Effect of modification on physicochemical and biological properties of haptoglobin. VI. Reaction with azlactone of p-nitrobenzoyl-valine.

The azlactone of p-nitrobenzoyl-valine (Nbz-Val) has been used for modification of xi-amino groups of lysine in haptoglobin type 1-1, in hemoglobin, and in the haptoglobin-hemoglobin complex. By the use of this reagent 95% of amino groups in haptoglobin and 90% in hemoglobin have been blocked without any changes in peroxidase activity of the formed complexes: Nbz-Val.haptoglobin with hemoglobin, Nbz-Val. hemoglobin with haptoglobin, and Nbz-Val.(haptoglonin-hemoglobin). After reduction and reoxidation, Nbz-Val.haptoglobin was found to retain 90% of peroxidase activity when complexed with hemoglobin. Beta chains separated either from haptoglobin or Nbz-Val.haptoglobin showed 15% of peroxidase activity in the complex with hemoglobin, alpha chains of the same origin were completely inactive. Whereas recombination of haptoglobin from alpha and beta chains resulted in 42% hemoglobin-binding capacity, renaturation of Nbz-Val.haptoglobin from separated subunits was found to proceed with almost 100% yield. In immunodiffusion with rabbit anti-haptoglobin or anti-Nbz-Val.haptoglobin sera, preparations of haptoglobin and Nbz-Val.haptoglobin after reduction and reoxidation or after recombination from separated subunits gave similar precipitation arcs showing the reaction of immunological identity.     

Rate of nitric oxide scavenging by hemoglobin bound to haptoglobin

Cell-free hemoglobin, released from the red cell, may play a major role in regulating the bioavailability of nitric oxide. The abundant serum protein haptoglobin, rapidly binds to free hemoglobin forming a stable complex accelerating its clearance. The haptoglobin gene is polymorphic with two classes of alleles denoted 1 and 2. We have previously demonstrated that the haptoglobin 1 protein–hemoglobin complex is cleared twice as fast as the haptoglobin 2 protein–hemoglobin complex. In this report, we explored whether haptoglobin binding to hemoglobin reduces the rate of nitric oxide scavenging using time-resolved absorption spectroscopy. We found that both the haptoglobin 1 and haptoglobin 2 protein complexes react with nitric oxide at the same rate as unbound cell-free hemoglobin. To confirm these results we developed a novel assay where free hemoglobin and hemoglobin bound to haptoglobin competed in the reaction with NO. The relative rate of the NO reaction was then determined by examining the amount of reacted species using analytical ultracentrifugation. Since complexation of hemoglobin with haptoglobin does not reduce NO scavenging, we propose that the haptoglobin genotype may influence nitric oxide bioavailability by determining the clearance rate of the haptoglobin–hemoglobin complex. We provide computer simulations showing that a twofold difference in the rate of uptake of the haptoglobin–hemoglobin complex by macrophages significantly affects nitric oxide bioavailability thereby providing a plausible explanation for why there is more vasospasm after subarachnoid hemorrhage in individuals and transgenic mice homozygous for the Hp 2 allele.     

Inhibition of prostaglandin synthase activity of sheep seminal vesicular gland by human serum haptoglobin

Serum haptoglobin was added to the reaction mixture of prostaglandin synthase (EC 1.14.99.1) and its inhibitory effect was studied [1-14C]Arachidonic acid was used as substrate and the enzyme activity was estimated by monitoring the radioactivity of the products after thin layer chromatography. With or without addition of hemoglobin to the reaction mixture, both the purified haptoglobin 1-1 and 2-2 showed inhibitory activity. In the presence of 5 microM hematin, however, inhibitory activity haptoglobin was not observed. Inhibition of prostaglandin synthesis in the system depended on the molar ratio of haptoglobin to hemoglobin in the reaction mixture. These results demonstrate that haptoglobin inhibits prostaglandin synthase by restricting available heme group for the enzyme activity through complexing with hemoglobin. However, haptoglobin did not inhibit completely the stimulatory effect of free hemoglobin. Relevant significant of this effect was discussed.     

