Hemopexin-mediated transport of heme into isolated rat hepatocytes

Hepatic injury in chronic iron overload. Role of lipid peroxidation

In both hereditary hemochromatosis and in the various forms of secondary hemochromatosis, there is a pathologic expansion of body iron stores due mainly to an increase in absorption of dietary iron. Excess deposition of iron in the parenchymal tissues of several organs (e.g. liver, heart, pancreas, joints, endocrine glands) results in cell injury and functional insufficiency. In the liver, the major pathological manifestations of chronic iron overload are fibrosis and ultimately cirrhosis. Evidence for hepatotoxicity due to iron has been provided by several clinical studies, however the specific pathophysiologic mechanisms for hepatocellular injury and hepatic fibrosis in chronic iron overload are poorly understood. The postulated mechanisms of liver injury in chronic iron overload include (a) increased lysosomal membrane fragility, perhaps mediated by iron-induced lipid peroxidation, (b) peroxidative damage to mitochondria and microsomes resulting in organelle dysfunction, (c) a direct effect of iron on collagen biosynthesis and (d) a combination of all of the above.

The hemopexin gene maps to the same location as the beta-globin gene cluster on human chromosome 11

Using human hemopexin cDNA clones isolated from lambda gt11 cDNA library as probes, we have carried out Southern blot analysis of a series of human-Chinese hamster somatic cell hybrids containing different combinations of human chromosomes. Synteny analysis revealed 100% concordance between the hemopexin gene and human chromosome 11. In situ hybridization of 3H-labeled hemopexin cDNA to metaphase chromosomes prepared from human lymphocytes further localized the gene to the region p15.4-p15.5, the same location as the beta-globin gene cluster.

Hemopexin is localized to human chromosome 11

Hemopexin, a plasma protein that migrates during electrophoresis with the beta-globulins, transports free heme to sites of its catabolism in the liver. A hemopexin cDNA clone has been utilized for mapping the hemopexin (HPX) gene to human chromosome 11 in the region pter----p11 by somatic cell hybrid analysis.

Fate of haem after parenteral administration of haem arginate to rabbits

Rabbits were injected either intravenously or intramuscularly with [14C]haem arginate and [59Fe]haem arginate (haem 5 mg kg-1). The main part (80%) of AUCINF of labelled haem was associated with the beta-phase, T1/2 being about 6 h. Only 1% of the haem dose had been taken up by the red blood cells. In contrast, the iron moiety from the haem molecule was effectively utilized. Thirty days post-injection of [59Fe]haem arginate, 40% of the dose after intravenous injection and 60% after intramuscular injection was circulating with the red cells. Radioactivity was shown to concentrate in the liver, where haem is mainly metabolized and eliminated. An accumulation of haem in the adrenals was also evident. Haem itself did not concentrate in the bone marrow, and a negligible amount of radioactivity was recovered from brain, implying a poor penetration of the blood brain barrier.

Bovine serum hemopexin: properties of the protein from a single animal

1. Hemopexin was isolated from bovine serum of a single animal in a yield of 0.5 mg/ml. 2. Bovine hemopexin was found to exist in two isoforms of mol. wt 68,000 and 65,000. 3. Treatment of hemopexin with glycopeptidase F yields a single band corresponding to a mol. wt of 51,000. 4. The protein binds heme on an equimolar ratio and shows a single component in reverse-phase high performance liquid chromatography. 5. The amino acid composition of bovine hemopexin compares with that of hemopexin isolated form other animals.

A rosette receptor assay with haem-microbeads. Demonstration of a haem receptor on K562 cells

A rosette assay was developed for the detection of haem receptor-bearing cells. Indicator particles were prepared by covalent binding of haem to acrylic microbeads. The new method was tested on K562 human erythroleukaemia cells, known to take up haem. In tests on several batches, 80-90% of the K562 cells were rosetted with haem-microbeads whereas mature erythrocytes were haem receptor-negative. Rosette formation was inhibited in a dose-dependent manner by micromolar concentrations of free haemin but not by albumin or transferrin. Uncoated microbeads or albumin-coated microbeads did not attach to K562 cells but transferrin-coated microbeads rosetted 50-70% of them. Diferric transferrin inhibited these rosettes, but haemin had no effect.

