Structural studies of haptoglobins. I. Hydrogen ion equilibria and optical rotatory dispersion of Hp 1-1 and Hp 2-2

Preparation and Properties of Nickel Hemoglobin

Hemoglobin A reconstituted with nickel protoporphyrin IX (NiHbA) has been prepared and characterized. Kinetics of its reaction with p-mercuribenzoate and with haptoglobin, absorption and circular dichroism spectra, and x-ray crystallographic properties have been investigated as probes of its structural conformation. The results suggest that NiHbA exists in a structure that is similar to the deoxy, or T-state of HbA. It is proposed that NiHbA and its derivatives may serve as a useful model for future studies of hemoglobin allosteric changes.

Inhibition of endothelium-dependent relaxation by hemoglobin in rabbit aortic strips: comparison between acellular hemoglobin derivatives and cellular hemoglobins

Hemoglobin (Hb)-based artificial oxygen carriers are supposed to induce vasoconstriction through the inactivation of endothelium-derived relaxing factor (EDRF). We examined the vasoconstrictive activity of acellular Hb and cellular Hb solutions in rabbit aortic strips. Unmodified Hb, pyridoxalated Hb, bovine unmodified Hb, haptoglobin-Hb complex (Hp-Hb), and polyoxyethylene glycol-conjugated Hb (PEG-Hb) were used as acellular Hbs having different molecular masses. Cellular Hbs included liposome-encapsulated Hb and red blood cells (RBC). In the first experiment, Hb (10 ng/ml to 1 mg/ml) was cumulatively added to the tissues in which steady-state relaxation was evoked by acetylcholine (ACh) after precontraction induced by phenylephrine. Although all Hb solutions induced a dose-dependent reversal of ACh-induced relaxation, the most potent vasoconstrictive effect was noted with acellular Hbs, and their contractile activities were almost the same independent of molecular mass. On the other hand, liposome-Hb and RBC showed reduced potencies in this order. These results indicate the importance of cellularity as the major factor determining Hb-related EDRF inactivation. In another experiment, the tissues were exposed to Hb at 0.01, 0.1, or 1 mg/ml for 30 min and ACh-induced relaxation was recorded after the complete removal of Hb in an organ bath chamber. Exposure to unmodified Hb at > 0.1-mg/ml concentrations significantly reduced the ACh-induced relaxation, whereas the relaxation was not affected by PEG-Hb, Hp-Hb, liposome-Hb, or RBC. These results suggest that unmodified Hb might be persistently associated with tissues and thereby inhibit ACh-induced relaxation. From these findings, we propose two attributes of Hb-related inhibition of endothelium-dependent relaxation: Acellular Hbs inhibit EDRF more efficiently in the luminal space than cellular Hbs, and unmodified Hb can also inhibit it adluminally and/or adventitially.

Hemoglobin binding with haptoglobin: delineation of the haptoglobin binding site on the alpha-chain of human hemoglobin

Previous studies from this laboratory employing a comprehensive synthetic overlapping peptide strategy showed that the alpha-chain of human hemoglobin (Hb) contains a single haptoglobin (HP) binding region residing within residues alpha 121-135. The present study describes a precise delineation of this Hp-binding site on the alpha-chain. Two overlapping peptides (alpha 111-125 and alpha 121-135) spanning this region and a panel of five peptides decreasing at the C-terminal from residue 135 by decrements of two residues (alpha 119-135, alpha 119-133, alpha 119-131, alpha 119-129, and alpha 119-127) were synthesized, purified, and characterized. Quantitative radiometric titration of 125I-labeled human HP (type 2-1) with adsorbents of each of these synthetic peptides showed that the peptide alpha 119-127 retained a Hp-binding activity equivalent to that of peptide alpha 121-135. This finding indicated that Lys-127 marked the C-terminal boundary of the binding site. Another panel of eight peptides was then synthesized, which had their C-terminus fixed at Lys-127 and increased at the N-terminus by one-residue increments from residue 122 up to residue 115 (alpha 122-127, alpha 121-127, alpha 120-127, alpha 119-127, alpha 118-127, alpha 117-127, alpha 116-127, and alpha 115-127). The binding of 125I-Hp to adsorbents of these peptides demonstrated that the N-terminal boundary of the site did not extend beyond Valine 121. It is, therefore, concluded that the Hp-binding site on the alpha-chain of human Hb comprises residues alpha 121-127.

