Roles of heme iron-coordinating histidine residues of human hemopexin expressed in baculovirus-infected insect cells

Circulating Iron in Patients with Sickle Cell Disease Mediates the Release of Neutrophil Extracellular Traps

Introduction

Neutrophils promote chronic inflammation and release neutrophil extracellular traps (NETs) that can drive inflammatory responses. Inflammation influences progression of sickle cell disease (SCD), and a role for NETs has been suggested in the onset of vaso-occlusive crisis (VOC). We aimed to identify factors in the circulation of these patients that provoke NET release, with a focus on triggers associated with hemolysis.

Methods

Paired serum and plasma samples during VOC and steady state of 18 SCD patients (HbSS/HbSβ0-thal and HbSC/HbSβ+-thal) were collected. Cell-free heme, hemopexin, and labile plasma iron have been measured in the plasma samples of the SCD patients. NETs formation by human neutrophils from healthy donors induced by serum of SCD patients was studied using confocal microscopy and staining for extracellular DNA using Sytox, followed by quantification of surface coverage using ImageJ.

Results

Eighteen patients paired samples obtained during VOC and steady state were available (11 HbSS/HbSβ0-thal and 7 HbSC/HbSβ+-thal). We observed high levels of systemic heme and iron, concomitant with low levels of the heme-scavenger hemopexin in sera of patients with SCD, both during VOC and in steady state. In our in vitro experiments, neutrophils released NETs when exposed to sera from SCD patients. The release of NETs was associated with high levels of circulating iron in these sera. Although hemin triggered NET formation in vitro, addition of hemopexin to scavenge heme did not suppress NET release in SCD sera. By contrast, the iron scavengers deferoxamine and apotransferrin attenuated NET formation in a significant proportion of SCD sera.

Discussion

Our results suggest that redox-active iron in the circulation of non-transfusion-dependent SCD patients activates neutrophils to release NETs, and hence, exerts a direct pro-inflammatory effect. Thus, we propose that chelation of iron requires further investigation as a therapeutic strategy in SCD.

A Model Peptide Reveals Insights into the Interaction of Human Hemopexin with Heme

Under hemolytic conditions, toxic heme is scavenged by hemopexin. Recently, the heme-binding properties of hemopexin have been reassessed, which revealed a KD of ~ 0.32 nM as well as a stoichiometry of one to two heme molecules binding to hemopexin. A 66mer hemopexin-derived peptide that spans over three heme-binding motifs was used to verify the earlier suggested heme-recruiting mechanism. Herein, we employed spectroscopic and computational methods to substantiate the hypothesis of more than one heme molecule binding to hemopexin and to analyze the heme-binding mode. Both, hemopexin and the 66mer peptide, were found to bind heme in mixed penta- and hexacoordinated states, which strongly indicates that heme binding follows distinct criteria and increases rigidity of the peptide-heme complex. Additional in silico molecular dynamics simulations support these experimental findings and, thus, contribute to our understanding of the molecular basis of the heme-hemopexin interaction. This analysis provides further details for consideration of hemopexin in biomedical applications.

Revisiting the interaction of heme with hemopexin

In hemolytic disorders, erythrocyte lysis results in massive release of hemoglobin and, subsequently, toxic heme. Hemopexin is the major protective factor against heme toxicity in human blood and currently considered for therapeutic use. It has been widely accepted that hemopexin binds heme with extraordinarily high affinity of <1 pM in a 1:1 ratio. However, several lines of evidence point to a higher stoichiometry and lower affinity than determined 50 years ago. Here, we re-analyzed these data. SPR and UV/Vis spectroscopy were used to monitor the interaction of heme with the human protein. The heme-binding sites of hemopexin were characterized using hemopexin-derived peptide models and competitive displacement assays. We obtained a K _D value of 0.32 ± 0.04 nM and the ratio for the interaction was determined to be 1:1 at low heme concentrations and at least 2:1 (heme:hemopexin) at high concentrations. We were able to identify two yet unknown potential heme-binding sites on hemopexin. Furthermore, molecular modelling with a newly created homology model of human hemopexin suggested a possible recruiting mechanism by which heme could consecutively bind several histidine residues on its way into the binding pocket. Our findings have direct implications for the potential administration of hemopexin in hemolytic disorders.