Cross-linking of hemoglobin, haptoglobin, and hemoglobin-haptoglobin complex with bifunctional imidoesters.

Dimethyl adipimidate was used to cross-link the polypeptides within hemoglobin, haptoglobin, and hemoglobin-haptoglobin complex. Cross-linked hemoglobin retained considerable ability to bind haptoglobin, although the amounts bound were reduced and the haptoglobin reaction could be used to fractionate the modified hemoglobin. With cross-links limited to intramolecular sites, hemoglobin showed four bands on polyacrylamide gel electrophoresis in sodium dodecyl sulfate, identified, with reference to the subunit polypeptides, as monomer, dimer, trimer, and tetramer. The dimer region consisted of at least two separable species. When hemoglobin-haptoglobin complex was cross-linked, a band of hemoglobin dimer was present, which demonstrates that at least two hemoglobin subunits have a close spatial relation when bound to haptoglobin. Some comparisons with adipimidate-reacted hemoglobin were made using malonimidate and suberimidate and some marked differences were noted.     

Binding of acellular, native and cross-linked human hemoglobins to haptoglobin: enhanced distribution and clearance in the rat

It is well established that hemoglobin resulting from red cell lysis binds to haptoglobin in plasma to form a complex. The increased molecular size precludes its filtration by the kidneys, redirecting it toward hepatocellular entry. Chemically cross-linked hemoglobins are designed to be resistant to renal excretion, even in the absence of haptoglobin. The manner in which binding to haptoglobin influences the pharmacokinetics of acellular cross-linked and native hemoglobins was investigated after intravenous injection of radiolabeled native human hemoglobin and trimesyl-(Lys82)beta-(Lys82)beta cross-linked human hemoglobin, at trace doses, into rats. Under these conditions, there is sufficient plasma haptoglobin for binding with hemoglobin. In vitro binding assayed by size-exclusion chromatography for bound and free hemoglobin revealed that, at <8 muM hemoglobin, native human hemoglobin was completely bound to rat haptoglobin, whereas only approximately 30% of trimesyl-(Lys82)beta-(Lys82)beta cross-linked hemoglobin was bound. Plasma disappearance of low doses (0.31 mumol/kg) of native and cross-linked hemoglobins was monoexponential (half-life = 23 and 33 min, respectively). The volume of distribution (40 vs. 19 ml/kg) and plasma clearance (1.22 vs. 0.4 ml.min(-1).kg(-1)) were higher for native than for cross-linked hemoglobin. Native and cross-linked human hemoglobins were found primarily in the liver, and not in the kidney, heart, lung, or spleen, mostly as degradation products. These pharmacokinetic findings suggest that the binding of hemoglobin to haptoglobin enhances its hepatocellular entry, clearance, and distribution.     

Reaction of haptoglobin with hemoglobin covalently cross-linked between the alpha beta dimers.

Hemoglobin tetramers which cannot split into alphabeta dimers, because they are covalently cross-linked between the beta chains across the polyphosphate binding site, form complexes with haptoglobin. The reaction is biphasic as measured by fluorescence quenching and peroxidase activity. A complex in which one of the alpha beta dimers of the cross-linked hemoglobin is bound to one of the sites in the divalent haptoglobin molecule, is formed reversibly during the initial fast phase. In the subsequent slower step, this product then either polymerizes, adds another cross-linked hemoglobin molecule or, in the presence of excess haptoglobin, combines with a second haptoglobin molecule. This latter complex, in which two haptoglobin molecules are bridged by a cross-linked hemoglobin tetramer, can still combine with normal alpha beta dimers at the vacant haptoglobin combining sites. In spite of the very low oxygen affinity of the cross-linked hemoglobin, combination with haptoglobin shifts if oxygen affinity to the very high value of the normal hemoglobin-haptoglobin complex.     