Functional and subunit assembly properties of hemoglobin Alberta (alpha 2 beta 2(101) Glu----Gly)

Hemoglobin Alberta has an amino acid substitution at position 101 (Glu----Gly), a residue involved in the alpha 1 beta 2 contact region of both the deoxy and oxy conformers of normal adult hemoglobin. Oxygen equilibrium measurements of stripped hemoglobin Alberta at 20 degrees C in the absence of phosphate revealed a high affinity (P50 = 0.75 mm Hg at pH 7), co-operative hemoglobin variant (n = 2.3 at pH 7) with a normal Bohr effect (- delta log P50/delta pH(7-8) = 0.65). The addition of inositol hexaphosphate resulted in a decrease in oxygen affinity (P50 = 8.2 mm Hg at pH 7), a slight increase in the value of n and an enhanced Bohr effect. Rapid mixing experiments reflected the equilibrium results. A rapid rate of carbon monoxide binding (l' = 7.0 X 10(5) M-1 S-1) and a slow rate of overall oxygen dissociation (k = 15 s-1) was seen at pH7 and 20 degrees C in the absence of phosphate. Under these experimental conditions the tetramer stability of liganded and unliganded hemoglobin Alberta was investigated by spectrophotometric kinetic techniques. The 4K4 value (the liganded tetramer-dimer equilibrium dissociation constant) for hemoglobin Alberta was found to be 0.83 X 10(-6) M compared to a 4K4 value for hemoglobin A of 2.3 X 10(-6) M, indicating that the Alberta tetramer was less dissociated into dimers than the tetramer of hemoglobin A. The values of 0K4 (the unliganded tetramer-dimer equilibrium dissociation constant) for hemoglobin Alberta and hemoglobin A were also measured and found to be 2.5 X 10(-8) M and 1.5 X 10(-10) M, respectively, demonstrating a greatly destabilized deoxyhemoglobin tetramer for hemoglobin Alberta compared to deoxyhemoglobin A. The functional and subunit dissociation properties of hemoglobin Alberta appear to be directly related to the dual role of the beta 101 residue in stabilizing the tetrameric form of the liganded structure, while concurrently destabilizing the unliganded tetramer molecule.

Uptake of iron from hemoglobin and the haptoglobin-hemoglobin complex by hemolytic bacteria

The abilities of Staphylococcus aureus and Streptococcus pyogenes to remove iron from mouse 59Fe hemoglobin that was either in free form or complexed with human haptoglobin, were evaluated. 59Fe hemoglobin from the amphibian Taricha granulosa was also used in free form or complexed with the amphibian's hemoglobin-binding proteins. Contrary to what was reported from a study using pathogenic Escherichia coli, haptoglobin failed to exhibit a bacteriostatic influence when complexed with hemoglobin. In our study, more 59Fe was removed by the bacteria from the haptoglobin-hemoglobin complex than from free mouse hemoglobin. The hemoglobin and hemoglobin-plasma protein complexes of Taricha were stripped of 59Fe at similar rates and extents by both bacterial species.

Structure of haptoglobin and the haptoglobin-hemoglobin complex by electron microscopy