HeMoQuest: a webserver for qualitative prediction of transient heme binding to protein motifs

Background

The notion of heme as a regulator of many physiological processes via transient binding to proteins is one that is recently being acknowledged. The broad spectrum of the effects of heme makes it important to identify further heme-regulated proteins to understand physiological and pathological processes. Moreover, several proteins were shown to be functionally regulated by interaction with heme, yet, for some of them the heme-binding site(s) remain unknown. The presented application HeMoQuest enables identification and qualitative evaluation of such heme-binding motifs from protein sequences.

Results

We present HeMoQuest, an online interface (http://bit.ly/hemoquest) to algorithms that provide the user with two distinct qualitative benefits. First, our implementation rapidly detects transient heme binding to nonapeptide motifs from protein sequences provided as input. Additionally, the potential of each predicted motif to bind heme is qualitatively gauged by assigning binding affinities predicted by an ensemble learning implementation, trained on experimentally determined binding affinity data. Extensive testing of our implementation on both existing and new manually curated datasets reveal that our method produces an unprecedented level of accuracy (92%) in identifying those residues assigned “heme binding” in all of the datasets used. Next, the machine learning implementation for the prediction and qualitative assignment of binding affinities to the predicted motifs achieved 71% accuracy on our data.

Conclusions

Heme plays a crucial role as a regulatory molecule exerting functional consequences via transient binding to surfaces of target proteins. HeMoQuest is designed to address this imperative need for a computational approach that enables rapid detection of heme-binding motifs from protein datasets. While most existing implementations attempt to predict sites of permanent heme binding, this application is to the best of our knowledge, the first of its kind to address the significance of predicting transient heme binding to proteins.

Haptoglobin improves shock, lung injury, and survival in canine pneumonia

During the last half-century, numerous antiinflammatory agents were tested in dozens of clinical trials and have proven ineffective for treating septic shock. The observation in multiple studies that cell-free hemoglobin (CFH) levels are elevated during clinical sepsis and that the degree of increase correlates with higher mortality suggests an alternative approach. Human haptoglobin binds CFH with high affinity and, therefore, can potentially reduce iron availability and oxidative activity. CFH levels are elevated over approximately 24-48 hours in our antibiotic-treated canine model of S. aureus pneumonia that simulates the cardiovascular abnormalities of human septic shock. In this 96-hour model, resuscitative treatments, mechanical ventilation, sedation, and continuous care are translatable to management in human intensive care units. We found, in this S. aureus pneumonia model inducing septic shock, that commercial human haptoglobin concentrate infusions over 48-hours bind canine CFH, increase CFH clearance, and lower circulating iron. Over the 96-hour study, this treatment was associated with an improved metabolic profile (pH, lactate), less lung injury, reversal of shock, and increased survival. Haptoglobin binding compartmentalized CFH to the intravascular space. This observation, in combination with increasing CFHs clearance, reduced available iron as a potential source of bacterial nutrition while decreasing the ability for CFH and iron to cause extravascular oxidative tissue injury. In contrast, haptoglobin therapy had no measurable antiinflammatory effect on elevations in proinflammatory C-reactive protein and cytokine levels. Haptoglobin therapy enhances normal host defense mechanisms in contrast to previously studied antiinflammatory sepsis therapies, making it a biologically plausible novel approach to treat septic shock.

Heme as a Target for Therapeutic Interventions

Heme is a complex of iron and the tetrapyrrole protoporphyrin IX with essential functions in aerobic organisms. Heme is the prosthetic group of hemoproteins such as hemoglobin and myoglobin, which are crucial for reversible oxygen binding and transport. By contrast, high levels of free heme, which may occur in various pathophysiological conditions, are toxic via pro-oxidant, pro-inflammatory and cytotoxic effects. The toxicity of heme plays a major role for the pathogenesis of prototypical hemolytic disorders including sickle cell disease and malaria. Moreover, there is increasing appreciation that detrimental effects of heme may also be critically involved in diseases, which usually are not associated with hemolysis such as severe sepsis and atherosclerosis. In mammalians homeostasis of heme and its potential toxicity are primarily controlled by two physiological systems. First, the scavenger protein hemopexin (Hx) non-covalently binds extracellular free heme with high affinity and attenuates toxicity of heme in plasma. Second, heme oxygenases (HOs), in particular the inducible HO isozyme, HO-1, can provide antioxidant cytoprotection via enzymatic degradation of intracellular heme. This review summarizes current knowledge on the pathophysiological role of heme for various diseases as demonstrated in experimental animal models and in humans. The functional significance of Hx and HOs for the regulation of heme homeostasis is highlighted. Finally, the therapeutic potential of pharmacological strategies that apply Hx and HO-1 in various clinical settings is discussed.

