Hemoglobin covalently bridged across the polyphoshate binding site

Pulse Oximetry: The Working Principle, Signal Formation, and Applications

Pulse oximeters are routinely used in various medical-grade and consumer-grade applications. They can be used to estimate, for example, blood oxygen saturation, autonomic nervous system activity and cardiac function, blood pressure, sleep quality, and recovery through the recording of photoplethysmography signal. Medical-grade devices often record red and infra-red light-based photoplethysmography signals while smartwatches and other consumer-grade devices usually rely on a green light. At its simplest, a pulse oximeter can consist of one or two photodiodes and a photodetector attached, for example, a fingertip or earlobe. These sensors are used to record light absorption in a medium as a function of time. This time-varying absorption information is used to form a photoplethysmography signal. In this chapter, we discuss the working principles of pulse oximeters and the formation of the photoplethysmography signal. We will further discuss the advantages and disadvantages of pulse oximeters, which kind of applications exist in the medical field, and how pulse oximeters are utilized in daily health monitoring.

Conjugated Human Hemoglobin as a Physiological Oxygen Carrier - Pyridoxalated Hemoglobin Polyoxyethylene Conjugate (PHP)

Problems associated with specific physiological properties of Hb-based blood substitutes, such as a low P50, short plasma half-life and nephrotoxicity are still major issues to be addressed. Extensive investigations aimed at overcoming these problems have resulted in the preparation of pyridoxalated-hemoglobin-polyoxyethylene conjugate (PHP). PHP was developed from human hemoglobin by two major chemical modifications; pyridoxylation for the purpose of lowering the oxygen affinity (P50 of 19.5 ± 1.2 mmHg), and coupling with polyoxyethylene (POE) to increase its molecular weight (to approximately 90 kdaltons). The circulating half-life of PHP is about 40 hours in dogs. Toxicologicai and physiological studies including renal function assessments have demonstrated that PHP does not have untoward effects on major organ functions. Its efficacy in transporting oxygen has been shown in ET and intracoronary perfusion, and in in vitro studies with sickle cells. Studies to date suggest that PHP is a promising candidate as a physiological oxygen carrier. In this paper the properties of PHP, its safety and efficacy aspects, and its potential as a clinical oxygen carrier are reviewed based on studies conducted in the Author's laboratory.

Serum Antibody Titers in Rats Receiving Repeated Small Subcutaneous Injections of Hemoglobin or Polyhemoglobin: A Preliminary Report

Cross-linking hemoglobin (Hb) into Polyhemoglobin (PolyHb) for use as an artificial blood substitute may affect its antigenicity. To investigate this, male Sprague-Dawley rats are immunized with one of the following: rat stroma-free Hb (rSFHb), rat PolyHb (rPolyHb), human stroma-free Hb (hSFHb), and human PolyHb (hPolyHb). Antibody titers are quantified using a double antibody radioimmunoassay. These results show that more antibodies are produced to hPolyHb than to hSFHb, whereas rSFHB and rPolyHb are relatively non-antigenic. Thus, under homologous conditions, cross-linking hemoglobin does not significantly increase its antigenicity, whereas under heterologous conditions the molecule becomes more antigenic.

Biologically active polymer nanosystems

The methods of synthesis of biologically active nanostructured systems based on functional and natural polymers are reviewed. The formation of nanosystems in the process of interaction between synthetic water-soluble polyelectrolytes and amphiphilic ionic surfactants is discussed. The influence of structure and stability of these systems on their biological activity is considered. The complexation between DNA and polycations with the formation of compacted DNA molecules, and the transport of resulting complexes into the cells are discussed. The data on nanostructuring of hemoglobin using polyfunctional crosslinkers and the data on the use of the obtained nanoparticles as oxygen-transporting blood substitutes are summarized. Using nanodisperse silver stabilized with poly(vinylpyrrolidone) as an example it was demonstrated, that transferring silver into nanodisperse state results in widening its bioactivity.

Hemoglobin dendrimers: functional protein clusters

A cluster of cross-linked hemoglobin tetramers was prepared by conjugating active esters to amino groups of a starburst dendrimer. Comparison of oxygen-binding properties of the protein cluster to those of nonclustered species reveals an increase in affinity and a decrease in cooperativity in the cluster. The altered oxygen-binding properties are assigned to protein-protein interactions.

