Haem transport to the liver by haemopexin. Receptor-mediated uptake with recycling of the protein

Effect of extracellular hemin on hemoglobin and ferritin content of erythroleukemia cells

Mouse (MEL) and human (K-562) erythroleukemia cell lines can be induced to undergo erythroid differentiation, including hemoglobin (Hb) synthesis, by extra cellular hemin. In order to study the effect of extracellular hemin on intracellular ferritin and Hb content, we have used Mossabauer spectroscopy to measure the amount of 57Fe incorporated into ferritin or Hb and a fluorescent enzyme-linked immunosorbent assay (ELISA) to measure the ferritin protein content. When K-562 cells were cultured in the presence of a 57Fe source either as transferrin or citrate, in the absence of a differentiation inducer, all the intracellular 57Fe was detected in ferritin. When the cells were cultured in the presence of 57Fe-hemin, 57Fe was found in both ferritin and Hb. 57Fe in ferritin increased rapidly, and after 2 days it reached a plateau at 5 X 10(-14) g/cell. 57Fe in Hb increased linearly with time and reached the same value after 12 days. Addition of other iron sources such as iron-saturated transferrin, iron citrate, or iron ammonium citrate caused a much lower increase in ferritin protein content as compared to hemin. When K-562 cells were induced by 57Fe-hemin in the presence of 56Fe-transferrin, 57Fe was found to be incorporated in equal amounts into both ferritin and Hb. However, when the cells were induced by 56Fe-hemin in the presence of 57Fe-transferrin, 57Fe was incorporated only into ferritin, but not into Hb, which contained 56Fe iron. These results indicate that in K-562 cells, when hemin is present in the culture medium it is preferentially incorporated into Hb, regardless of the availability of other extra- or intracellular iron sources such as transferrin or ferritin. In MEL cells induced to differentiate by dimethylsulfoxide (DMSO) a different pattern of iron incorporation was observed; 57Fe from both transferrin and hemin was found to incorporate in ferritin as well as in Hb.

Behaviour in a silica-based high-performance liquid gel permeation chromatographic column of the apo- and holo-forms of the haem-binding proteins haemopexin, histidine-rich glycoprotein, globin and albumin

The elution of the apo- and holo-forms of four haem-binding proteins was studied using a TSK G 3000 SW HPLC column. Apo-haemopexin had a higher apparent molecular size [68,000 daltons (d)] than histidine-rich glycoprotein (HRG) (66,000 d) in gel chromatography, contrasting with the values in sodium dodecyl sulphate electrophoresis, 84,000 d for HRG and 69,000 d for haemopexin. The elution of the haem complexes of both proteins correlated better with their true molecular weights. Saturation of albumin with haem did not significantly influence its elution. The peaks were more symmetrical for the holo- than the apo-proteins, except for globin/haemoglobin. The results indicated that the apo-forms of haemopexin and HRG had affinity for the column matrix. HRG, which has several haem binding sites, was retained more than haemopexin, which binds only one haem. Free haem itself was bound to the silica column but could be released by globin. HRG had a tendency to polymerize after haem binding, in contrast to haemopexin, which remained monomeric. Globin was eluted from the column with an apparent molecular size of 16,000 d and after saturation with haem with a molecular size of 31,000 d.

Serum histidine-rich glycoprotein levels are decreased in acquired immune deficiency syndrome and by steroid therapy

Using radial immunodiffusion serum histidine-rich glycoprotein (HRG) levels were measured in acquired immune deficiency syndrome (AIDS) patients, in end-stage renal disease (ESRD) patients after renal transplantation and immunosuppressive steroid therapy, and in asthma and chronic obstructive pulmonary disease (COPD) patients treated with steroids. Compared with controls (12.5 +/- 3.0 mg/dl), HRG levels were significantly decreased in patients with AIDS (5.7 +/- 1.8 mg/dl, P less than 0.005): in patients with ESRD after renal transplantation with steroid therapy (4.4 +/- 1.1 mg/dl, P less than 0.005); and in asthmatic and COPD patients receiving steroids in acute (7.6 +/- 2.9 mg/dl, P less than 0.005) or chronic (7.4 +/- 3.0 mg/dl, P less than 0.025) high-dose regimens. In contrast, levels of hemopexin, another serum glycoprotein synthesized by the liver, were not lowered in these patients. These results show that serum HRG levels are selectively decreased in AIDS and in patients treated with immunosuppressive steroids.

