Disposal of plasma heme in normal man and patients with intravascular hemolysis

The anticoagulant effect of Apis mellifera phospholipase A2 is inhibited by CORM-2 via a carbon monoxide-independent mechanism

Bee venom phospholipase A₂ (PLA₂) has potential for significant morbidity. Ruthenium (Ru)-based carbon monoxide releasing molecules (CORM) inhibit snake venoms that are anticoagulant and contain PLA₂. In addition to modulating heme-bearing proteins with carbon monoxide, these CORM generate reactive Ru species that form adducts with histamine residues resulting in changes in protein function. This study sought to identify anticoagulant properties of bee venom PLA₂ via catalysis of plasma phospholipids required for thrombin generation. Another goal was to determine if Ru-based CORM inhibit bee venom PLA₂ via carbon monoxide release or via potential binding of reactive Ru species to a key histidine residue in the catalytic site of the enzyme. Anticoagulant activity of bee venom PLA₂ was assessed via thrombelastography with normal plasma. Bee venom PLA₂ was then exposed to different CORM and a metheme forming agent and anticoagulant activity was reassessed. Using Ru, boron and manganese-based CORM and a metheme forming agent, it was demonstrated that it was unlikely that carbon monoxide interaction with a heme group attached to PLA₂ was responsible for inhibition of anticoagulant activity by Ru-based CORM. Exposure of PLA₂ to a Ru-based CORM in the presence of histidine-rich human albumin resulted in loss of inhibition of PLA₂. Ru-based CORM likely inhibit bee venom PLA₂ anticoagulant activity via formation of reactive Ru species that bind to histidine residues of the enzyme.

Apohemoglobin-haptoglobin complex attenuates the pathobiology of circulating acellular hemoglobin and heme

Haptoglobin (Hp) is the plasma protein that binds and clears cell-free hemoglobin (Hb), whereas apohemoglobin (apoHb, i.e., Hb devoid of heme) can bind heme. Therefore, the apoHb-Hp protein complex should facilitate holoHb-apoHb αβ-dimer exchange and apoHb-heme intercalation. Thus, we hypothesized that apoHb-Hp could facilitate both Hb and heme clearance, which, if not alleviated, could have severe microcirculatory consequences. In this study, we characterized apoHb-Hp and Hb/heme ligand interactions and assessed their in vivo consequences. Hb exchange and heme binding with the apoHb-Hp complex was studied with transfer assays using size-exclusion high-performance liquid chromatography coupled with UV-visible spectrophotometry. Exchange/transfer experiments were conducted in guinea pigs dosed with Hb or heme-albumin followed by a challenge with equimolar amounts of apoHb-Hp. Finally, systemic and microcirculatory parameters were studied in hamsters instrumented with a dorsal window chamber via intravital microscopy. In vitro and in vivo Hb exchange and heme transfer experiments demonstrated proof-of-concept Hb/heme ligand transfer to apoHb-Hp. Dosing with the apoHb-Hp complex reversed Hb- and heme-induced systemic hypertension and microvascular vasoconstriction, reduced microvascular blood flow, and diminished functional capillary density. Therefore, this study highlights the apoHb-Hp complex as a novel therapeutic strategy to attenuate the adverse systemic and microvascular responses to intravascular Hb and heme exposure.NEW & NOTEWORTHY This study highlights the apoHb-Hp complex as a novel therapeutic strategy to attenuate the adverse systemic and microvascular responses to intravascular Hb and heme exposure. In vitro and in vivo Hb exchange and heme transfer experiments demonstrated proof-of-concept Hb/heme ligand transfer to apoHb-Hp. The apoHb-Hp complex reverses Hb- and heme-induced systemic hypertension and microvascular vasoconstriction, preserves microvascular blood flow, and functional capillary density. In summary, the unique properties of the apoHb-Hp complex prevent adverse systemic and microvascular responses to Hb and heme-albumin exposure and introduce a novel therapeutic approach to facilitate simultaneous removal of extracellular Hb and heme.

Iron Supplementation Alters Heme and Heme Oxygenase 1 (HO-1) Levels In Pregnant Women in Ghana

BACKGROUND

Iron supplementation is recommended for pregnant women to meet their iron requirement for a healthy pregnancy. The benefits and risks of universal iron supplementation during pregnancy in malaria endemic countries are currently being debated. As part of a broader study that focused on the effect of heme/HO-1 on pregnancy outcomes in malaria in pregnancy, we determined the association between iron supplementation and free heme levels in blood of pregnant women with and without malaria in Ghana. We hypothesized that pregnant women with malaria who took iron supplements will have higher levels of Heme/HO-1 than those who did not take iron supplements.