Molecular engineering of proteins with predefined function. Part I: Design of a hemoglobin-based oxygen carrier

Molecular engineering refers to a collection of complex, computer-based methods used to study molecular structures and properties. These methods include ones for determining properties as well as for accessing prior knowledge about them. Applying these methods, one can generate, manipulate and calculate the energy involved with the three-dimensional conformation of a given molecule. These computational tools were utilized in this study, to design cross-linking reagents for cell-free Hb, for the purpose of O(2)-carriers development. Hb, when removed from the red blood cell, misses some of its functional characteristics required. Yet, these characteristics can be rebuilt into the Hb molecule by appropriate chemical modifications. These modifications have been devised to prevent dimer formation, increase the retention time in circulation, and decrease the high oxygen affinity of free Hb. The reagent reported in this study, namely, oxidized-NAD (o-NAD), has been designed to fulfill both criteria of retention time and oxygen affinity, in a single package. Feasibility of the cross-linking reaction of o-NAD with Hb was assessed by studying the docking process of o-NAD within the 2,3-DPG pocket of Hb. In this study, we provide an insight into how the overall factors involved with the potential energy calculations contribute to the hydrogen bonding network, formed within the complex. Conformational search analysis has shown a high proximity, of functional moieties on the Hb molecule, to reactive groups on the o-NAD molecule suggested. This is an important step in the design and later synthesis of O(2)-carrying materials to be used as blood substitutes.

Red blood cell substitutes

Soluble polymerized haemoglobin (polyhaemoglobin) is now in a phase III clinical trials. Patients have received up to 20 units (10 litres) in trauma surgery and other surgery. Polyhaemoglobin can be stored for more than 1 year. Haemoglobin solutions have no blood group antigen and can be used as a 'universal donor' oxygen carrier. They can also be sterilized. With a circulation half-life of 24 hours they are undergoing trials for peri-operative use. For conditions with potential for ischaemia-reperfusion injuries, a new polyhaemoglobin-superoxide dismutase-catalase, which can reduce oxygen radicals, is being developed. Recombinant human haemoglobin has been tested in clinical trials, and a new type of recombinant human haemoglobin that has low affinity for nitric oxide is being developed for clinical trials. To increase the circulation time, artificial red blood cells have been prepared with a bilayer lipid membrane (haemoglobin liposomes) or with a biodegradable polymer membrane-like polylactide (haemoglobin nanocapsules). Synthetic chemicals such as perfluorochemicals are also being developed and tested in clinical trials as red blood cell substitutes.

Modified hemoglobin blood substitutes: present status and future perspectives

Biotechnological techniques of cross-linking and microencapsulation of hemoglobin result in blood substitutes that can replace red blood cells. Unlike red blood cells they can be sterilized by pasteurization, ultrafiltration and chemical means. This removes microorganisms responsible for AIDS, hepatitis, etc. Since they are free of red blood cell blood group antigens, there is no need for cross-matching or typing. This saves time and facilities and allows on-the-spot transfusion such as the infusion of salt solution. Furthermore, they can be stored for a long time. Hemoglobin for modification can be extracted from human red blood cells. Other sources of hemoglobin include bovine hemoglobin and recombinant human hemoglobin. Clinical trials are ongoing testing the possible uses of cross-linked hemoglobin in cardiac, orthopedic, trauma and other types of surgery. It is also being tested for the replacement of lost blood in severe bleeding due to trauma or other causes. Cross-linked hemoglobins are first generation blood substitutes that only fulfil some of the functions of red blood cells. New generations of more complete red blood cell substitutes are being developed. These include cross-linked hemoglobin-catalase-superoxide dismutase and microencapsulated hemoglobin-enzyme systems.

Artificial cells with emphasis on cell encapsulation of genetically engineered cells

Artificial cells are prepared in the laboratory for medical and biotechnological applications. Encapsulated cells are being studied for the treatment of diabetes, liver failure, and other conditions. More recently, there have been extensive studies into the use of encapsulated genetically engineered cells for gene therapy. We recently found that daily orally administered artificial cells, each containing a genetically engineered microorganism, can lower the elevated urea level in uremic rats to normal levels. This may solve the final obstacle of the lack of an effective oral urea removal system for the simple and inexpensive oral treatment of uremia. This is important because 85% of the world's uremic population cannot afford standard dialysis. Other areas of artificial cell application include use in hemoperfusion. Red blood cell substitutes based on modified hemoglobin are already in Phase 3 clinical trials in patients. Artificial cells containing enzymes are being developed for clinical trial in hereditary enzyme deficiency disease and other diseases. They are also being investigated for drug delivery and for use in other applications in biotechnology, chemical engineering, and medicine.