Homology with hemopexin suggests a possible scavenging function for S-protein/vitronectin

S-protein is an abundant plasma protein which has recently been shown to be identical to vitronectin and serum spreading factor [(1985) EMBO J. 4, 3153-3157]. It therefore has multiple binding sites for terminal complement complexes, thrombin-antithrombin III, heparin, and a specific cell receptor. In this report a structural and sequence homology with hemopexin is described which suggests that the principle function of S-protein could be as a scavenging molecule, clearing spent complement and coagulation complexes from the circulation.

Isolation of the haemopexin-haem receptor from pig liver cells

Isolated pig liver plasma membranes interact specifically with the haemopexin-haem complex (Kd 4.4 X 10(-7) M). Affinity chromatography was used to isolate a membrane component which binds this complex with high affinity. Pig serum haemopexin was first isolated by affinity chromatography on haemin-Sepharose followed by HPLC gel filtration. Liver membranes solubilized with Triton X-100 were incubated with haemin-Sepharose saturated with haemopexin, and as a control, with affinity gel lacking haemopexin. SDS-poly-acrylamide gel electrophoresis of the eluted protein indicated that from the haemin-Sepharose emerglow-molecular-mass haemin-binding proteins whereas the eluate from haemopexin-haemin-Sepharose contained an additional 71 kDa protein, which did not bind free haemin. This protein appears to represent the haemopexin-haem receptor or a part of it. Haem from the haemopexin complex, as also free haemin, was accepted by a binder in the plasma membrane, which in gel filtration behaved like an 80 kDa molecule. This component probably represents a second functional subunit of the haemopexin-haem receptor.

The reactivity of the disulfide bonds of human serum haemopexin

The reactivity of the disulfide bonds of the specific haeme-binding plasma protein-human haemopexin has been studied with 2-mercaptoethanol. A molecule of haemopexin has six intrachain disulfide bridges (Takahashi et al., 1985) or which four are reactive while the remaining two can be reduced in the presence of greater than or equal to 4M urea. Disruption of the four reactive disulfide bonds in apohaemopexin abolishes the haeme binding ability. In equimolar haeme-haemopexin complex only one disulfide is reactive which suggests a large change in the tertiary structure of this protein on haeme binding.

Intracellular distribution of haem after uptake by different receptors. Haem-haemopexin and haem-asialo-haemopexin

How the interaction of haemopexin with two different receptors affects its subsequent metabolism and 'intracellular' haem transport was examined by using mesohaem-haemopexin and mesohaem-asialo-haemopexin. The physical properties of the two haem proteins, including their absorption and c.d. spectra, are similar. Binding studies in vitro showed that haem-asialo-haemopexin interacts with both the haemopexin-specific and galactose-specific receptors on liver plasma membranes, but that haem-haemopexin interacts only with the haemopexin receptor. In vivo haem-asialo-haemopexin rapidly interacts with the liver via the galactose-specific receptor, since the protein is extensively catabolized and uptake is blocked by asialofetuin. Haem iron from haem-asialo-haemopexin is not accumulated in the liver to the same extent as from intact haem-haemopexin, and the native sialylated protein is not proteolysed. Moreover, after fractionation of homogenized liver by using colloidal-silica gradients, liver-associated haem-haemopexin and haem-asialo-haemopexin produced distinctly different patterns for both protein and ligand, consistent with their uptake by two distinct receptors. These results demonstrate that the interaction of haemopexin with different receptors influences its subsequent metabolic fate and that haem iron from haem-haemopexin is efficiently conserved only if it enters the liver cell via the specific haemopexin receptor.

A characterization of haem uptake and intracellular distribution by isolated hepatocytes

The uptake and intracellular distribution of haem by isolated rat hepatocyte suspensions was studied. An increase in cell haem content occurred after a challenge with 5, 10 or 20 microM haem, supplied as methaemalbumin. The rate of haem uptake was temperature dependent; no non-specific binding occurred. Intracellular haem distribution data are consistent with a rapid association of haem with the endoplasmic reticulum fraction prior to its accumulation in the cytosol and at the mitochondrion.

