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Biomedical subjects

W T Morgan

Publications and source records attributed to W T Morgan.

At least 91 records · Page 5Linked to original sources
PubMed1979

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.

Animals↗
PubMed1978

Human serum histidine-rich glycoprotein. I. Interactions with heme, metal ions and organic ligands.

The 3.8 S alpha2-histidine-rich glycoprotein of human serum is composed of two non-identical subunits, each of which contains carbohydrate. The far ultraviolet circular dichroism spectrum of alpha2-histidine glycoprotein indicates that the protein has little alpha-helix but apparently appreciable amounts of beta-sheet and non-regular structures. alpha2-Histidine-rich glycoprotein binds heme with concomitant changes in the electrophoretic mobility of the protein, in the fluorescence of tryptophan residues, and in the absorption and optical activity of the heme chromophore. By fluorescence quenching, the stoichiometry of binding is 1 heme per alpha2-histidine-rich glycoprotein molecule with an apparent Kd near 1.5 muM; however, by changes in absorbance, the interaction of 9 to 10 additional heme molecules with the alpha protein can be detected. The absorption spectra of heme . alpha2-histidine-rich glycoprotein complexes resemble those of low-spin hemoproteins. The ellipticity induced in the heme chromophore on binding by alpha2-histidine-rich glycoprotein increases linearly up to about 10 hemes bound per mol protein. No change in the conformation of alpha2-histidine-rich glycoprotein was indicated by circular dichroism when one or two heme molecules are bound by the protein. alpha2-Histidine-rich glycoprotein does not effectively compete with human serum albumin for heme, suggesting that alpha2-histidine-rich glycoprotein has no major function in serum heme transport. Nonetheless, the binding of heme by alpha2-histidine-rich glycoprotein provides a means of studying the structure of this protein using the heme chromophore as a probe. alpha2-Histidine-rich glycoprotein also binds other organic molecules including bilirubin, diaquocobinamide, Cibacron blue F3GA and rose bengal, and certain divalent metals. It is of interest that copper, zinc, nickel, cadmium and cobalt effectively inhibit the binding of heme by alpha2-histidine-rich glycoprotein, whereas other divalent metals tested, including calcium, magnesium and manganese do not appreciably affect the heme-alpha2-histidine-rich glycoprotein interaction.

Amino Acids↗
PubMed1978

Interaction of rabbit hemopexin with bilirubin.

The interaction of hemopexin with bilirubin was characterized by spectrophotometric, fluorimetric and circular dichroic techniques. Hemopexin rapidly forms an equimolar complex with libirubin that has an apparent dissociation constant Kd, of 7.5.10(-7) M. The association alters the absorption band of bilirubin near 150 nm, quenches the fluorescence of tryptophan residues of hemopexin, enhances the fluorescence of bilirubin, and induces strong ellipticity extrema in bilirubin of --60 . 10(3) deg . cm2 . dmol-1 at 465 nm and +70 . 10(3) deg . cm2 . dmol-1 at 415 nm. However, the conformation-sensitive ellipticity aband at 231 nm of hemopexin is not altered. In displacement experiments using circular dichroism, heme readily replaced bound bilirubin, indicating that bilirubin and heme are bound at the same site on hemopexin. Even at molar ratios of hemopexin to albumin of 3 to 1, human serum albumin removes bilirubin from hemopexin. Hemopexin is thus unlikely to have a role in the transport of bilirubin in serum.

Animals↗
PubMed1976

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.

Apoproteins↗
PubMed1976

The aromatic and heme chromophores of rabbit hemopexin. Difference absorption and fluorescence spectra.