Modified hemoglobin-based blood substitutes: crosslinked, recombinant and encapsulated hemoglobin

Native hemoglobin in the form of stroma-free hemoglobin cannot be used as blood substitute. Hemoglobin has to be modified either molecularly or encapsulated. First generation molecularly modified ultrapure hemoglobins are now in clinical trial--some in Phase III. There are a number of these. Polyhemoglobin is formed by crosslinking hemoglobin molecules intermolecularly and intramolecularly. A crosslinked single hemoglobin molecule is formed by crosslinking hemoglobin intramolecularly. Recombinant hemoglobin from E.coli is formed by fusion of the subunits of each hemoglobin molecule. Conjugated hemoglobin is formed by crosslinking each hemoglobin molecule to soluble polymers. A second generation system formed by crosslinking hemoglobin-superoxide dismutase-catalase is being developed. A third generation hemoglobin-based blood substitute is based on microencapsulated hemoglobin, artificial red blood cells, that more closely resemble a complete red blood cell.

Hemoglobin based oxygen carriers: how much methemoglobin is too much?

The oxidized form of hemoglobin, methemoglobin, is unable to deliver oxygen to tissues. Hemoglobin based oxygen carriers generally lack the natural oxidative-reductive machinery present within the red blood cell that converts methemoglobin to hemoglobin. This study examines tolerable levels of methemoglobin that can be present in solutions of polyethylene glycol (PEG) conjugated bovine hemoglobin without compromising its ability to deliver oxygen. Rodents were exchange-transfused to 30% of their estimated blood volume with solutions of six grams percent PEG-hemoglobin containing varying concentrations of PEG-methemoglobin. Tissue oxygenation was measured by an oxygen dependant phosphorescence quenching method. This study also looked at the level of methemoglobin formation following a top loaded infusion of low methemoglobin containing PEG-hemoglobin. Results of the oxygenation study showed that PEG-methemoglobin levels at or below 10% did not significantly alter the ability of solutions to deliver oxygen to intestines, liver, spleen and kidney. However, PEG-methemoglobin levels greater than 10% resulted in a significant decrease in PEG-hemoglobin's ability to oxygenate tissues. In addition, methemoglobin levels remain low (< 10%) for a substantial period of time following PEG-hemoglobin administration.

Conformation of the sebacyl beta1Lys82-beta2Lys82 crosslink in T-state human hemoglobin

The crystal structure of human T state hemoglobin crosslinked with bis(3,5-dibromo-salicyl) sebacate has been determined at 1.9 A resolution. The final crystallographic R factor is 0.168 with root-mean-square deviations (RMSD) from ideal bond distance of 0.018 A. The 10-carbon sebacyl residue found in the beta cleft covalently links the two betaLys82 residues. The sebacyl residue assumes a zigzag conformation with cis amide bonds formed by the NZ atoms of betaLys82's and the sebacyl carbonyl oxygens. The atoms of the crosslink have an occupancy factor of 1.0 with an average temperature factor for all atoms of 34 A2. An RMSD of 0.27 for all CA's of the tetramer is observed when the crosslinked deoxyhemoglobin is compared with deoxyhemoglobin refined by using a similar protocol, 2HHD [Fronticelli et al. J. Biol. Chem. 269: 23965-23969, 1994]. Thus, no significant perturbations in the tertiary or quaternary structure are introduced by the presence of the sebacyl residue. However, the sebacyl residue does displace seven water molecules in the beta cleft and the conformations of the beta1Lys82 and beta2Lys82 are altered because of the crosslinking. The carbonyl oxygen that is part of the amide bond formed with the NZ of beta2Lys82 forms a hydrogen bond with side chain of beta2Asn139 that is in turn hydrogen-bonded to the side chain of beta2Arg104. A comparison of the observed conformation with that modeled [Bucci et al. Biochemistry 35:3418-3425, 1996] shows significant differences. The differences in the structures can be rationalized in terms of compensating changes in the estimated free-energy balance, based on differences in exposed surface areas and the observed shift in the side-chain hydrogen-bonding pattern involving beta2Arg104, beta2Asn139, and the associated sebacyl carbonyl group.