Hemoglobin binding to deglycosylated haptoglobin

The carbohydrate portion of polymeric haptoglobin was gradually removed by exoglycosidases in order to investigate its role in complex formation between haptoglobin and hemoglobin. Total removal of sialic acid diminished the haptoglobin-hemoglobin complex formation 15%. Removal of about 25% of the galactose residues from asialohaptoglobin, i.e., about 40% of the total weight of the carbohydrate moiety, totally inhibited the ability of haptoglobin to form complex with hemoglobin and react with haptoglobin-specific antibodies. Liberation of further galactose residues resulted in slow precipitation of the protein. Removal of a similar part of the carbohydrate moiety from haptoglobin-hemoglobin complex did not liberate hemoglobin from it, and the complex reacted with haptoglobin antibodies. The combined data indicate that the carbohydrate portion is essential for the functionally active form of polymeric haptoglobin to complex with hemoglobin, but it hardly has any direct role in the binding event, and other factors are responsible for the stability of the complex.     

Hemoglobin-haptoglobin receptor in rat liver plasma membrane

The presence of a receptor specific for the hemoglobin . haptoglobin complex is demonstrated in rat liver plasma membranes. Hemoglobin . haptoglobin complex, administered intravenously to rats, was cleared from the circulation at a constant rate with exclusive incorporation of the molecule into hepatocytes. This incorporation was unaffected by the simultaneous injection of asialoglycoprotein or heme . hemopexin complex. In vitro experiments with isolated liver plasma membranes indicated the absence of competitive binding of these molecules to the membrane and suggested that this receptor might recognize an altered conformation of the haptoglobin moiety of the complex resulting from the binding with hemoglobin. These observations suggest that the mechanism of recognition and binding of hemoglobin . haptoglobin complex by the receptor is different from that of the asialoglycoprotein receptor or heme . hemopexin receptor.     

Haptoglobin-hemoglobin binding involves the heavy chain of haptoglobin and the α and β chains of hemoglobin

Previous studies from this laboratory have demonstrated unambiguously that the isolated β chain of human adult hemoglobin binds human haptoglobin (Hp). In the present work, the ability of the isolated subunits of haptoglobin and hemoglobin to form complexes is investigated. In quantitative radiometric adsorbent titrations, the H chain of haptoglobin bound to hemoglobin whereas the L chain had no binding activity. Also, the H chain of haptoglobin bound to the isolated α and β subunits of hemoglobin, but its binding to the α or β chain was less than the binding it exhibits to hemoglobin. The isolated L chain was able to reassociate with the H chain to form a complex that binds to hemoglobin or its subunits. Although the L chains had no binding activity, its association with the H chain increased the binding of the latter to Hb or its isolated α and β subunits suggesting a more indirect role for the L chain in haptoglobin-hemoglobin interactions.     

Degradation of Host Heme Proteins by Lysine- and Arginine-Specific Cysteine Proteinases (Gingipains) of Porphyromonas gingivalis

Porphyromonas gingivalis can use hemoglobin bound to haptoglobin and heme complexed to hemopexin as heme sources; however, the mechanism by which hemin is released from these proteins has not been defined. In the present study, using a variety of analytical methods, we demonstrate that lysine-specific cysteine proteinase of P. gingivalis (gingipain K, Kgp) can efficiently cleave hemoglobin, hemopexin, haptoglobin, and transferrin. Degradation of hemopexin and transferrin in human serum by Kgp was also detected; however, we did not observe extensive degradation of hemoglobin in serum by Kgp. Likewise the beta-chain of haptoglobin was partially protected from degradation by Kgp in a haptoglobin-hemoglobin complex. Arginine-specific gingipains (gingipains R) were also found to degrade hemopexin and transferrin in serum; however, this was observed only at relatively high concentrations of these enzymes. Growth of P. gingivalis strain A7436 in a minimal media with normal human serum as a source of heme correlated not only with the ability of the organism to degrade hemoglobin, haptoglobin, hemopexin, and transferrin but also with an increase in gingipain K and gingipain R activity. The ability of gingipain K to cleave hemoglobin, haptoglobin, and hemopexin may provide P. gingivalis with a usable source of heme for growth and may contribute to the proliferation of P. gingivalis within periodontal pockets in which erythrocytes are abundant.     