METHODS

A total of 337 women were recruited for this study. Blood samples were collected for malaria diagnosis and heme/HO-1 measurement. Quantification of heme was done using a heme colorimetric assay kit and HO-1 levels were performed using Enzyme-Linked Immunosorbent Assay (ELISA) on plasma samples.

RESULTS

Malaria positive iron supplemented women, in their third trimester, had significantly higher median levels of heme 59.3(43.1 - 60.4) than non-malaria iron supplemented women 35.7(33.0 - 62.2), p = 0.026. Also, malaria positive iron supplemented women had significant higher median levels of HO-16.2(IQR 4.9 - 8.1) than pregnant women who did not take iron supplements 2.9 (IQR 2.1 - 3.8), p = <0.001.

CONCLUSION

Although iron supplementation may be highly beneficial and improve pregnancy outcomes for iron deficient or anemic mothers, it is also likely that iron supplementation for pregnant women who are not iron deficient may put this group of women at risk for adverse pregnancy outcomes. Findings from this study sheds light on the effect of iron supplementation on malaria derived heme in pregnancy, which may inform how iron supplementation is recommended for pregnant women who are not iron deficient.

Heme: Modulator of Plasma Systems in Hemolytic Diseases

Hemolytic diseases such as sickle-cell disease, β-thalassemia, malaria, and autoimmune hemolytic anemia continue to present serious clinical hurdles. In these diseases, lysis of erythrocytes causes the release of hemoglobin and heme into plasma. Extracellular heme has strong proinflammatory potential and activates immune cells and endothelium, thus contributing to disease pathogenesis. Recent studies have revealed that heme can interfere with the function of plasma effector systems such as the coagulation and complement cascades, in addition to the activity of immunoglobulins. Any perturbation in such functions may have severe pathological consequences. In this review we analyze heme interactions with coagulation, complement, and immunoglobulins. Deciphering such interactions to better understand the complex pathogenesis of hemolytic diseases is pivotal.

Hemopexin and haptoglobin: allies against heme toxicity from hemoglobin not contenders

The goal here is to describe our current understanding of heme metabolism and the deleterious effects of "free" heme on immunological processes, endothelial function, systemic inflammation, and various end-organ tissues (e.g., kidney, lung, liver, etc.), with particular attention paid to the role of hemopexin (HPX). Because heme toxicity is the impetus for much of the pathology in sepsis, sickle cell disease (SCD), and other hemolytic conditions, the biological importance and clinical relevance of HPX, the predominant heme binding protein, is reinforced. A perspective on the function of HPX and haptoglobin (Hp) is presented, updating how these two proteins and their respective receptors act simultaneously to protect the body in clinical conditions that entail hemolysis and/or systemic intravascular (IVH) inflammation. Evidence from longitudinal studies in patients supports that HPX plays a Hp-independent role in genetic and non-genetic hemolytic diseases without the need for global Hp depletion. Evidence also supports that HPX has an important role in the prognosis of complex illnesses characterized predominantly by the presence of hemolysis, such as SCD, sepsis, hemolytic-uremic syndrome, and conditions involving IVH and extravascular hemolysis (EVH), such as that generated by extracorporeal circulation during cardiopulmonary bypass (CPB) and from blood transfusions. We propose that quantitating the amounts of plasma heme, HPX, Hb-Hp, heme-HPX, and heme-albumin levels in various disease states may aid in the diagnosis and treatment of the above-mentioned conditions, which is crucial to developing targeted plasma protein supplementation (i.e., "replenishment") therapies for patients with heme toxicity due to HPX depletion.