Spectrophotometric and fluorimetric techniques were employed to charcterize the environment of the heme chromophore of rabbit hemopexin and to monitor changes in the environment of aromatic amino acid residues induced by the interaction of hemopexin with porphyrins and metalloporphyrins. Difference spectra showed maxima at 292 and 285 nm when hemopexin binds heme or deuteroheme but not deuteroporphyrin. These maxima are attributed to alterations in the local environment of tryptophan and tyrosine residues. Spectro-photometric titrations of the tyrosine residues of hemopexin, heme-hemopexin and hemopexin in 8 M urea showed apparent pK values at 11.4, 11.7, and 10.9 respectively. Perturbation difference spectra produced by 20% v/v ethylene glycol are consistent with the exposure of 6-8 of the 14 tyrosine residues and 6-8 of the 15 tryptophan residues of rabbit hemopexin to this perturbant. Only small differences were found between the perturbation spectra of apo- and heme-hemopexin near 290 nm, suggesting that slight or compensating changes in the exposure to solvent of tryptophan chromophores occur. In the Soret spectral region, the exposure of heme in the heme-hemopexin complex to ethylene glycol was 0.7, relative to the fully exposed heme peptide of cytochrome c. The fluorescence quantum yields of rabbit apo- and heme-hemopexin were estimated to be 0.06 and 0.03, respectively, compared to a yield of 0.13 for L-tryptophan. Iodide quenched 50% of the fluorescence of the deuteroheme-hemopexin complex. Cesium was not an effective quencher. Modification of approximately, 4 tryptophan residues with N-bromosuccinimide also decreased the relative fluorescence of apo-hemopexin by 50% and concomitantly reduced the heme-binding ability of the protein by 70%. The existence of sterically unhindered tryptophan residues in either apo- heme-hemopexin is unlikely since no charge transfer compelxes between these proteins and N-methylnicotinamide were detected.

Animals↗
PubMed1976

The binding and transport of heme by hemopexin.

Hemopexin binds proto-, meso-, or deutero-ferriheme with high affinity, forming an equimolar, low-spin complex. The ferroheme-hemopexin complex, which coordinates with CO and readily autoxidizes, is also low-spin. Formation of the ferriheme-hemopexin complex requires essential histidine and tryptophan residues and induces changes in the protein's tertiary structure. These changes may be important for the uptake of the heme-hemopexin complex by hepatocytes. Hemopexin also binds other porphyrins including protoporphyrin IX, and uro- and coproporphyrins I and III in a 1:1 molar ratio, but they are readily displaced by heme and do not produce discernable changes in the protein's conformation. In preliminary experiments, a selective interaction in vitro between heme-hemopexin and isolated rat hepatocytes has been demonstrated. This information is used as the basis for proposed models of the heme-binding site of hemopexin and of the interaction of heme-hemopexin with the parenchymal cells of the liver.

Animals↗
PubMed1976

Immunochemical observations on the human blood group P system.

Haemagglutination inhibition experiments were carried out with anti-P1, anti-Pk and anti-P sera in an attempt to increase understanding of the chemical, genetical and serological relationships within the P system. The test-substances comprised a glycoprotein with human blood group P1 and Pk activity isolated from sheep hydatid cyst fluid, fragments isolated from the partial acid hydrolysis products of the P1Pk active glycoprotein, glycolipids, monosaccharides and di- and oligo-saccharides of known structure. The trisaccharide alphaGal(1 leads to 4)betaGal(1 leads to 4)GlcNAc isolated from the glycoprotein hydrolysis products, and earlier established as the P1 determinant (Cory et al., 1974), was the only low molecular weight compound that gave strong inhibition with human, rabbit and goat anti-P1 sera. A disaccharide alphaGal(1 leads to 4)Gal, also isolated from the glycoprotein hydrolysis products, failed to react with anti-P1 reagents but inhibited human anti-Pk sera as strongly as the trisaccharide. The glycolipid alphaGal(1 leads to 4)betaGal(1 leads to 4)Glc-Cer (Ceramide trihexoside) and other oligosaccharides containing alphaGal(1 leads to 4)Gal at the non-reducing terminal were also strong inhibitors of anti-Pk sera. Oligosaccharides with terminal alpha-galactosyl residues joined in other positional linkages gave definite, although less strong, inhibition. The inhibition results suggest a close structural relationship between the P1 and Pk determinants and indicate that the specificity of anti-Pk sera is less closely delineated than that of anti-P1. Human anti-P sera differed markedly from anti-P1 and anti-Pk and were not inhibited by any of the compounds containing alpha-galactosyl residues. The glycolipid betaGalNAc(1 leads to 3)alphaGal(1 leads to 4)betaGal(1 leads to 4)Glc-Cer (globoside) strongly inhibited the anti-P sera. The inhibition of anti-Pk and anti-P sera by ceramide trihexoside and globoside, respectively, confirms the observations of Naiki & Marcus (1974) and supports their conclusions that Pk is the precursor of P. The genetic relationship of the P1 antigen to P and Pk is not clear but biosynthetic pathways are discussed that might explain the absence of P1, Pk and P antigens in individuals of the p phenotype.

Blood Group Antigens↗
PubMed1975

Interaction of rabbit hemopexin with rose bengal and photooxidation of the rose bengal-hemopexin complex.