The physiological and histopathological response of dogs to exchange transfusion with polyethylene glycol-modified bovine hemoglobin (PEG-Hb)

The performance of polyethylene glycol-modified bovine hemoglobin (PEG-Hb) was evaluated in dogs following the replacement of 30% or 50% of their blood volume with PEG-Hb or lactated Ringer's solution (LRS). Dogs fully instrumented with catheters and blood pressure probes were transfused by simultaneous bleeding from the jugular vein and infusion of PEG-Hb or LRS via the cephalic vein. Animals were monitored for abnormal behavior and clinical signs for fourteen days. No mortalities, overt toxicity, changes in body weight, food consumption or ophthalmology, or discernable trends in hematology, blood chemistry coagulation, urinalysis or hemodynamic parameters that could be attributed to PEG-Hb were noted. Blood gas analyses were steady and within physiological ranges. Dose-related histopathological findings of vacuolated histiocytes in the femoral bone marrow, splenic parenchyma, the medulla of the mesenteric and mandibular lymph nodes, and vacuolated sinusoidal cells in the liver and the renal tubular epithelial cells were believed to be related to the phagocytosis and degradation of PEG-Hb by the reticulo-endothelial system. The maintenance of high oxygen levels in the circulation for the two-week treatment period, as well as the insignificant physiological and histopathological findings indicate that PEG-Hb could be a successful blood substitute.

Bovine hemoglobin cross-linked through the beta chains: functional and structural aspects

2-Nor-2-formylpyridoxal (NFPLP) has been synthesized and coupled to bovine Hb according to the procedure developed by Benesch and Benesch. The reaction of bovine Hb with NFPLP leads to a cross-linkage between the beta subunits, which greatly stabilizes the low affinity T state of the molecule and simultaneously abolishes the tendency of the tetramer to dissociate into alpha beta dimers. The functional properties, examined from both the equilibrium and kinetic points of view, indicate that the chemical modification affects the O2 affinity, abolishes cooperativity, and induces a slight decrease of the Bohr effect. From modeling studies we are confronted with two different structural alternatives; the cross-link of beta chains may be formed between lysine 82 of beta2 and the N terminus of methionine 2 of beta1 or between the two lysine 82 residues of both beta2 chains. Digestion of modified beta globin chains and isolation of the cross-linked peptide have showed that NFPLP cross-links Met-beta2 and Lys-beta82. This allowed discussion in some detail of the molecular basis of the Bohr effect of the modified bovine hemoglobin. On the whole, NFPLP-modified bovine Hb could be considered as a first step toward the synthesis of a potential blood substitute.

A starch-hemoglobin resuscitative compound

A resuscitative compound in freeze-dried form has been synthesized between a modified starch and a tetremerically stabilized hemoglobin. In order to complex the hemoglobin, the starch has been prepared in mono-, di-, tri-, and tetra-aldehyde moieties. The hemoglobin was stabilized with low molecular weight diacids. Electrophoretic densitometric patterns indicate compound formation. The resulting polymers were characterized with respect to oxygen transport (biotonometry), Hill constant and P50. The in vivo evaluation indicates that these compounds are effective in exchange-transfusion experiments with rats to a level of about 85% replacement of whole blood. The final product is a cost-effective acellular resuscitative compound which can be stored in freeze-dried form at room temperature for extended periods of time. This artificial blood substitute can be reconstituted upon the addition of water.

Quantitative transformation of hemoglobin into stable tetramers

A method is described for the preparation of human or bovine hemoglobin with a covalent bridge, formed by bispyridoxal-tetraphosphate, between the beta chains. The yield is 95% of the total hemoglobin. The location of the two molecules of bispyridoxal-tetraphosphate in the tetramer has been established. The functional properties of the cross-linked hemoglobin, its stability, and particularly, the simplicity of the method for its preparation, make it a promising candidate for an acellular blood substitute.

Current status of injectable oxygen carriers

In this review the current status of what commonly are termed "blood substitutes" is discussed. The term blood substitute is a misnomer because the formulations under development at this time transport respiratory gases but do not perform the metabolic, regulatory, and protective functions of blood. Either hemoglobin or a perfluorochemical form the base to transport oxygen; the advantages and disadvantages of each base are discussed. The availability of a blood substitute in the U.S. will require approval by the Food and Drug Administration (FDA) and, by law, both its efficacy and safety must be demonstrated prior to approval. Showing efficacy of any blood substitute is complicated by the oxygen reserve and the compensatory mechanisms to acute blood loss in man. The challenge is to prove that the administration of these formulations offer clinical advantages compared with replacement of volume alone. Several efficacy models, the most attractive among them being perioperative hemodilution, should provide data that would bring these formulations into clinical practice. When hemoglobin is not within the favorable environment of the red cell, whether the hemoglobin is derived from expression vectors developed through recombinant biotechnology or from lysed human red cells, it acquires a left-shifted oxygen disassociation curve. Further, because the tetramer disassociates when injected intravenously and the resulting dimers are cleared rapidly from the circulation by the kidneys, intravascular dwell time is brief. Hemoglobins have been modified chemically and linked intramolecularly, intermolecularly, and to macromolecules to correct these problems. While these manipulations have normalized the p50 and extended the dwell time significantly, some toxicity problems remain unresolved. The binding of nitric oxide to hemoglobin preparations and the presumably resultant systemic and pulmonary hypertension observed in animals may be the most difficult to overcome, although the implications of these reactions in man is poorly understood. Perfluorochemicals (PFC) provide a fundamentally different and simpler approach to oxygen transport than hemoglobin formulations. Typically, the PFCs used are liquids composed of 8 to 10 carbon atoms that dissolve oxygen and obey Henry's law. Thus, the recipient's inspired oxygen and cardiac output assume importance. Because they are insoluble in water, PFCs are administered as emulsions, that is, as small droplets about 0.1 to 0.2 microns in diameter. In this respect, they are very similar to the lipid emulsions widely used for parenteral nutrition. Egg yolk phospholipid and poloxamers are most commonly used as emulsifiers. PFCs are not metabolized and are excreted unchanged by the lungs, following temporary storage by the monocyte-macrophage system (MMS).(ABSTRACT TRUNCATED AT 400 WORDS)