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.     

Regulation of CD163 associated casein kinase II activity is haptoglobin genotype dependent

Free hemoglobin is now recognized as a major mediator of a variety of vascular diseases. The abundant serum protein haptoglobin irreversibly binds to hemoglobin and promotes the uptake of hemoglobin via the macrophage CD163 receptor. The haptoglobin gene is polymorphic in man with two common alleles denoted 1 and 2. The haptoglobin genotype specifies the nature of the response of the macrophage to free hemoglobin. Hp 1-Hb complexes stimulate an anti-inflammatory macrophage phenotype while Hp 2-Hb complexes do not. We have previously demonstrated that Hp 1-Hb induced anti-inflammatory cytokine production is critically dependent on casein kinase II. In this study we set out to determine whether the amount or the activity of casein kinase II associated with CD163 was altered by the binding of Hp 1-1-Hb to CD163. Our results indicate that casein kinase II activity is increased by the binding of Hp 1-1-Hb to CD163.     

Hemoglobin binding by isolated polymeric proteins from human haptoglobin types 2-1 and 2-2. Some suggested polymer subunit compositions.

1. Some of the individual members of the polymeric series of proteins from human haptoglobin types 2-1 and 2-2 were isolated by gel electrophoresis. By reacting this purified material with less than an equivalent amount of hemoglobin and analyzing the result by electrophoresis, the number of haptoglobin-hemoglobin complexes could be clearly counted. For the haptoglobin 2-1 series, the number of complexes formed was n+1, where n is the serial order, in decreasing electrophoretic mobility, of the haptoglobin polymeric form used. For the haptoglobin 2-2 series, the number of complexes was n+2. 2. For the first three members of haptoglobin 2-1 series, the haptoglobin-hemoglobin composition of the complexes was estimated from scans of the unstained gels. The data indicated that this series consists of 2,3,4... alpha beta haptoglobin subunits, each of which can combine with an alpha beta subunit of hemoglobin.     

Two hemoglobin-binding proteins identified in the plasma of the amphibian Taricha granulosa

The plasma of the amphibian newt Taricha granulosa has been shown to be devoid of haptoglobin. Upon hemolysis, Taricha albumin and another protein associate with hemoglobin. The acute-phase response to inflammation observed in birds and mammals appears to be absent in Taricha. Taricha hemoglobin failed to bind to human haptoglobin. Taricha hemoglobin not only failed to dissociate into alpha beta dimers as did human Hb, but formed alpha beta octamers.     

A unique loop extension in the serine protease domain of haptoglobin is essential for CD163 recognition of the haptoglobin-hemoglobin complex

Haptoglobin and haptoglobin-related protein are homologous hemoglobin-binding proteins consisting of a complement control repeat (alpha-chain) and a serine protease domain (beta-chain). Haptoglobin-hemoglobin complex formation promotes high affinity binding of hemoglobin to the macrophage scavenger receptor CD163 leading to endocytosis and degradation of the haptoglobin-hemoglobin complex. In contrast, complex formation between haptoglobin-related protein and hemoglobin does not promote high affinity interaction with CD163. To define structural components of haptoglobin important for CD163 recognition, we exploited this functional difference to design and analyze recombinant haptoglobin/haptoglobin-related protein chimeras complexed to hemoglobin. These data revealed that only the beta-chain of haptoglobin is involved in receptor recognition. Substitution of 4 closely spaced amino acid residues of the haptoglobin beta-chain (valine 259, glutamate 261, lysine 262, and threonine 264) abrogated the high affinity receptor binding. The 4 residues are encompassed by a part of the primary structure not present in other serine protease domain proteins. Structural modeling based on the well characterized serine protease domain fold suggests that this sequence represents a loop extension unique for haptoglobin and haptoglobin-related protein. A synthetic peptide representing the haptoglobin loop sequence exhibited a pronounced inhibitory effect on receptor binding of haptoglobin-hemoglobin.     