Heme-induced neutrophil extracellular traps contribute to the pathogenesis of sickle cell disease

Sickle cell disease (SCD) is characterized by recurring episodes of vascular occlusion in which neutrophil activation plays a major role. The disease is associated with chronic hemolysis with elevated cell-free hemoglobin and heme. The ensuing depletion of heme scavenger proteins leads to nonspecific heme uptake and heme-catalyzed generation of reactive oxygen species. Here, we have identified a novel role for heme in the induction of neutrophil extracellular trap (NET) formation in SCD. NETs are decondensed chromatin decorated by granular enzymes and are released by activated neutrophils. In humanized SCD mice, we have detected NETs in the lungs and soluble NET components in plasma. The presence of NETs was associated with hypothermia and death of these mice, which could be prevented and delayed, respectively, by dismantling NETs with DNase I treatment. We have identified heme as the plasma factor that stimulates neutrophils to release NETs in vitro and in vivo. Increasing or decreasing plasma heme concentrations can induce or prevent, respectively, in vivo NET formation, indicating that heme plays a crucial role in stimulating NET release in SCD. Our results thus suggest that NETs significantly contribute to SCD pathogenesis and can serve as a therapeutic target for treating SCD.

Pathological conditions involving extracellular hemoglobin: molecular mechanisms, clinical significance, and novel therapeutic opportunities for α(1)-microglobulin

Hemoglobin (Hb) is the major oxygen (O(2))-carrying system of the blood but has many potentially dangerous side effects due to oxidation and reduction reactions of the heme-bound iron and O(2). Extracellular Hb, resulting from hemolysis or exogenous infusion, is shown to be an important pathogenic factor in a growing number of diseases. This review briefly outlines the oxidative/reductive toxic reactions of Hb and its metabolites. It also describes physiological protection mechanisms that have evolved against extracellular Hb, with a focus on the most recently discovered: the heme- and radical-binding protein α(1)-microglobulin (A1M). This protein is found in all vertebrates, including man, and operates by rapidly clearing cytosols and extravascular fluids of heme groups and free radicals released from Hb. Five groups of pathological conditions with high concentrations of extracellular Hb are described: hemolytic anemias and transfusion reactions, the pregnancy complication pre-eclampsia, cerebral intraventricular hemorrhage of premature infants, chronic inflammatory leg ulcers, and infusion of Hb-based O(2) carriers as blood substitutes. Finally, possible treatments of these conditions are discussed, giving a special attention to the described protective effects of A1M.

Heme interacts with c1q and inhibits the classical complement pathway

C1q is the recognition subunit of the first component of the classical complement pathway. It participates in clearance of immune complexes and apoptotic cells as well as in defense against pathogens. Inappropriate activation of the complement contributes to cellular and tissue damage in different pathologies, urging the need for the development of therapeutic agents that are able to inhibit the complement system. In this study, we report heme as an inhibitor of C1q. Exposure of C1q to heme significantly reduced the activation of the classical complement pathway, mediated by C-reactive protein (CRP) and IgG. Interaction analyses revealed that heme reduces the binding of C1q to CRP and IgG. Furthermore, we demonstrated that the inhibition of C1q interactions results from a direct binding of heme to C1q. Formation of complex of heme with C1q caused changes in the mechanism of recognition of IgG and CRP. Taken together, our data suggest that heme is a natural negative regulator of the classical complement pathway at the level of C1q. Heme may play a role at sites of excessive tissue damage and hemolysis where large amounts of free heme are released.

Transitory renal failure following rapid administration of a relatively large amount of hematin in a patient with acute intermittent porphyria in clinical remission

Transitory renal failure occurred in a patient with acute intermittent porphyria in clinical remission following i.v. administration of 1 000 mg hematin. The clinical and biochemical picture suggested "acute tubular necrosis", which was followed by a prompt and complete return of renal function without any late sequelae. The renal failure is thought to have resulted from the presence of circulating free hematin, formed as a result of rapid administration of such a relatively large amount. Such a complication has not occurred in patients given hematin for acute porphyric relapse, in whom much smaller amounts have been infused.

Functional variability of antibodies upon oxidative processes

Reactive oxygen species (ROS) released from activated phagocytes are involved in the innate immune defense against pathogens. However, when released in excess and when the antioxidant systems are impaired, ROS may induce cellular and tissue damage and dissociation of iron ions or iron containing compounds (heme) from protein-bound state. Free iron ions and free heme are prooxidative. Immunoglobulins usually perform their biological functions at sites of inflammation, where they may encounter reactive oxygen species and/or redox active compounds. It has been demonstrated that the exposure of some antibodies to heme, to transition metal ions or to reactive oxygen species induces an appearance of new binding specificities for various autoantigens. This review article is devoted to the interplay between redox active agents and antibodies. The biological significance of the appearance of new antigen binding specificities on antibodies after exposure to redox-active agents is discussed.