Purified and Recombinant Hemopexin: Protease Activity and Effect on Neutrophil Chemotaxis

Infusion of the heme-binding protein hemopexin has been proposed as a novel approach to decrease heme-induced inflammation in settings of red blood cell breakdown, but questions have been raised as to possible side effects related to protease activity and inhibition of chemotaxis. We evaluated protease activity and effects on chemotaxis of purified plasma hemopexin obtained from multiple sources as well as a novel recombinant fusion protein Fc-hemopexin. Amidolytic assay was performed to measure the protease activity of several plasma-derived hemopexin and recombinant Fc-hemopexin. Hemopexin was added to the human monocyte culture in the presence of lipopolysaccharides (LPS), and also injected into mice intravenously (i.v.) 30 min before inducing neutrophil migration via intraperitoneal (i.p.) injection of thioglycolate. Control groups received the same amount of albumin. Protease activity varied widely between hemopexins. Recombinant Fc-hemopexin bound heme, inhibited the synergy of heme with LPS on tumor necrosis factor (TNF) production from monocytes, and had minor but detectable protease activity. There was no effect of any hemopexin preparation on chemotaxis, and purified hemopexin did not alter the migration of neutrophils into the peritoneal cavity of mice. Heme and LPS synergistically induced the release of LTB4 from human monocytes, and hemopexin blocked this release, as well as chemotaxis of neutrophils in response to activated monocyte supernatants. These results suggest that hemopexin does not directly affect chemotaxis through protease activity, but may decrease heme-driven chemotaxis and secondary inflammation by attenuating the induction of chemoattractants from monocytes. This property could be beneficial in some settings to control potentially damaging inflammation induced by heme.

Haptoglobin or Hemopexin Therapy Prevents Acute Adverse Effects of Resuscitation After Prolonged Storage of Red Cells

BACKGROUND

Extracellular hemoglobin and cell-free heme are toxic breakdown products of hemolyzed erythrocytes. Mammals synthesize the scavenger proteins haptoglobin and hemopexin, which bind extracellular hemoglobin and heme, respectively. Transfusion of packed red blood cells is a lifesaving therapy for patients with hemorrhagic shock. Because erythrocytes undergo progressive deleterious morphological and biochemical changes during storage, transfusion of packed red blood cells that have been stored for prolonged intervals (SRBCs; stored for 35-40 days in humans or 14 days in mice) increases plasma levels of cell-free hemoglobin and heme. Therefore, in patients with hemorrhagic shock, perfusion-sensitive organs such as the kidneys are challenged not only by hypoperfusion but also by the high concentrations of plasma hemoglobin and heme that are associated with the transfusion of SRBCs.

METHODS

To test whether treatment with exogenous human haptoglobin or hemopexin can ameliorate adverse effects of resuscitation with SRBCs after 2 hours of hemorrhagic shock, mice that received SRBCs were given a coinfusion of haptoglobin, hemopexin, or albumin.

RESULTS

Treatment with haptoglobin or hemopexin but not albumin improved the survival rate and attenuated SRBC-induced inflammation. Treatment with haptoglobin retained free hemoglobin in the plasma and prevented SRBC-induced hemoglobinuria and kidney injury. In mice resuscitated with fresh packed red blood cells, treatment with haptoglobin, hemopexin, or albumin did not cause harmful effects.

CONCLUSIONS

In mice, the adverse effects of transfusion with SRBCs after hemorrhagic shock are ameliorated by treatment with either haptoglobin or hemopexin. Haptoglobin infusion prevents kidney injury associated with high plasma hemoglobin concentrations after resuscitation with SRBCs. Treatment with the naturally occurring human plasma proteins haptoglobin or hemopexin may have beneficial effects in conditions of severe hemolysis after prolonged hypotension.