The human serum protein, haptoglobin, forms a stable, irreversible complex with hemoglobin. Haptoglobin is composed of two H chains, which are connected via two smaller L chains to give a protein of 85,000 Mr. In the complex, each H chain binds an alpha beta dimer of hemoglobin for a total molecular weight of 150,000. The scanning transmission electron microscope has been used to derive new information about the shape and structure of haptoglobin and hemoglobin, and about their relative orientation in the complex. The micrographs of negatively stained images show that haptoglobin has the shape of a barbell with two spherical head groups, which are the H chains. These are connected by a thin filament with a central knob, which corresponds to the L chains. The overall length of the molecule is about 124(+/- 8) A and the interhead distance is 87 (+/- 7) A. In the haptoglobin-hemoglobin complex, the head groups are ellipsoidal and under optimal staining conditions bilobal . Thus, the alpha beta dimers are binding to the H chains, but off the long axis of the barbell by 127 degrees in a trans configuration. This angle considerably restricts the region on the surface of the H chain structure that can contain the hemoglobin binding site. The interhead group distance for complex is 116.5(+/- 6.3) A or 30 A greater than for haptoglobin. The N terminus of the beta chain was located on the trans off-axis configured barbell structure of complex by using a hemoglobin that was crosslinked between the alpha beta dimers in the region of the beta N terminus. The distances and angles that are measured on the micrographs for the native and crosslinked complex molecules permit the directions of two of the alpha beta dimer ellipsoid axes to be assigned. Taken together, these data provide an approximate relative orientation for the binding of the alpha beta dimer to the H chain of haptoglobin.

Globin-chain affinity chromatography on Sepharose-haptoglobin: a new method of study of hemoglobin synthesis in reticulocytes, in bone marrow and in colonies of erythroid precursors

In the present work we have developed an affinity chromatography system, using haptoglobin bound covalently to Sepharose 4B, to purify hemoglobin from soluble non-heme proteins. Sepharose-haptoglobin specifically binds hemoglobin. It exhibits the same characteristics in its interactions with hemoglobin and alpha or beta hemoglobin chains as does haptoglobin in solution. Globin chains can be eluted from the Sepharose-haptoglobin after removal of the heme. This method has allowed accurate measurements of globin-chain synthesis in blood and bone marrow samples and in culture of early erythroid precursors.

Elevated Hb F associated with beta-thalassaemia trait: haemoglobin synthesis in reticulocytes and in blood BFU-E

The red cells of a patient heterozygous for beta-thalassaemia contained 19% fetal Hb. Study of his family suggested that the proband had inherited the Swiss type of hereditary persistence of fetal Hb (HPFH) from his mother who is not thalassaemic and possessed 1.37% of Hb and 11% F-cells. Studies of globin synthesis showed a similar imbalance in the heterocellular HPFH-beta-thalassaemia compound heterozygotes and in the heterozygous beta-thalassaemic members of the family. Age stratification of the red cells showed a slight enrichment in Hb F and a decreased Hb A2 level in the older cell populations. Hb F production in the BFU-E colonies of the proband was higher than that found in vivo and in other beta-thalassaemic heterozygotes in culture. Study of single erythroid burst colonies showed a marked heterogeneity in Hb F synthesis from one colony to another, while the pool of free alpha-chains remained of similar magnitude. It is suggested that in the proband, the HPFH gene, which is in trans with respect to the beta-thal-gene, increases the size of the F-cell population and its activity is carried on at the expense of the normal beta A gene.

Haemoglobin binding with haptoglobin. Unequivocal demonstration that the beta-chains of human haemoglobin bind to haptoglobin

Haptoglobin binding to haemoglobin and its isolated alpha- and beta-chains was studied by use of a highly sensitive solid-phase radiometric assay. As expected, adsorbents of haemoglobin bound 125I-labelled haptoglobin more efficiently than did adsorbents of the alpha-chain. However, unexpectedly, adsorbents of the beta-chain were found to be essentially identical with those of the alpha-chain in their ability to bind haptoglobin. These results demonstrate, unequivocally, the ability of beta-chains to bind to haptoglobin, and indicate that this assay is particularly convenient and useful for studying haptoglobin interactions with haemoglobin and its alpha- and beta-chains.

Is immunochemical determination of haptoglobin phenotype dependent?

Immunochemical methods have been used to determine the concentration of haptoglobins. The dependence on the phenotype was tested with highly purified Hp 2-1, Hp 2-2 and Hp 1-1, by immunonephelometry and radial immunodiffusion (RID). Measurements with three different instruments: automated immunonephelometer (AIP, Technicon), laser nephelometer (LN, Behring) and immunochemistry system (ICS, Beckman) were performed. For each type of apparatus antisera against a pool of haptoglobins were provided by the respective manufacturers. Some experiments were done with an antiserum to the haptoglobin heavy chain prepared in the laboratory. This study shows that haptoglobin determination depends neither on the physical geometry of the instruments or on the type of antiserum used in this work. In contrast, the data display a dependence on haptoglobin phenotype. When Hp 2-1, the most common phenotype, is taken as a standard, thd values obtained for Hp 2-2 are in good agreement with those obtained for Hp 2-1. However, the values obtained for Hp 1-1 are overestimated unless they are corrected by an experimental factor which has been determined in this study.