Synthesis and secretion of hemopexin in primary cultures of rat hepatocytes. Demonstration of an intracellular precursor of hemopexin

Secretion of hemopexin (20% carbohydrate) and its dependence on glycosylation was studied in primary rat hepatocyte cultures in comparison to the secretion of transferrin (5% carbohydrate). In pulse-chase experiments with [35S]methionine half of the labeled hemopexin was secreted in 30 min. By contrast, it took approximately 50 min for secretion of half of the transferrin. Tunicamycin treatment of cultures significantly delayed the secretion of hemopexin but not that of transferrin. During the pulse period a prominent intracellular precursor of hemopexin, smaller than the mature protein, was evident. It is concluded that the extent of glycosylation of a secretory protein is not necessarily a determinant of the transit time required for intracellular processing and secretion. In the case of hemopexin the glycosylation apparently facilitates the secretion although it is not an absolute prerequisite for the exocytosis of this protein.

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.

Differential interaction of porphyrins used in photoradiation therapy with ferrochelatase

The mechanism of porphyrin accumulation by tumours is not yet established. If metabolism aids porphyrin elimination, tumours, unlike normal tissues, may not metabolize porphyrins used clinically, such as proto-, haemato-, OO'-diacetyl-haemato- and monohydroxyethyl-monovinyl-deutero-porphyrin. Proto-, haemato- and monohydroxyethyl-monovinyl-deutero-porphyrin are substrates for the mitochondrial enzyme ferrochelatase (EC 4.99.1.1), which can form haem analogues from exogenous porphyrins. The Km values for proto-, haemato- and monohydroxyethyl-monovinyl-deutero-porphyrin are 11, 22 and 23 microM respectively. However, OO'-diacetyl-haematoporphyrin is an effective competitive inhibitor with Ki of 11 microM. Hepatic ferrochelatase specific activity is 5.9 and 5.5 nmol of haem/h per mg of protein respectively in normal Buffalo rat and in those bearing the extrahepatic Morris 7288C hepatoma, and is only 0.13 nmol/h per mg in the hepatomas. Therefore low ferrochelatase activity in cancerous cells may provide one means whereby some porphyrins accumulate in tumours, and the ability of certain porphyrins to act as ferrochelatase inhibitors may provide another.

Structure of human hemopexin: O-glycosyl and N-glycosyl sites and unusual clustering of tryptophan residues

The primary structure of human hemopexin is being deduced from sequence analysis of a series of peptides obtained from chemical and enzymatic digests of the protein. Human hemopexin consists of about 440 amino acid residues. It has five sites of attachment of glucosamine oligosaccharides at the signal sequence of Asn-X-Thr/Ser. A unique structural feature is the virtual blocking of the amino-terminal threonine residue, which is O-linked to a galactosamine oligosaccharide that has not previously been identified in this protein. The galactosamine oligosaccharide and one glucosamine oligosaccharide are located in the amino-terminal region, three of the glucosamine oligosaccharides are in the middle region, and one glucosamine oligosaccharide is in the carboxyl-terminal region of the protein. Two of the five glucosamine oligosaccharides are present in a histidine-rich sequence of the middle region of the protein, in which the histidines flank beta-turns presumably at the surface of hemopexin. Clusters of tryptophan residues occur in four regions, each of which contains three or four tryptophan residues separated by 0-12 other residues. This clustering is significant because both histidine and tryptophan have been implicated in the binding of heme. A computer analysis did not identify significant matches of human hemopexin to any protein, including cytochromes and other heme-binding proteins, which suggests that the human hemopexin gene evolved from a unique primordial gene differing from those of other heme-binding proteins.

Haematoporphyrin and OO'-diacetylhaematoporphyrin binding by serum and cellular proteins. Implications for the clearance of these photochemotherapeutic agents by cells

Haematoporphyrin derivative (HpD), a mixture of porphyrins, is currently used as a photochemotherapeutic agent in the treatment of neoplasias. The interaction of purified components of HpD with serum and cellular proteins was investigated using absorption and fluorescence spectroscopy. The interactions of haematoporphyrin and OO'-diacetylhaematoporphyrin with human albumin and with haemopexin, the two major serum porphyrin-binding proteins, show stoichiometries of 1 mol of porphyrin bound per mol of protein. The apparent dissociation constants, Kd, are in the range of 1-2 microM for albumin and 3-4 microM for haemopexin. These two major components of HpD would, after intravenous injection, bind to albumin and circulate in serum as albumin complexes. Free porphyrin rather than porphyrin bound to albumin interacts with Morris hepatoma tissue culture cells. A rapid high-affinity saturable transport system operates at free porphyrin concentrations of less than 2 microM. In addition, fluorescence spectra show that components in rat liver cytosol can bind haematoporphyrin and OO'-diacetylhaematoporphyrin and distinguish these binders from those present in rat serum.