Identification of a warm-temperature acclimation-associated 65-kDa protein encoded by a temperature- and infection-responsive gene in the Kumgang fat minnow Rhynchocypris kumgangensis

Water temperature is one of the most important factors in fish physiology; thus, it is important to identify genes that respond to changes in water temperature. In this study, we identified a warm- temperature acclimation-associated 65-kDa protein (Wap65) in the Kumgang fat minnow Rhynchocypris kumgangensis, a small, cold-freshwater fish species endemic to Korea. Kumgang fat minnow Wap65-1 (kmWap65-1) was cloned using polymerase chain reaction (PCR)-based strategies, and was found to be highly homologous with teleost Wap65-1 and mammalian hemopexin, a heme-binding protein that transfers plasma heme into hepatocytes. kmWap65-1 mRNA was expressed mainly in the liver and its expression levels were significantly increased by both short- and long-term exposure to high temperature, which was evaluated by real-time quantitative PCR. Furthermore, the expression levels of kmWap65-1 were highly elevated by exposure to bacterial lipopolysaccharide. These results indicate that kmWap65-1 expression is associated with environmental stresses such as increases in water temperature and bacterial infection. J. Exp. Zool. 325A:65-74, 2016. © 2015 Wiley Periodicals, Inc.

Molecular cloning, characterization and expression analysis of a novel wap65-1 gene from Plecoglossus altivelis

Warm temperature acclimation associated 65-kDa protein 1 (WAP65-1) is a specific fish plasma glycoprotein that is possibly involved in various physiological or pathological processes. In this study, we obtained the cDNA and genomic DNA sequences of the Plecoglossus altivelis wap65-1 (Pawap65-1) gene. Multiple sequence alignment showed that Pawap65-1 is similar in structure to wap65-1 in fish. Phylogenetic analysis revealed that Pawap65-1 is most closely related to that of a rainbow trout. Pawap65-1 transcripts are present in various tissues and are most abundant in the liver. We expressed recombinant PaWAP65-1 in Escherichia coli and raised antiserum against it in mouse. Western blot analysis revealed that the higher molecular mass of PaWAP65-1 in blood plasma was caused by post-translational N-glycosylation. Quantitative real-time quantitative PCR (qPCR) and Western blot analysis data showed that the hepatic mRNA and blood plasma levels of PaWAP65-1 were both influenced by warm temperature acclimation and cadmium exposure, but not by Listonella anguillarum infection, hypo-osmotic, or cold temperature acclimation. In conclusion, our data reveals that PaWAP65-1 is a stress-related protein, and may play a role in fish acclimation to warm temperature and cadmium exposure.

An alternative view of the proposed alternative activities of hemopexin

Hemopexin is a plasma protein that plays a well-established biological role in sequestering heme that is released into the plasma from hemoglobin and myoglobin as the result of intravascular or extravascular hemolysis as well as from skeletal muscle trauma or neuromuscular disease. In recent years, a variety of additional biological activities have been attributed to hemopexin, for example, hyaluronidase activity, serine protease activity, pro-inflammatory and anti-inflammatory activity as well as suppression of lymphocyte necrosis, inhibition of cellular adhesion, and binding of divalent metal ions. This review examines the challenges involved in the purification of hemopexin from plasma and in the recombinant expression of hemopexin and evaluates the questions that these challenges and the characteristics of hemopexin raise concerning the validity of many of the new activities proposed for this protein. As well, an homology model of the three-dimensional structure of human hemopexin is used to reveal that the protein lacks the catalytic triad that is characteristic of many serine proteases but that hemopexin possesses two highly exposed Arg-Gly-Glu sequences that may promote interaction with cell surfaces.

Emergence of the acute-phase protein hemopexin in jawed vertebrates

When released from damaged erythrocytes free heme not only provides a source of iron for invading bacteria but also highly toxic due to its ability to catalyze free radical formation. Hemopexin (Hx) binds free heme with very high-affinity and thus protects against heme toxicity, sequesters heme from pathogens, and helps conserve valuable iron. Hx is also an acute-phase serum protein (APP), whose expression is induced by inflammation. To date Hx has been identified as far back in phylogeny as bony fish where it is called warm-temperature acclimation-related 65 kDa protein (WAP65), as serum protein levels are increased at elevated environmental temperatures as well as by infection. During analysis of nurse shark (Ginglymostoma cirratum) plasma we isolated a Ni(2+)-binding serum glycoprotein and characterized it as the APP Hx. We subsequently cloned Hx from nurse shark and another cartilaginous fish species, the little skate Leucoraja erinacea. Functional analysis showed shark Hx, like that of mammals, binds heme but is found at unusually high levels in normal shark serum. As an Hx orthologue could not be found in the genomes of jawless vertebrates or lower deuterostomes it appears to have arisen just prior to the emergence of jawed vertebrates, coincident with the second round of genome-wide duplication and the appearance of tetrameric hemoglobin (Hb).