Investigation of two deoxygenated haemoglobin-haptoglobin complexes by Mössbauer spectroscopy

Haemoglobin Haptoglobin complexes formed when [Hp+]/[Hb]= 1/1 and [Hp]/[Hb] =2/1 were investigated by 57Fe Mössbauer spectroscopy. Both samples gave a spectrum consisting of a single quadrupole doublet. The temperature dependence of the quadrupole splitting was also identical for both samples. This proves that in both samples the nearest neighbour environment of the iron atom must be the same. A comparison with earlier investigations on myoglobin and haemoglobin indicates that the electronic structure of iron in the HbHp-complexes is similar to that in myoglobin.

Studies on hemoglobin tryptophanyl contact residues in the haptoglobin-hemoglobin complex

Hemoglobin and apohemoglobin bind heptoglobin in the same molar ratio. Structural studies on haptoglobin-hemoglobin complex do not suggest any important structural changes in either protein upon binding. However, when apohemoglobin is bound to haptoglobin, a marked reduction in secondary structure, attributed to unfolding of globin chains, has been observed. Here we describe some properties of the haptoglobin-apohemoglobin (Hp-apoHb) complex, prepared by isoelectric focusing in the presence of an excess of haptoglobin. This complex does not exhibit the irreversibility of complexes obtained with hemoglobin in identical experimental conditions. The 'freezing' of the conformation of apohemoglobin upon binding to haptoglobin has been studied by fluorescence quenching experiments carried out in the presence of 8 M acrylamide. Changes in conformation of haptoglobin upon binding to apohemoglobin have been detected by titration of the exposed tryptophans using N-bromosuccinimide. Comparison of the additivity of exposed tryptophans in the complexes reveal that two tryptophans become inaccessible in the complex formation of haptoglobin with hemoglobin but not with apohemoglobin. These tryptophans, probably located on the alpha1beta2 contact interface of hemoglobin, have been tentatively identified as Trp-C3(37)beta.

Haptoglobin--haemoglobin interaction. Heterogeneity of the haptoglobin 1-1 molecule in its binding affinity for horse R/E-hemoglobin

The formation of two different complexes when haptoglobin (Hp) and haemoglobin (Hb) are mixed in a 1:1 molar ratio is demonstrated by isoelectrofocusing. In these two complexes, the affinity of Hp for Hb is shown to be different, since Hb can be displaced only from one of the complexes, by a further addition of Hp. This is confirmed by a quantitative study of the reaction stoichiometry, when [Hp]/[Hb] = 1 and [Hp]/[Hb] greater than 1, which allows an evaluation of the amount of each complex formed. All these data cannot be explained other than by the existence of two forms of Hp molecule and a reaction scheme which fits these experiments is proposed.

Thermal and solvent perturbation as probes of conformational heterogeneity of haptoglobin 1-1

The shifts which occur in the ultraviolet spectra of haptoglobin 1-1 (Hp) and a conformational isomer with lower affinity for hemoglobin (Hp), in response to temperature and solvent changes, have been measured by difference spectra. The thermal difference spectra provide evidence for a different participation of tryptophyl residues in the conformational stability of Hp and Hp. Solvent perturbations have allowed an estimation to be made of the number of surface chromophores in the native Hp and Hp. These results are compatible with the existence of two Hp conformational isomers with different free energy, separated by a high-potential barrier.

Kinetics of the reaction between oxygen and haemoglobin bound to haptoglobin

The kinetics of the reaction between O(2) and haemoglobin bound to haptoglobin 1-1 were investigated by the stopped-flow and temperature-jump techniques. The reaction, which follows second-order kinetics in the lower concentration range, becomes independent of O(2) concentration above about 150mum-O(2).