Antibodies Use Heme as a Cofactor to Extend Their Pathogen Elimination Activity and to Acquire New Effector Functions

Various pathological processes are accompanied by release of high amounts of free heme into the circulation. We demonstrated by kinetic, thermodynamic, and spectroscopic analyses that antibodies have an intrinsic ability to bind heme. This binding resulted in a decrease in the conformational freedom of the antibody paratopes and in a change in the nature of the noncovalent forces responsible for the antigen binding. The antibodies use the molecular imprint of the heme molecule to interact with an enlarged panel of structurally unrelated epitopes. Upon heme binding, monoclonal as well as pooled immunoglobulin G gained an ability to interact with previously unrecognized bacterial antigens and intact bacteria. IgG-heme complexes had an enhanced ability to trigger complement-mediated bacterial killing. It was also shown that heme, bound to immunoglobulins, acted as a cofactor in redox reactions. The potentiation of the antibacterial activity of IgG after contact with heme may represent a novel and inducible innate-type defense mechanism against invading pathogens.

Iron Ions and Haeme Modulate the Binding Properties of Complement Subcomponent C1q and of Immunoglobulins

The complement system and circulating antibodies play a major role in the defence against infection. They act at the sites of inflammation, where the harsh microenvironment and the oxidative stress lead to the release of free iron ions and haeme. The aim of this study was to analyse the consequences of the exposure of C1q and immunoglobulins to iron ions or haeme. The changes in target recognition by C1q and in the rheumatoid factor activity of the immunoglobulins were investigated. The exposure of C1q to ferrous ions increased its binding to IgG and to IgM. In contrast, haeme inhibited C1q binding to all studied targets, especially to IgG1 and C-reactive protein. Thus, the haeme released as a result of tissue damage and oxidative stress may act as a negative feedback regulator of an inappropriate complement triggering as seen in ischaemia-reperfusion tissue injury. The results also show that iron ions and haeme were able to reveal rheumatoid factor activity of IgG. The modulation of the C1q-target binding as well as the revealing of rheumatoid factor activity of IgG by exposure to redox-active agents released at the sites of inflammation may have important consequences for the understanding of the immunopathological mechanisms of inflammatory and autoimmune diseases.

Mechanism of horseradish peroxidase-catalyzed heme oxidation and polymerization (β-hematin formation)

Horseradish peroxidase (HRP) catalyzes the polymerization of free heme (beta-hematin formation) through its oxidation. Heme when added to HRP compound II (FeIV=O) causes spectral shift from 417 nm (Compound II) to 402 nm (native, FeIII) indicating that heme may be oxidized via one-electron transfer. Direct evidence for one-electron oxidation of heme by HRP intermediates is provided by the appearance of an E.s.r signal of a 5,5-dimethyl-1-pyrroline N-oxide (spin trap)-heme radical adduct (a1H=14.75 G, a2H=4.0 G) in E.s.r studies. Heme-polymerization by HRP is inhibited by spin trap indicating that one-electron oxidation product of heme ultimately leads to the formation of heme-polymer. HRP, when incubated with diethyl pyrocarbonate (DEPC), a histidine specific reagent, shows concentration dependent loss of heme-polymerization indicating the role of histidine residues in the process. We suggest that HRP catalyzes the formation of heme-polymer through one-electron oxidation of free heme.

Free heme toxicity and its detoxification systems in human

Severe hemolysis or myolysis occurring during pathological states, such as sickle cell disease, ischemia reperfusion, and malaria results in high levels of free heme, causing undesirable toxicity leading to organ, tissue, and cellular injury. Free heme catalyzes the oxidation, covalent cross-linking and aggregate formation of protein and its degradation to small peptides. It also catalyzes the formation of cytotoxic lipid peroxide via lipid peroxidation and damages DNA through oxidative stress. Heme being a lipophilic molecule intercalates in the membrane and impairs lipid bilayers and organelles, such as mitochondria and nuclei, and destabilizes the cytoskeleton. Heme is a potent hemolytic agent and alters the conformation of cytoskeletal protein in red cells. Free heme causes endothelial cell injury, leading to vascular inflammatory disorders and stimulates the expression of intracellular adhesion molecules. Heme acts as a pro-inflammatory molecule and heme-induced inflammation is involved in the pathology of diverse conditions; such as renal failure, arteriosclerosis, and complications after artificial blood transfusion, peritoneal endometriosis, and heart transplant failure. Heme offers severe toxic effects to kidney, liver, central nervous system and cardiac tissue. Although heme oxygenase is primarily responsible to detoxify free heme but other extra heme oxygenase systems also play a significant role to detoxify heme. A brief account of free heme toxicity and its detoxification systems along with mechanistic details are presented.