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

Heme is vital to our aerobic universe. Heme cellular content is finely tuned through an exquisite control of synthesis and degradation. Heme deficiency is deleterious to cells, whereas excess heme is toxic. Most of the cellular heme serves as the prosthetic moiety of functionally diverse hemoproteins, including cytochromes P450 (P450s). In the liver, P450s are its major consumers, with >50% of hepatic heme committed to their synthesis. Prosthetic heme is the sine qua non of P450 catalytic biotransformation of both endo- and xenobiotics. This well-recognized functional role notwithstanding, heme also regulates P450 protein synthesis, assembly, repair, and disposal. These less well-appreciated aspects are reviewed herein.

A simple and reliable protocol for mouse serum proteome profiling studies by use of two-dimensional electrophoresis and MALDI TOF/TOF mass spectrometry

Background

Unravelling the serum proteome is the subject of intensified research. In this regard, two-dimensional electrophoresis coupled with MALDI MS analysis is still one of the most commonly used method. Despite some improvements, there is the need for better protocols to enable comprehensive identification of serum proteins.

Here we report a combination of two proteomic strategies, zoom in acidic and neutral part of 2-D gels and an application of two optimised matrix preparations for MALDI-MS analyses to simplify serum proteome mapping.

Results

Mouse serum proteins were separated by 2-D electrophoresis at the pH ranges 3–10 and 4–7, respectively. Then in gel tryptic digests were analysed by MALDI-MS. Notably, sample-matrix preparations consisted of either a thin-layer α-ciano-4-hydroxycinnamic acid (CHCA) matrix deposition or a matrix-layer 2,5-dihydroxybenzoic acid (DHB). This enabled an identification of 90 proteins. The herein reported method enhanced identification of proteins by 32% when compared with previously published studies of mouse serum proteins, using the same approaches. Furthermore, experimental improvements of matrix preparations enabled automatic identification of mouse proteins, even when one of the two matrices failed.

Conclusion

We report a simple and reliable protocol for serum proteome analysis that combines an optimized resolution of 2-D gels spots and improved sample-matrix preparations for MALDI-MS analysis. The protocol allowed automated data acquisition for both CHCA and DHB and simplified the MS data acquisition therefore avoiding time-consuming procedures. The simplicity and reliability of the developed protocol may be applied universally.

Expression of warm temperature acclimation-related protein 65-kDa (Wap65) mRNA, and physiological changes with increasing water temperature in black porgy, Acanthopagrus schlegeli

We isolated the warm temperature acclimation-related protein 65-kDa (Wap65) cDNA from the liver of black porgy and investigated the expression by increasing water temperature in black porgy, Acanthopagrus schlegeli. Black porgy Wap65 full-length cDNA consists of 1,338 nucleotides, including an open reading frame, predicted to encode a protein of 425 amino acids and showed high homology to pufferfish (79%), Medaka (73%), carp (70%), and goldfish (68%) Wap65. Increase in water temperature (20 degrees C --> 30 degrees C; 1 degrees C/day) induced the rise of Wap65 mRNA expression in liver of black porgy. Also, the levels of cortisol and glucose in plasma were significantly higher at 30 degrees C than at 20 degrees C. To determine the high water temperature stressor specificity of the induction of Wap65, black porgy were transferred from seawater (SW) to freshwater (FW) for 24 hr. Wap65 expression was not detected when the fish were transferred from SW to FW (in fish transferred from SW to FW), although the levels of cortisol and glucose in plasma were increased. These results suggest that increase in Wap65 gene is related to high water temperature stress and play important roles in high water temperature environment of black porgy.