Cross-linking hemoglobin by design: lessons from using molecular clamps

The development of a red cell substitute by chemical modification of hemoglobin has been approached as a systematic, iterative process. Acyl phosphate methyl esters were designed as anionic electrophiles to permit selective acylation of amino groups in the cationic site of hemoglobin which binds polyanions. Kinetic studies with systematically substituted acyl phosphates and amines show that the reaction is controlled by a reversible addition step followed by an irreversible elimination step. Acyl phosphate methyl esters which are derivatives of rigid dicarboxylic acids introduce cross-links in human hemoglobin between amino groups in the beta subunits (epsilon-NH2-Lys-82, alpha-NH2-Val-1) and permit correlation of oxygen binding properties with cross-link structure. The data suggest that the cross-link maintains cooperativity while reducing overall oxygen affinity by lowering the affinity of the R form for oxygen rather than by perturbing the R,T equilibrium of native hemoglobin. Materials produced from deoxyhemoglobin with a cross-link between positions 1 and 82 of the two beta units have appropriate oxygen affinity for red cell substitutes. The use of a trifunctional cross-linker, trimesyl tris(methyl phosphate) selectively produces hemoglobin with the desired 1-82 connection in good yield. The reagent is readily prepared and the properties of this chemically modified hemoglobin are suitable for trial as a red cell substitute, closely resembling those of optimized materials produced by recombinant technology. Further work is producing new chemicals and providing structural information.

Prolongation of the intravascular retention of hemoglobin modified with a long-chain fatty acid derivative

To develop hemoglobin (Hb) derivatives with an increased circulatory half-life, Hb was chemically modified with long chain fatty acid analogs. One compound, sodium 1-hexadecyl 6-(2-iodoacetamido)hexyl phosphate, specifically modified the Cys-93 beta residues of human hemoglobin (HbA) as determined by sulfhydryl titration analysis. The resulting modified Hb derivative, FAHbA, was isolated and was shown to have a two-fold longer circulatory half-life than native HbA in a rat low-dose acute transfusion model.

Consequences of chemical modifications on the free radical reactions of human hemoglobin

Hemoglobin-based oxygen carriers (HBOCs) are candidates for use as blood substitutes and resuscitation fluids. We determined that HBOCs of specific types differ in their ability to generate or interact with free radicals. The differences do not correlate with oxygen affinity. Detailed comparisons with unmodified human hemoglobin, HbA0, were carried out with two cross-linked derivatives: HbA-FMDA, produced by the reaction of human oxyhemoglobin with fumaryl monodibromoaspirin, and HbA-DBBF, produced by the reaction of human deoxyhemoglobin with bis(3,5-dibromosalicyl) fumarate. Both derivatives had lower oxygen affinity than unmodified HbA0. As previously reported, exposure of oxyhemoglobin to H2O2 causes generation of free radicals capable of generating formaldehyde from dimethyl sulfoxide. Relative to the reaction catalyzed by 50 microM HbA (18.0 +/- 3.5 nmol/30 min/ml), the formaldehyde formation was roughly 70% for HbA-DBBF and 50% for HbA-FMDA under comparable conditions. More profound differences are exhibited at lower hemoglobin concentrations. Spectral changes of the HBOCs during the reaction differ qualitatively and occur at different rates. The HBOCs also differ in rates of hemoglobin-catalyzed NADPH oxidation and aniline hydroxylation, reactions mediated by reactive oxygen species. These results show that stereochemical differences brought about by chemical cross-linking alter the ability of HBOCs to generate radicals and to react with activated oxygen species. These studies also show that the ability of hemoglobin to produce activated species of oxygen can be enhanced or suppressed independently of oxygen affinity.