Haptoglobin and CD163: captor and receptor gating hemoglobin to macrophage lysosomes

Abstract

The plasma protein haptoglobin and the endocytic hemoglobin receptor HbSR/CD163 are key molecules in the process of removing hemoglobin released from ruptured erythrocytes. Hemoglobin in plasma is instantly bound with high affinity to haptoglobin – an interaction leading to the recognition of the complex by HbSR/CD163 and endocytosis in macrophages. The haptoglobin-dependent HbSR/CD163 scavenging system for hemoglobin clearance prevents toxic effects of hemoglobin in plasma and kidney and explains the decrease in the haptoglobin plasma concentration in patients with accelerated hemolysis. The HbSR/CD163 activity may be of quantitative importance for iron uptake in macrophages in general and for some iron-associated pathological processes, e.g. the atherogenesis-promoting oxidation of LDL leading to foam cell formation and apoptosis in the vessel wall.     

Binding of human hemoglobin and its polypeptide chains with haptoglobin coupled to an agarose matrix.

The interactions of human haptoglobin covalently linked to agarose with human hemoglobin and with p-chloromercuribenzoic-acid-treated alpha and beta chains (alpha* and beta* chains) were studied by flow chromatography and equilibrium binding. The results indicate that in solid state, haptoglobin maintains the same binding characteristics as in solution, the order of binding affinities being: hemoglobin greater than alpha* chain greater than beta* chain. The study of the binding parameters of the alpha* chain shows an heterogeneity of binding sites on the haptoglobin and an average affinity constant Ka of 3.6 X 10(4)l/mol.     

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.     

Interaction of sheep haptoglobin with trypsin inhibitors

It was found that polymeric sheep haptoglobin C interacts with duck egg ovomucoid and with maize trypsin inhibitor. These inhibitors do not block the region in haptoglobin C molecule which is responsible for the formation of its complex with hemoglobin. The binding of the natural protein inhibitors is suggestive of homology of the haptoglobin site involved in the interaction with the substrate-specific site of trypsin. It is assumed that the regions in these protein molecules adjacent to the active and specific sites also possess a high degree of homology.     

Intraplaque hemorrhage

Intraplaque hemorrhage is a common feature of atherosclerotic plaques and is considered one of the identifying features of complex lesions preceding acute ischemic events. The cause of intraplaque hemorrhage is most often secondary to rupture of neovessels, which have invaded the plaque. However, inflammation and metabolic factors such as diabetes may also precipitate hemorrhage from mature microvessels by damaging the endothelium. The mechanism by which hemorrhage destabilizes the plaque is in large part secondary to the action of hemoglobin released from red blood cells at the site of the hemorrhage. Hemoglobin is a potent pro-inflammatory agent by virtue of its ability to promote formation of ROS. The major defense mechanism against the toxic effects of extracorpuscular hemoglobin is the protein haptoglobin, which tightly binds to hemoglobin and prevents it from catalyzing oxidative reactions. There exists a common allelic polymorphism in the haptoglobin gene, which has recently been strongly associated with the risk of cardiovascular disease in multiple independent cohorts. The protein products of the two different haptoglobin alleles differ in their ability to serve as an antioxidant against hemoglobin and also to activate the CD163 receptor. This review presents a unifying hypothesis whereby the haptoglobin genotype is proposed to modulate the response to intraplaque hemorrhage and thereby play a fundamental role in determining the morphological and metabolic features of complex plaques preceding acute ischemic events.