The warm temperature acclimation protein Wap65 as an immune response gene: its duplicates are differentially regulated by temperature and bacterial infections

The warm temperature acclimation related 65kDa protein (Wap65) in teleost fish shares high structural similarities with mammalian hemopexins. Recent studies using microarray analysis indicated that this temperature acclimation protein may also be involved in immune responses. To provide evidence of its potential involvement in immune responses after bacterial infections, we have identified and characterized two types of Wap65 genes in channel catfish, referred to as Wap65-1 and Wap65-2, respectively. While Wap65-1 and Wap65-2 are both structurally similar to the mammalian hemopexins, they exhibit highly differential patterns of spatial expression. Wap65-1 was expressed in a wide range of tissues, whereas Wap65-2 was only expressed in the liver. Their regulation with warm temperature and bacterial infections was also highly different: Wap65-1 was constitutively expressed, whereas Wap65-2 was highly regulated by both warm temperature and bacterial infections, and warm temperature and bacterial infections appeared to synergistically induce the expression of Wap65-2. The great contrast of expression patterns and regulation of the two catfish Wap65 genes suggested both neofunctionalization and partitioning of their functions. Phylogenetic analysis indicated that the duplicated catfish Wap65 genes were evolved not only from whole genome duplication, but also from tandem, intrachromosomal gene duplications. Taken together, the results of this study suggest that Wap65 genes are not only important for its classical role as a warm temperature acclimation protein, but more importantly, may also function as an immune response protein.

The occurrence of two types of hemopexin-like protein in medaka and differences in their affinity to heme

Full-length cDNA clones encoding two types of hemopexin-like protein, mWap65-1 and mWap65-2, were isolated from the HNI inbred line of medaka Oryzias latipes. The deduced amino acid sequence of mWap65-2 resembled mammalian hemopexins more closely than that of mWap65-1. Histidine residues required for the high affinity of hemopexins for hemes were conserved in mWap65-2, but not in mWap65-1. Surprisingly, mWap65-1, but not mWap65-2, showed heme-binding ability as revealed by hemin-agarose affinity chromatography, even though mWap65-1 lacked the essential histidine residues. Furthermore, RT-PCR analysis of different tissues demonstrated that the transcripts of mWap65-2 were restricted to liver, whereas those of mWap65-1 were found in various tissues including liver, eye, heart and brain. Quantitative RT-PCR revealed that transcripts of mWap65-2 were expressed earlier than those of mWap65-1 during ontogeny. However, the accumulated mRNA levels of both mWap65-1 and mWap65-2 did not differ significantly in fish acclimated to either 10 degrees C or 30 degrees C for 5 weeks. These characteristics suggest that the two proteins have different physiological functions and that mWap65-2 is not a hemopexin.

Heme binding by hemopexin: evidence for multiple modes of binding and functional implications

Hemopexin binds 1 mol of heme per mol with high affinity (Kd < 1 pM) in a low-spin complex and acts as a transport vehicle for the heme. Circular dichroism (CD) spectroscopy was used to examine the heme environment in the ferri-, ferro-, and CO-ferro complexes of four iron tetrapyrroles [meso-, proto-, deutero-, and (2-vinyl, 4-hydroxymethyl)-deutero-heme] with three species (human, rabbit, and rat) of hemopexin. All ferri-heme-hemopexin complexes exhibit a band of positive ellipticity near the Soret maximum, except for the human ferri-protoheme hemopexin complex, which has a bisignate spectrum. The ferro-heme and CO-ferro-heme complexes display a variety of spectra, demonstrating redox- and ligand-linked shifts in conformation that alter the environment of the heme. The rabbit mesoheme-N-domain complexes have absorbance spectra almost indistinguishable from those of intact hemopexin, but present CD spectra that are distinctly different. However, adding the C-domain to mesoheme-N-domain restores most of the CD characteristics of the intact hemopexin complexes.

Two heme-binding domains of heme-regulated eukaryotic initiation factor-2alpha kinase. N terminus and kinase insertion

In heme deficiency, protein synthesis in reticulocytes is inhibited by activation of heme-regulated alpha-subunit of eukaryotic initiation factor-2alpha (eIF-2alpha) kinase (HRI). Previous studies indicate that HRI contains two distinct heme-binding sites per HRI monomer. To study the role of the N terminus in the heme regulation of HRI, two N-terminally truncated mutants, Met2 and Met3 (deletion of the first 103 and 130 amino acids, respectively), were prepared. Met2 and Met3 underwent autophosphorylation and phosphorylated eIF-2alpha with a specific activity of approximately 50% of that of the wild type HRI. These mutants were significantly less sensitive to heme regulation both in vivo and in vitro. In addition, the heme contents of purified Met2 and Met3 HRI were less than 5% of that of the wild type HRI. These results indicated that the N terminus was important but was not the only domain involved in the heme-binding and heme regulation of HRI. Heme binding of the individual HRI domains showed that both N terminus and kinase insertion were able to bind hemin, whereas the C terminus and the catalytic domains were not. Thus, both the N terminus and the kinase insertion, which are unique to HRI, are involved in the heme binding and the heme regulation of HRI.