An avian serum alpha 1-glycoprotein, hemopexin, differing significantly in both amino acid and carbohydrate composition from mammalian (beta-glycoprotein) counterparts

We report here on physicochemical characteristics of chicken hemopexin, which can be isolated by heme-agarose affinity chromatography [Tsutsui, K., & Mueller, G. C. (1982) J. Biol. Chem. 257, 3925-3931], in comparison with representative mammalian hemopexins of rat, rabbit, and human. The avian polypeptide chain appears to be slightly longer (52 kDa) than the human, rat, or rabbit forms (49 kDa), and also the glycoprotein differs from the mammalian hemopexins in being an alpha 1-glycoprotein instead of a beta 1-glycoprotein. This distinct electrophoretic mobility probably arises from significant differences in the amino acid composition of the chicken form, which, although lower in serine and particularly in lysine, has a much higher glutamine/glutamate and arginine content, and also a higher proline, glycine, and histidine content, than the mammalian hemopexins. Compositional analyses and 125I concanavalin A and 125I wheat germ agglutinin binding suggest that chicken hemopexin has a mixture of three fucose-free N-linked bi- and triantennary oligosaccharides. In contrast, human hemopexin has five N-linked oligosaccharides and an additional O-linked glycan blocking the N-terminal threonine residue [Takahashi, N., Takahashi, Y., & Putnam, F. W. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 2021-2025], while the rabbit form has four N-linked oligosaccharides [Morgan, W. T., & Smith, A. (1984) J. Biol. Chem. 259, 12001-12006]. In keeping with the finding of a simpler carbohydrate structure, the avian hemopexin exhibits only a single band on polyacrylamide gel electrophoresis under both nondenaturing and denaturing conditions, whereas the hemopexins of the three mammalian species tested show several bands.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of secretion of proteins into bile: studies in the perfused rat liver

Employing the in situ perfused rat liver, we examined the origins and mechanisms of transport of proteins into bile. First, utilizing polyacrylamide gels, we noted that many biliary proteins co-migrated with dominant serum proteins. Upon liver perfusion with serum-free medium, most proteins disappeared from the biliary profile; one major biliary protein that was not present in serum, identified as secretory component, remained. Kinetic analysis of the disappearance half-lives of the biliary proteins suggested that some serum proteins enter bile by a slow (20 to 30 min; transcellular) route, while others utilize both slow and rapid (5 min; paracellular) routes. In biosynthetic labeling experiments, secretion of newly synthesized proteins into bile was delayed about 20 min when compared with secretion of proteins into the perfusion medium and comprised less than 1% of the total secreted proteins. When a new liver was inserted into the perfusion medium containing newly synthesized secreted proteins, only two proteins, hemopexin and an unidentified protein, were transported into the bile from the perfusion medium; other biliary proteins were presumed to come directly from the hepatocyte. This latter group included some proteins that were secreted into the perfusion medium as well as into bile, and others, e.g., secretory component, that were secreted only into bile. Based on our results we have defined six pathways for entry of proteins into bile.

The effect of metalloporphyrins and heme liposomes on delta-aminolevulinate synthase activity in rat liver

Heme administration causes inhibition of delta-aminolevulinate synthase (ALAS), best tested in the allylisopropylacetamide (AIA)-treated rat, a model for hepatic porphyrias. Because heme suspended in aqueous media (for injection) is unstable and has adverse effects on coagulation, alternate therapeutic modalities are being explored. The present study tries to answer two questions: 1) are any heme analogs as effective inhibitors of ALAS as heme is; and 2) does heme administration in the form of liposomes increase its effectiveness? None of the liposome compositions tested, even if containing lactosylceramide for preferential hepatocyte uptake, was more effective in inhibiting AIA-induced ALAS activity than heme in buffer. As for the function of the heme analogs, although deuteroheme and heme dimethyl ester proved ineffective, mesoheme and cobalt protoporphyrin were nearly as effective as heme itself, indicating that both hydrophobic side chains in positions 2 and 4 and free propionate groups at 6 and 7 are essential for ALAS inhibition, as is the presence of a central cobalt or iron atom.

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.

Structural studies of glycans isolated from rat plasma hemopexin

After exhaustive pronase digestion, purification by gel filtration and affinity chromatography on concanavalin A, three glycopeptide fractions were obtained from rat hemopexin. Two fractions (I and II) were concanavalin A non-reactive and one (III) was concanavalin A reactive. On the basis of carbohydrate composition, methylation analysis and proton nuclear magnetic resonance spectroscopy, the primary structure of the glycan in fraction III is proposed as being a mixture of mono- and di-sialo-diantennae of the N-glycosidic, N- acetyllactosamine type. Hydrazinolysis of glycopeptides not binding to concanavalin A yielded mixtures of oligosaccharides for both fractions. These oligosaccharides were separated by HPLC; the molar composition of each of them is given. These data suggest that rat hemopexin contains, among others, a diantennary structure bearing three sialic acid residues.