Purification and characterization of liposomes encapsulating hemoglobin as potential blood substitutes

In view of the desirability to increase the survival time of the liposome-based artificial red blood cells in vivo, the variables influencing optimum hemoglobin capture and preservation for the bovine hemoglobin-loaded liposomes (LEHb) are investigated. In order to predict the in vivo response, the necessary experiments for the in vitro system characterization have been carried out. The liposomes are prepared by the Reverse Phase Evaporation technique and then purified using a Sepharose 4B column. The purified LEHbs display a unimodal size distribution in the submicron range with a volume average diameter of 0.115 microns and a particle count of 1.25* 10(15) per ml of suspension. Analysis of the lipi/Hb content of the liposomes reveals that the variations in the ratio of Hb encapsulated to lipid entrapped (Hb/L)f as a function of the initial Hb concentration ([Hb]o) is insignificant compared to the net augmentation of (Hb/L)f as a function of the increasing initial lipid to Hb loading ([L]o). Meanwhile high [Hb]o s are necessary for the preservation of oxyhemoglobin.

Preparation and characterization of crosslinked and polymerized hemoglobin solutions

In 1982 we synthesized 2-Nor-2-formylpyridoxal 5'-phosphate (NFPLP) and subsequently showed that coupling of the beta chains of hemoglobin (Hb) by this organic phosphate compound according to Benesch et al. (1) lowers the oxygen affinity and prolongs the retention time in the circulation of rats and rabbits with a factor 3 by prevention of excretion via the kidneys. Optimal conditions for the purification of HbNFPLP either by ion-exchange chromatography or by heat treatment were established with recoveries of 70% and 85%, respectively. By extrapolation from the data in rats and rabbits a half life of about 8 hours can be expected in the circulation of humans. However, under some conditions a further prolongation is required. The aim of further modification of HbNFPLP was to achieve a retention time of about 24 hours. Polymerization with glutaraldehyde to polyHbNFPLP resulted in a mixture of polymers of different size. We determined the optimal degree of polymerization with respect to the effects on vascular retention time, oncotic activity, viscosity and oxygen affinity. Depending on the degree of polymerization we found in rats a 5- to 7- fold increase in vascular half-life compared to native Hb. The change in oxygen affinity was found to be independent of the polymer size (P50 = 18-22 mmHg). A limiting factor for polymerization is the increase in viscosity, which was dramatic when large polymers (greater than 300 kD) were present in the preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Recombinant haemoglobin in the development of red-blood-cell substitutes

Increasing concern over viral contamination of blood is spurring the development of a blood substitute which can effectively replace the oxygen-carrying capabilities of transfused erythrocytes. Solutions of chemically modified haemoglobin represent one option being evaluated for this role. More recently, recombinant-DNA techniques have enabled production of human haemoglobin in host expression systems, and progress is being made towards the creation of a genetically engineered molecule incorporating the properties required of a blood substitute.

Blood substitutes based on modified hemoglobin prepared by encapsulation or crosslinking: an overview

Modified hemoglobin consists of (1) encapsulated hemoglobin and (2) crosslinked hemoglobin (polyhemoglobin, intramolecularly cross-linked hemoglobin and conjugated hemoglobin). There have been new advances in all types of modified hemoglobins. Modified hemoglobins are effective in hemorrhagic shock. However, it is important to define hemorrhagic shock models and experimental designs. Important progress has been made in research on vasoactivities, organ perfusion, organ preservation, biodistribution, hematology, complement activation immunology and other areas. A preclinical screening test may bridge the gap between animal safety studies and injection into human. Potential new sources of hemoglobin included bovine hemoglobin, recombinant human hemoglobin and synthetic heme.