Neutrophil activation by heme: implications for inflammatory processes

Heme, a ubiquitous iron-containing compound, is present in large amounts in many cells and is inherently dangerous, particularly when it escapes from intracellular sites. The release of heme from damaged cells and tissues is supposed to be higher in diseases such as malaria and hemolytic anemia or in trauma and hemorrhage. We investigated here the role of free ferriprotoporphyrin IX (hemin) as a proinflammatory molecule, with particular attention to its ability to activate neutrophil responses. Injecting hemin into the rat pleural cavity resulted in a dose-dependent migration of neutrophils, indicating that hemin is able to promote the recruitment of these cells in vivo. In vitro, hemin induced human neutrophil chemotaxis and cytoskeleton reorganization, as revealed by the increase of neutrophil actin polymerization. Exposure of human neutrophils to 3 microM hemin activated the expression of the chemokine interleukin-8, as demonstrated by quantitative reverse-transcription polymerase chain reaction, indicating a putative molecular mechanism by which hemin induces chemotaxis in vivo. Brief incubation of human neutrophils with micromolar concentrations of hemin (1-20 microM) triggered the oxidative burst, and the production of reactive oxygen species was directly proportional to the concentration of hemin added to the cells. Finally, we observed that human neutrophil protein kinase C was activated by hemin in vitro, with a K(1/2) of 5 microM. Taken together, these results suggest a role for hemin as a proinflammatory agent able to induce polymorphonuclear neutrophil activation in situations of clinical relevance, such as hemolysis or hemoglobinemia.

Cellular protection mechanisms against extracellular heme. heme-hemopexin, but not free heme, activates the N-terminal c-jun kinase

Hemopexin protects cells lacking hemopexin receptors by tightly binding heme abrogating its deleterious effects and preventing nonspecific heme uptake, whereas cells with hemopexin receptors undergo a series of cellular events upon encountering heme-hemopexin. The biochemical responses to heme-hemopexin depend on its extracellular concentration and range from stimulation of cell growth at low levels to cell survival at otherwise toxic levels of heme. High (2-10 microM) but not low (0.01-1 microM) concentrations of heme-hemopexin increase, albeit transiently, the protein carbonyl content of mouse hepatoma (Hepa) cells. This is due to events associated with heme transport since cobalt-protoporphyrin IX-hemopexin, which binds to the receptor and activates signaling pathways without tetrapyrrole transport, does not increase carbonyl content. The N-terminal c-Jun kinase (JNK) is rapidly activated by 2-10 microM heme-hemopexin, yet the increased intracellular heme levels are neither toxic nor apoptotic. After 24 h exposure to 10 microM heme-hemopexin, Hepa cells become refractory to the growth stimulation seen with 0.1-0.75 microM heme-hemopexin but HO-1 remains responsive to induction by heme-hemopexin. Since free heme does not induce JNK, the signaling events, like phosphorylation of c-Jun via activation of JNK as well as the nuclear translocation of NFkappaB, G2/M arrest, and increased expression of p53 and of the cell cycle inhibitor p21(WAF1/CIP1/SDI1) generated by heme-hemopexin appear to be of paramount importance in cellular protection by heme-hemopexin.

Myocyte injury by hemin

Effects of free hemin on myocardium were investigated using a model of neonatal myocyte primary cultures. Cells were subjected to free hemin at concentrations up to 20 microM and equilibrated for 5 h at 37 degrees C. Distribution of hemin in media, cell sarcolemma, and cell interior was evaluated. Time-dependent reduction in beating rate was monitored throughout the entire concentration range of administrated hemin. With time and in a hemin concentration-dependent manner, arrhythmic beatings which were followed by loss of contractility were observed. In parallel, morphologic changes appeared from granulation to complete loss of cell integrity. At the concentration range studied, hemin also induced a biphasic release of cytosolic enzymes. In the first phase, the fraction of enzyme released was dependent of the ratio of hemin:cells and was correlated with the amount of nonviable cells as monitored by a trypan blue test. In the second phase, the fraction of released enzyme was much larger than that of nonviable cells. The data are interpreted as an indication of complete loss of cytosolic content due to sarcolemma damage in first phase and partial damage to cell interior in the prolonged second phase. It is concluded that in similarity with other amphipathic molecules, free hemin is toxic to the myocardium.

