linking is preferred to modulate both oxygen affinity and molecular properties of PEGylated haemoglobin, and cross-linking outside the central cavity could only modulate molecular properties of PEGylated haemoglobin. It is suggested that PEGylation induces a hydrodynamic drag on haemoglobin and this plays a role in the microcirculatory properties of PEGylated haemoglobin.

Extension Arm Facilitated PEGylation of Hemoglobin: Correlation of the Properties with the Extent of PEGylation

Human hemoglobin (Hb) conjugated with six copies of PEG-5K is nonhypertensive. The hexaPEGylated Hb exhibits molecular size homogeneity in spite of the chemical heterogeneity with respect to the sites of conjugation (Manjula et al., 2005). In the present study, Hb conjugated with an average of 4, 6, 8 and 10 copies of PEG-5K chains have been generated using the extension arm facilitated PEGylation protocol. Except for the tetraPEGylated Hb, all the other products exhibit molecular size homogeneity. The molecular, colligative and functional properties of PEG-Hb conjugates have been correlated with the extent of PEGylation. The results imply that six copies of PEG-5K chains are accommodated on Hb without significant crowding on the molecular surface. As more copies of PEG-5K chains are conjugated to form octa and deca PEGylated Hb, the PEG-chains conjugated appear to undergo transition from a mushroom (compact) to a brush-like conformation (extended conformation) with a concomitant decrease in the propensity of the molecule to transition from oxy to deoxy conformation in the presence of allosteric effectors. The viscosity and the colloidal osmotic pressure of Hb increase with the number of the PEG-chains conjugated in an exponential fashion. The composition of the PEGylated Hb generated appears to be controlled by (i) high reactivity of thiol groups of the extension arms on Hb with maleimide-PEG, (ii) increase in the viscosity of the reaction mixture as the level of PEGylation increases and (iii) increased resistance induced by the PEG-shell of PEGylated Hb to accommodate more PEG-chains as the level of PEGylation increases. Potential implications of extent of PEGylation on the oxygen delivery by PEG-Hb conjugate in vivo have been discussed.

Influence of the chemistry of conjugation of poly(ethylene glycol) to Hb on the oxygen-binding and solution properties of the PEG-Hb conjugate

Our recent studies on PEG-Hb [poly(ethylene glycol)-Hb] conjugates generated by thiolation-mediated maleimide-chemistry based PEGylation demonstrated that the vasoactivity of the PEG-Hb conjugates is a function of the configuration of the PEG chains on the surface of the protein and is independent of the PEG/protein-mass ratio [Manjula, A. G. Tsai, Intaglietta, H.-C. Tsai, Ho, Smith, Perumalsamy, Kanika, Friedman and Acharya (2005) Protein J. 24, 133-146]. A Hb conjugated with six PEG5k chains (SP-PEG5k)6-Hb, was vasoinactive. In an attempt to understand whether the chemistry of conjugation of PEG to Hb has any influence on the modulation of its functional and solution properties, we have now generated a new hexaPEGylated-Hb, (propyl-PEG5k)6-Hb, by reductive alkylation chemistry. CD (circular dichroism) spectral measurements indicated that the overall secondary structure of Hb is not adversely influenced upon PEGylation. (Propyl-PEG5k)6-Hb exhibited an increased O2 affinity with decreased co-operativity and decreased modulation by allosteric effectors comparable with that of (SP-PEG5k)6-Hb, although its Cys-93(b) is not derivatized as in the latter. On a molecular mass basis, PEG linked to Hb by reductive alkylation increased its COP (colloidal osmotic pressure) more efficiently than when linked by thiolation-mediated maleimide-chemistry. These results demonstrate that the functional properties of PEG-Hb conjugates may be a direct consequence of surface decoration of Hb with PEG, but are independent of the site (pattern) and/or the chemistry of PEGylation. However the solution properties of PEGylated Hb are influenced by the site (pattern) and/or the chemistry of PEGylation and the presence or absence of an 'extension arm' between the conjugating site of Hb and the PEG chain.