Hemoglobin tetramers stabilized by a single intramolecular cross-link

A specific intramolecular cross-link was introduced into bovine and human hemoglobin by reaction of the deoxyhemoglobin with the dialdehyde, bispyridoxal tetraphosphate (bisPL)P4, followed by reduction with NaBH4. The yield of cross-linked hemoglobin is 80% in both cases, using 1 mol of (bisPL)P4 per mol of Hb. The crosslink is confined to the beta chains, where it connects the N-terminal residue (valine and methionine, respectively) to a lysine on the other beta chain across the central cavity. The stereochemical requirements for the reaction were probed by using a rigid analogous cross-linking reagent, as well as with a mutant Hb, which has a shorter distance between the residues to which the cross-link is attached. Introduction of the cross-link into human and bovine Hb results in a five-fold and four-fold reduction in the oxygen affinity and a decrease in the Bohr Effect by 1/3 and 1/2, respectively. Oxygenation remains cooperative, albeit with a decreased Hill coefficient. The cross-linked hemoglobins are oxidized more rapidly to the ferric form, but their resistance to heat denaturation is increased. The stability of the link between the beta chains and their hemes is 10 times greater in both cross-linked hemoglobins that in their native counterparts. The possible application of this chemical modification for the preparation of hemoglobin-based blood substitutes is discussed.

Covalent fixation of polymer-linked benzene hexacarboxylate onto human haemoglobin

The reaction of human deoxy and oxyhaemoglobin with a macromolecular effector, monomethoxypolyoxyethylene-linked benzene hexacarboxylate, in the presence of a water soluble carbodiimide, produces under defined conditions, the same conjugates preferentially acylated at the two valines beta 1. The oxygen affinity of both these conjugates is decreased by approximately 5-fold compared with that of native Hb (at pH 7.2, in 0.05 M Tris buffer, 25 degrees C, P50: 20.1 and 20.7 Torr versus about 4 Torr for Hb). This difference appears to be due to an overstabilization of the T state probably together with a decrease of the oxygen affinity of the R state. Addition of IHP to the conjugate solutions does not influence the P50 but addition of IHP to the reaction mixtures before the coupling limits the substitution of Hb by the macromolecular effector, to 20% (instead of 100% in absence of IHP). The cooperativity curve is shifted to the right with an Nmax of 3 at about 90% oxygen saturation, which corresponds to a potential release of 48% of oxygen at pH 7.2, 25 degrees C, between 100 and 40 Torr, compared with 40% for blood. Such kinds of conjugates especially those obtained from oxyhaemoglobin which are easily prepared, could be of a great interest as non-diffusing oxygen carriers in transfusional and perfusional fluids.

Oxygen equilibrium properties of highly purified human adult hemoglobin cross-linked between 82 beta 1 and 82 beta 2 lysyl residues by bis(3,5-dibromosalicyl) fumarate

We investigated oxygen equilibrium properties of highly purified human adult hemoglobin cross-linked between lysine-82 beta 1 and lysine-82 beta 2 by a fumaryl group, which is prepared by reaction of the CO form with bis(3,5-dibromosalicyl) fumarate. The cross-linked hemoglobin preparation isolated by the previous purification method, namely, gel filtration in the presence of 1 M MgCl2 followed by ion-exchange chromatography, was found to be contaminated with about 20% of an electrophoretically silent impurity that shows remarkably high affinity for oxygen. This impurity was separated from the desired cross-linked hemoglobin by a newly developed purification method, which utilizes a difference between the authentic hemoglobin and the impurity in reactivity of the sulfhydryl groups of cysteine-93 beta toward N-ethylmaleimide under a deoxygenated condition. After this purification procedure, the oxygen equilibrium properties of purified cross-linked hemoglobin in the absence of organic phosphate became very similar to those of unmodified hemoglobin with respect to oxygen affinity, cooperativity, and the alkaline Bohr effect. The functional similarity between the cross-linked hemoglobin and unmodified hemoglobin allows us to utilize this cross-linking for preparing asymmetric hybrid hemoglobin tetramers, which are particularly useful as intermediately liganded models. Previous studies on this type of cross-linked hemoglobin should be subject to reexamination due to the considerable amount of the impurity.

Chemically modified and recombinant hemoglobin blood substitutes

Several intramolecularly cross-linked hemoglobins having properties useful as blood substitutes have been developed. At least one of these, HbXL99 alpha, is amenable to large-scale production. This hemoglobin, and perhaps other cross-linked derivatives as well, is sufficiently heat stable to achieve complete viral inactivation. This makes it possible to use human blood as a starting material. Preliminary studies on the use of HbXL99 alpha to perfuse the heart during coronary angioplasty appear promising (Rossen et al. 1987). For large-volume blood replacement, a derivative having a longer intravascular retention time would be desirable. The development of more selective cross-linking agents for the polymerization of hemoglobin would be useful for this purpose. The expression of human hemoglobin in E. coli (Nagai and Thogersen 1984, 1987; Hoffman et al. 1989) and in transgenic mice (Behringer et al. 1989; Ryan et al. 1990) has been achieved. The E. coli system should prove useful for the design of hemoglobin mutants having specifically tailored properties for use as blood substitutes. Adequate supplies of donated blood will likely be available for at least the next decade for the production of chemically modified hemoglobin derivatives. If the supply of human blood later becomes limiting, large-scale production of human hemoglobin should be feasible in transgenic pigs or cows. The economics of this process could be enhanced by producing other blood proteins of commercial value, e.g., human albumin and factor VIII, in the same animal.