In vitro studies on some parameters of the binding of the rat hemopexin--heme complex with the hepatic membrane receptor

The binding of [125I]Hpx--heme with the rat hepatic plasma membrane receptor was studied at 37 degrees C as well as different parameters such as plasma membrane concentration, calcium dependence, optimal pH and specific binding. A Scatchard plot revealed the existence of one binding for [125I]Hpx--heme on the isolated liver plasma membrane with a Kd = 3.2 X 10(-8) M.

Characterization of transferrin binding and specificity of the placental transferrin receptor

This study systematically examined the characteristics of specific binding of adult diferric transferrin to its receptor using a Triton X-100 solubilized preparation from human placentas as the receptor source. The following information was obtained. The ionic strength for maximal binding is in the range of 0.1-0.3 M NaCl. The pH optimum for specific binding extends over the range, from pH 6.0-10.0. Specific binding of diferric transferrin is not affected by 2.5 approximately 50 mM CaCl2 or by 10 mM EDTA. Triton X-100 in the concentration range of 0.02-3.0% does not affect specific binding. Specific binding is saturated within 10 min at 25 or 37 degrees C in the presence of excess amounts of diferric transferrin. The binding is reversible and the dissociation of diferric transferrin from the transferrin receptor is complete within 40 min at 25 degrees C. Apotransferrin, both adult and fetal, showed less binding than the holotransferrin species by competitive binding assay in the presence of 10 mM EDTA independent of up to 20 mM CaCl2. A 1500-fold molar excess of adult and fetal apotransferrin is required to give 40% inhibition for 125I-labeled diferric transferrin binding. Since calcium ion is not a factor, and since apotransferrin has such high binding affinity for iron (Ka = 1 X 10(24], this experiment suggests that the EDTA was necessary to prevent conversion of apotransferrin to holotransferrin from available iron in the reaction system. The specificity of the transferrin receptor for transferrin was examined by competitive binding studies in which 125I-diferric transferrin binding was measured in the presence of a series of other proteins. The proteins tested in the competitive binding studies were classified into three groups; in the first group were human serum albumin and ovalbumin; in the second group were proteins containing iron ions, such as hemoglobin, hemoglobin-haptoglobin complex, heme-hemopexin complex, ferritin, and diferric lactoferrin; in the third group were the metal-binding serum proteins, ceruloplasmin and metallothionein. None of these proteins except ferritin showed inhibition of diferric transferrin binding to the receptor. The effect of ferritin was small since a 700- to 1500-fold molar excess of ferritin is required for 50% inhibition of binding of diferric transferrin to the receptor.

Hemin binding to serum proteins and the catalysis of interprotein transfer

The reaction of hemin (Hm) with human hemopexin (Hx) has been studied in a mixed dimethyl sulfoxide (Me2SO)-water solvent system and in aqueous caffeine solutions. In both media, the kinetics could be described by a single, second-order process: (formula - see text) with k = 1.8 X 10(6) M-1 s-1 in 40% Me2SO-water [pH 7.4, mu = 0.2 M (NaCl)] and k = 3.9 X 10(7) M-1 s-1 in water [pH 7.4 mu = 0.2 M (NaCl), [caffeine] = 0.025 M]. The reaction shows an ionic strength dependence consistent with a residual 1+ to 2+ charge in the vicinity of the binding region of the protein. The kinetics of the transfer of hemin from albumin to hemopexin (formula - see text) were studied as a function of concentration, ionic strength, pH, and temperature. In experiments conducted at 3 less than or equal to [Alb]0/[Hx]0 less than or equal to 20 where the transfer kinetics are first order, k' = 5 X 10(-3) S-1 at mu = 0.3 M (NaCl), pH 7.1; the reaction is strongly dependent on ionic strength and choice of electrolyte. The addition of imidazole catalyzes this transfer process via a ligand-mediated pathway with k' = 5 X 10(-3) + 21[Im]T2. At [Alb]0/[Hx]0 = 92, the noncatalyzed transfer reaction is second order. From the kinetic analysis of the reaction under these conditions, an estimate is made of the distribution of hemin between the two proteins at concentration levels which are characteristic of serum. The association of hemin and hemopexin is approximately 30 times faster than that of hemin and albumin, a finding consistent with the recycling function of hemopexin during heme transport to the liver parenchymal cells.