Novel heme-binding component in the serum of the channel catfish (Ictalurus punctatus)

The serum of the channel catfish (Ictalurus punctatus) was examined for heme- and hemoglobin-binding proteins. Electrophoretic mobility retardation assays failed to detect a hemoglobin-binding material similar to mammalian haptoglobin; however, a heme-binding component (not previously described) was identified in catfish serum. The heme-binding component was purified by gel filtration chromatography; electrophoretic analyses suggested it to be composed of two polypeptide subunits of molecular masses about 115 and 98 kDa. This composition is inconsistent with hemopexin, the known heme-binding serum protein of mammals. Although it was not fully saturated with heme, the catfish component contained detectable heme in normal sera. When complexed by the binding material, heme was used as an iron source by isolates of the bacterial Gram-negative genus Aeromonas; the capacity of other bacteria to use the complex was not tested. The physiological function of the catfish heme-binding serum protein is presently not clear.

Heme inhibits cartilage metabolism and growth in vitro

The present study was designed to investigate the potential role of heme as a direct inhibitor of cartilage metabolism and growth. We used the embryonic chick pelvic rudiment bioassays and the hypophysectomized rat cartilage sulfation bioassay, both sensitive to growth factors and growth inhibitors. In the chick bioassay pelvic rudiment growth was inhibited when heme was added to the culture medium at 0.1 mmol/L (after five days in culture, cartilage weight 224% +/- 11% of initial with normal serum, but 141% +/- 3% of initial with serum plus heme 0.25 mmol/L); the heme-induced growth inhibition was promptly reversible when cartilages were placed in heme-free medium. This was due, at least in part, to heme-induced inhibition of (1) [35S] sulfate incorporation into proteoglycans by as little as 0.05 mmol/L heme (66% +/- 5% of assay buffer); (2) [14C] leucine incorporation into proteins by greater than or equal to 0.05 mmol/L heme (85% +/- 6% of assay buffer); and (3) [3H] uridine incorporation by greater than or equal to 0.10 mmol/L heme (50% +/- 4% of assay buffer). In the rat cartilage sulfation bioassay (the literature "standard" bioassay for skeletal growth studies) a dose-dependent inhibition of [35S] sulfate incorporation occurred with greater than or equal to 0.01 mmol/L added heme (82% +/- 3% of assay buffer). Heme caused a dose-dependent and reversible inhibition of cartilage metabolism and growth, and it may have a novel role in the pathophysiology of growth retardation associated with some chronic disease.

Tetrapyrroles as inhibitors of normal cartilage metabolism: relative potency of different compounds

The present study was designed to explore the role of different tetrapyrroles as inhibitors of cartilage metabolism. We studied the effects of tetrapyrroles on the incorporation of [35S]sulfate into proteoglycans, [14C]-leucine into protein, and [3H]uridine into the RNA of normal cartilage from two different vertebrate classes using the embryonic chicken pelvic rudiment bioassay and the hypophysectomized rat costal cartilage bioassay, both very sensitive to cartilage growth factors and growth inhibitors. We compared the relative potencies of the following compounds: both metalloporphyrins (heme and chlorophyllin), linear tetrapyrroles (bilirubin), and heme proteins (hemoglobin, myoglobin, and cytochrome c). Hemoglobin and heme were the most potent inhibitors of rat cartilage metabolism, and bilirubin was a far more potent inhibitor of embryonic chick cartilage metabolism. Chlorophyllin had moderate inhibitory activity, especially on chick cartilage, whereas cytochrome c was inactive in these bioassays. Surprisingly, myoglobin was relatively ineffective despite its close similarity to heme and hemoglobin. The bilirubin-induced inhibition of sulfate incorporation into chick cartilage was partially prevented when glutathione was included in the incubation medium, suggesting that a free-radical mechanism may be involved. There were significant differences in the sensitivity of the two cartilages studied, indicating there may be species-dependent sensitivity to different tetrapyrroles.

Minimal haemolytic effects from 0.8% chlormethiazole infusion in volunteer subjects. Chlormethiazole infusion and haemolysis

A solution of 0.8% chlormethiazole ethanedisulphonate was infused into the antecubital vein of 14 healthy volunteers for periods ranging from 60 to 120 minutes to examine any haemolytic effects in vivo. Plasma haptoglobin and haemopexin concentrations were measured before, during and after infusion. No evidence of intravascular haemolysis of any clinical significance was found.