Effect of aromatic isothiocyanates on the functional properties of human hemoglobin. Role of the stereochemistry of the charged group

The effect of chemical modification of hemoglobin with six derivatives of benzene isothiocyanate has been studied. The negatively charged reagents (isothiocyanates of benzoic and benzenesulfonic acids) markedly inhibit the interaction of hemoglobin with allosteric effectors such as H+, Cl- and organic phosphates; the affinity for heme ligands in the absence of effectors is reduced but cooperativity is maintained, making these modified hemoglobins suitable models for a possible 'blood substitute'. The only uncharged reagent tested (isothiocyanate of benzenesulfonamide) increases the oxygen affinity of hemoglobin and affects only slightly the interaction with heterotropic ligands; its potential use as an antisickling drug is under study.

Cooperative ligand binding of crosslinked hemoglobins at very high temperatures

Human hemoglobin was reacted with the bifunctional reagent bis(3,5-dibromosalicyl) fumarate to yield a derivative (Hb alpha alpha) crosslinked between the two alpha-chains; when the reaction was carried out with HbA already crosslinked between the two beta-chains by 2-nor-2-formylpyridoxal 5'-phosphate, a doubly crosslinked derivative (Hb alpha alpha beta beta) was obtained. We have observed that both modified hemoglobins are extremely stable up to temperatures of at least 85 degrees C. The carbon monoxide binding kinetics of both crosslinked hemoglobins, studied at temperatures between 15 and 85 degrees C, by means of stopped flow and flash photolysis techniques, show that the ligand-linked allosteric transition is maintained even at the highest temperatures. These results are also relevant to the mechanism of thermal unfolding of human hemoglobin, since they show that dissociation into alpha beta dimers (and exposure of the relatively hydrophobic dimer-dimer interfaces) is an obligatory step in the irreversible denaturation of deoxy and carbon monoxy hemoglobin.

Reexamination of the polymerization of pyridoxylated hemoglobin with glutaraldehyde

Pyridoxylated adult human hemoglobin (HbAo) was prepared using a one molar equivalent of pyridoxal 5-phosphate (PLP) per heme and reduced with either NaCNBH3 or NaBH4. A separate sample was pyridoxylated and passed through a mixed-bed ion exchange column without reduction. All three preparations had a P50 of 29 +/- 2 torr and a cooperativity of n = 2.4 +/- 0.1. These preparations, in both the oxy and deoxy forms, were then treated with 7 equivalents of glutaraldehyde per tetramer at pH 6.8 at 4 degrees C and at room temperature. The polymerization invariably reduced the P50 to 18 +/- 2 torr with Hill coefficients of less than 2. These solutions, with or without further reduction using NaCNBH3, all retained the PLP in differing amounts (2-3 moles/tetramer). Methemoglobin concentrations were increased during the polymerization reaction. The normal pyridoxylation procedure, using sodium borohydride reduction, resulted in a number of different molecular species. Polymerization with glutaraldehyde caused a further proliferation of molecular species that could not be separated by anion exchange chromatography or by isoelectric focusing. The extent of polymerization, estimated by gel exclusion chromatography and SDS polyacrylamide gel electrophoresis, was from 40 to 50%. Analysis of the reverse phase chromatograms, which separate the heme and the alpha- and beta-chains, showed extensive polymerization and distribution of the radioactively labeled PLP on the protein for all preparations. All of the polymerized and pyridoxylated samples were unstable, and showed different chromatographic patterns after storage at 4 degrees C for 1 month. Attempts to stabilize these preparations by further reduction with NaCNBH3 gave products with a lower P50 and lower cooperativity. When the reactions were conducted with a purified HbAo, heterogeneity was somewhat decreased compared to the normally used stroma-free hemoglobin, but a large number of molecular species were still formed.

A versatile, highly reactive, cross-linking reagent: 2,2'-sulfonylbis[3-methoxy-(E,E)-2-propenenitrile]

Adequate aqueous stability and cross-linking ability of the novel title reagent, recently discovered in this laboratory, have been demonstrated by comparison of its rate of hydrolysis with the rate of reaction with an amine nucleophile and by cross-linking deoxy- and oxyhemoglobins, as an example.