Transferrin protein and iron uptake by cultured hepatocytes

The binding and uptake of 59Fe-loaded 3H-labelled rat transferrin by cultured rat hepatocytes was investigated. At 4 degrees C, there is no evidence for a specific binding of transferrin which could be related to the association of neo-synthesized transferrin with plasma membrane receptors. At 37 degrees C, iron uptake is much more important than transferrin uptake; it proceeds linearly over the time of incubation, is largely proportional to the extracellular transferrin concentration, and is compatible with uptake by fluid phase endocytosis. The difference observed between iron and transferrin uptake implies the existence of a mechanism allowing the reutilization of transferrin after iron delivery.

Purification of pig serum haemopexin by haemin-Sepharose affinity chromatography

Haemopexin was isolated from pig serum in pure form by affinity chromatography and ion-exchange chromatography. The affinity gel synthesized contained about 0.3 mumol/ml haemin covalently linked to AH-Sepharose 4B. The molecular weight of the protein was measured by polyacrylamide gel electrophoresis in the presence of SDS followed by sensitive silver staining. This procedure and also immunoelectrophoretic studies indicated purity. The mobility of haemopexin in polyacrylamide gels changed following reduction with dithiothreitol.

Fluorimetric study of the binding of protoporphyrin to haemopexin and albumin

Fluorescence spectra of protoporphyrin bound to its most affinitive site on human serum albumin, bound to human haemopexin and dissolved in human plasma reveal that, when present in plasma, at least 90% of this porphyrin is bound to albumin. Human serum albumin binds protoporphyrin with an affinity KA = 3 X 10(9)M-1 in phosphate-buffered saline. The affinity of haemopexin for protoporphyrin is 4 times smaller. From these data it is concluded that less than 1% of plasma protoporphyrin is bound to haemopexin. Implications of the data for protoporphyrin transport and clearance are discussed.

Duchenne muscular dystrophy carrier detection using logistic discrimination: serum creatine kinase and hemopexin in combination

Creatine kinase (CK) activity and hemopexin concentration were measured in 208 serum samples from 104 normal females and 22 obligate carriers of Duchenne muscular dystrophy (DMD) 20-40 years old. Logistic discrimination was used to assess the effectiveness of the parameters alone or in combination in identifying DMD carriers. In this approach, a serum sample with particular CK, hemopexin, or a combination of CK and hemopexin values is given a probability that if drawn at random from a defined mixture of controls and carriers, it comes from a carrier. The carrier probability based on the biochemical tests can be directly combined with the carrier probability determined from a woman's pedigree to yield a final posterior probability that she is a carrier. When CK and hemopexin were considered individually, 65 and 27% of the carriers, respectively, could be distinguished from 95% of the controls. When the two tests were used in combination, 82% of the carriers could be distinguished from 95% of the controls. When the two-test method was applied to 93 possible carriers, 35 women were classified as carriers, whereas only 29 were identified using CK alone. This method can be extended to include other variables in order to further improve the identification of DMD carriers. It can also be applied to carrier detection in other genetic disorders.

Destruction of erythroleukemia, myelocytic leukemia and Burkitt lymphoma cells by photoactivated protoporphyrin

The effect of protoporphyrin on erythroid, myeloid and lymphoid leukemic cells and their destruction induced by the photoactivated porphyrin was studied. Friend erythroleukemic cells (FL) and myelocytic leukemic cells (ML) accumulated protoporphyrin in a cap or patch-like pattern observed by fluorescence microscopy. Photoactivated protoporphyrin induced the appearance of "holes" on the cell membrane demonstrated by scanning electron microscopy. On the other hand, Burkitt lymphoma (BL) and mastocytoma (MS) cells accumulated porphyrin intracellularly around the nuclear envelope and as circular profiles, respectively. Photoactivated protoporphyrin induced development of multiple blebs on the cell membrane, and even complete cell destruction. Cytotoxicity of protoporphyrin at short-term incubation periods was determined by [3H]thymidine and [3H]uridine incorporation. Protoporphyrin, unexposed to light, reduced the incorporation of both precursors only to a moderate extent. On the other hand, porphyrin-treated cells exposed to light showed complete inhibition of RNA and DNA synthesis. Long-term exposure of ML and BL cells to porphyrin in the dark induced a nearly 50% inhibition of RNA and DNA synthesis. Although the cytotoxic effect of protoporphyrin in the dark was lower than that of photoactivated porphyrin, this may possess a potential activity in vivo even without illumination.