Molecular cloning of a hemopexin-like cDNA from rainbow trout liver

Hemopexin is an abundant heme-binding serum protein synthesized in the liver and for which the sequence is determined only for mammals. Little is known about fish heme-binding proteins. We have cloned a rainbow trout complementary DNA that encodes a 445 amino acid polypeptide exhibiting an overall 30% homology with human hemopexin and 69% with the goldfish warm temperature acclimation-related 65-kDa protein. Structural homology, deduced from hydrophobic cluster analysis (HCA), was strong between the trout and human proteins since global HCA scores of 76% were obtained when the N- or C-terminal halves of the two proteins were compared. Moreover, several characteristics of hemopexin were found in the trout protein; finally, the trout hemopexin-like messenger RNA was specifically expressed in the liver. We conclude that the trout protein is a good candidate for a true fish hemopexin.

EPR studies on the effects of complexation of heme by hemopexin upon its reactions with organic peroxides

Hemopexin, a heme-binding serum glycoprotein, is thought to play an important role in the prevention of oxidative damage that may be catalysed by free heme. Through the use of EPR techniques, the generation of free radicals from organic hydroperoxides by heme and heme-hemopexin complexes, and the concomitant formation of high oxidation-state iron species has been studied; these species are implicated as causative agents in processes such as cardiovascular disease and carcinogenesis. From the rates of production of these species from both n-alkyl and branched hydroperoxides, it has been inferred that the dramatic reduction in the yield of oxidising species generated by heme upon its complexation with hemopexin arises from steric hindrance of the access of hydroperoxide to the bound heme.

The warm temperature acclimation-related 65-kDa protein, Wap65, in goldfish and its gene expression

cDNAs encoding a warm temperature acclimation-related protein (Wap65) were cloned from the muscle and hepatopancreas cDNA libraries of the warm temperature-acclimated goldfish Carassius auratus, and their nucleotide sequences containing 5'- and 3'-noncoding regions together with their polyadenylation signal were determined. The deduced amino acid sequence of Wap65 was 31% homologous to rat hemopexin. However, goldfish Wap65 lacked a few possible glycosylation sites and presumed functional histidine residues, implying that it may have different functions from hemopexin. Wap65 contained a leader peptide of 30 amino acids and a mature protein region of 415 amino acids. Southern blot analysis demonstrated that the protein is expressed by a single copy gene in the goldfish haploid genome. In RNA blot analysis using isolated cDNA clones, a single transcript of about 2.0 kilobases was detected in the hepatopancreas but not in brain, muscle, or hemocytes. The abundancy of this transcript markedly increased in the hepatopancreas as a result of warm temperature acclimation. Electrophoretic analysis of plasma proteins revealed a good correlation of plasma Wap65 levels to those of the corresponding transcript in the hepatopancreas, suggesting that serum Wap65 concentrations are regulated mainly by transcript levels in the hepatopancreas via the secretion process.

1.8 A crystal structure of the C-terminal domain of rabbit serum haemopexin

BACKGROUND

Haemopexin is a serum glycoprotein that binds haem reversibly and delivers it to the liver where it is taken up by receptor-mediated endocytosis. Haemopexin has two homologous domains, each having a characteristic fourfold internal sequence repeat. Haemopexin-type domains are also found in other proteins, including the serum adhesion protein vitronectin and various collagenases, in which they mediate protein-protein interactions.

RESULTS

We have determined the crystal structure of the C-terminal domain of haemopexin at 1.8 A resolution. The domain is folded into four beta-leaflet modules, arranged in succession around a central pseudo-fourfold axis. A funnel-shaped tunnel through the centre of this disc-shaped domain serves as an ion-binding site.

CONCLUSIONS

A model for haem binding by haemopexin is proposed, utilizing an anion-binding site at the wider end of the central tunnel, together with an associated cleft. This parallels the active-site location in other beta-propeller structures. The capacity to bind both cations and anions, together with the disc shape of the domain, suggests that such domains may be used widely for macromolecular recognition.