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

Rat [(59)Fe]haem-(125)I-labelled haemopexin complexes (700pmol/rat) associate rapidly and exclusively with the liver after intravenous injection into anaesthetized rats. The two isotopes exhibit different patterns of accumulation. Liver (125)I-labelled haemopexin is maximum 10min after injection (20+/-4.9pmol/g of liver) and then declines by 2h to the low values (about 3pmol/g of liver) seen after injection of the apoprotein. In contrast, [(59)Fe]haem accumulates in the liver for at least 2h. Haemopexin undergoes no extensive proteolysis during 2h of haem transport as shown by precipitation with acid (98%) and specific antiserum (92%) and by electrophoresis. Moreover, only 1-2% of the dose is located in extrahepatic tissues, and there is no significant urinary excretion of either (125)I or (59)Fe. Hepatic uptake at 10min is saturable, reaching 200pmol of haemopexin/g of liver and 350pmol of haem/g of liver at a dose of 9nmol/rat, whereas uptake of the apoprotein is 3-5% of the dose. This suggests that the interaction of haem-haemopexin with the liver is a specific receptor-mediated process. The complex probably interacts via the protein moiety, since the haem analogues mesohaem and deuterohaem do not affect association of the protein with the liver but the species of haemopexin does. Increasing amounts of protein are associated with the liver 5min after injection in the order: human>rabbit>rat, and haem uptake is consistently increased. For both rat and rabbit haemopexin saturation is reached at the same concentration of protein, i.e. 180-200pmol/g of liver, indicating that the different protein species bind to a common receptor. We propose that haemopexin transports haem to the liver by a specific receptor-mediated process and then returns to the circulation.

Immunological cross-reactions between heterologous hemopexins

1. The degree of immunological identity of the hemopexin of 64 mammals and non-mammals is determined by double diffusion in agar and two radioimmunoassays, employing antisera produced with the human or the rabbit protein. 2. Antigenic determinants are shared by the Eutherian mammals but not by the non-Eutherian mammals and lower animals. 3. The hemopexins of man and apes appear to be identical, whereas those of Old World monkeys lack one antigenic determinant, and New World monkeys lack at least two antigenic determinants.

Griseofulvin causing hyperhemopexinemia and hepatic proliferation in mice: an in vivo and in vitro study

Concentrations of hemopexin, a porphyrin-binding serum protein synthesized exclusively in the liver, increased significantly and concomitantly with levels of erythrocyte and liver protoporphyrin and coproporphyrin in mice made porphyric with 1% griseofulvin in the feed. Liver weights of porphyric mice increased remarkably. In comparison with controls, the ratio of the weight of normal to porphyric livers was at 10 days 1:2.1, at 21 days 1:2.8, and at 46 days 1:3.8. This increase in liver size was accompanied by increased cell division. The hepatic hyperplastic tissue fragments survived in vitro for several weeks and could be subcultured. The cultured cells, like those of the original liver, showed intense protoporphyrin fluorescence in the cytoplasm.

Transfer of heme from heme-albumin to hemopexin

Exchange of heme in vitro between two heme-binding serum proteins, albumin and hemopexin, was examined spectrophotometrically. Hemopexin, albumin and heme in molar ratios of 1 : 70 : 1 were incubated at 22 degrees C, pH 7.3. The heme was added as free heme, heme-hemopexin or methemalbumin. Due to the high affinity of hemopexin for heme, Kd near 10(-13) M, only negligible amounts of heme were transferred from hemopexin to albumin in 48 h. However, more than 80% of heme was transferred from methemalbumin to hemopexin within 24 h. Heme added to a 1 : 70 mixture of the apo-proteins is initially bound by albumin; but more than 90% is bound by hemopexin in 24 h. Addition of dithionite causes nearly all of the heme present, whether added as free heme or methemalbumin, to associate with hemopexin in 15 min. Albumin thus appears to have a much lower affinity for ferro- than for ferri-heme. Results obtained from similar experiments with human serum and human serum made hemopexin-free by immunoadsorption fully corroborate those obtained with mixtures of purified albumin and hemopexin. These observations suggest that the rate-limiting step in the heme transport function of hemopexin is the formation of the heme-hemopexin complex, rather than the uptake of the complex by the liver.