Rabbit hemopexin associates with rose bengal producing a hypochromic shift in the absorption spectrum of the dye; the extinction coefficient of the dye bound to heme-saturated hemopexin is approximately 20% lower than that of the dye bound to the apoprotein. The interaction of apo- and heme-saturated hemopexin with rose bengal was studied in detail by difference spectroscopy. Apo-hemopexin has one tight binding site for the dye with a dissociation constant in the micromolar range and a set of several weaker binding sites. In contrast, heme-saturated hemopexin has a very low affinity for the dye. Evidence that histidine residues of hemopexin participate in the binding of heme was obtained by photooxidation of hemopexin sensitized by rose bengal. Progressive modification of the 16 histidine residues of hemopexin is effected by illumination of the dye-hemopexin complexes. The midpoint of this pH-dependent reaction is at pH 6.8 +/- 0.1. In 15 min of irradiation, apo-hemopexin loses 50% of its ability to form a low spin hemichrome complex with deuteroheme while only 10% of the ligand coordination to heme iron of the deuteroheme-hemopexin is lost. At that time, approximately 2 more histidine residues are modified in apo-hemopexin than in deuteroheme-hemopexin, and no change is found in other potentially photolabile amino acid residues. The characteristic circular dichroism positive extremum at 231 nm of hemopexin also was decreased by photooxidation, and the loss was slower in the deuteroheme-hemopexin complex than in the apoprotein. When deuteroporphyrin IX was used as the photosensitizing agent, similar results were obtained.

Animals↗
PubMed1975

Interaction of rabbit hemoplexin with copro- and uroporphyrins.

Rabbit hemopexin forms equimolar complexes in vitro with the I and III isomers of both coproporphyrin and uroporphyrin. The apparent dissociation constants (Kd) of these complexes are estimated to be 4-10(-7) M for coproporphyrin-hemopexin and 10(-6) M for uroporphyrin-hemopexin by equilibrium dialysis and quenching of protein fluorescence. Results of competitive binding experiments suggest that all four porphyrins bind at the heme-binding site of hemopexin, and that the relative affinity of rabbit hemopexin for these porphyrins is: deuteroheme greater than coproporphyrin I or III greater than uroporphyrin I or III. These findings provide further evidence that hemopexin may function as a transport protein for circulating coproporphyrins as well as for heme.

Animals↗
PubMed1975

Catabolism of photo-oxidized and desialylated hemopexin in the rabbit.

Following injection of rabbit 125I-asialohemopexin, more than 90% of the protein-bound 125I was removed from the circulation of rabbits within 12 min. The amount of asialoprotein in the catabolic compartment reached a peak concentration (75 to 85%) 12 min after injection and was completely eliminated from this compartment within 2 hours. The degradation products were excreted into the urine, with 50 to 70% of the 125I eliminated during the first 24 hours and 90 to 95% excreted by 48 hours. Analysis of these data indicated an apparent first order rate constant for uptake of asialohemopexin of 0.32 min-1, for catabolism of 0.020 min-1 and for excretion of 0.054 to 0.093 hour-1. The plasma distribution curves of 125I-hemopexin, after the first 24 hours, showed essentially no difference. Both proteins were catabolized with an average T1/2 of 25 to 26 hours and a similar fractional catabolic rate. Simultaneous injection of heme and 125I-hemopexin resulted in rapid removal and catabolism of the protein. In contrast, injection of heme had little if any effect on the plasma radioactivity curve of photoinactivated 125I-hemopexin.

Animals↗
PubMed1975

Circular dichroism of C3a anaphylatoxin. Effects of pH, heat, guanidinium chloride, and mercaptoethanol on conformation and function.

Circular dichroism spectra for C3a anaphylatoxins (protein fragments generated enzymatically in serum during activation of the third component of complement (C3)) from human and porcine sources were compared in the region of 190 to 250 nm. The spectra were indistinguishable in this region, although an estimated difference of approximately 20% exists between the primary structures of human and porcine C3a. Calculations indicated a total of 40 to 45% alpha helical content based on either the 208 or 222 nm extremum of the CD spectra. Addition of either mercaptoethanol or 6 M guanidinium chloride to human C3a produced a marked decrease in the mean residue ellipticity centered at 222 nm without irreversibly affecting the biological activity. Simultaneous addition of mercaptoethanol and guanidinium chloride virtually eliminated the CD contribution at 222 nm and resulted in more than 90% inactivation of the anaphylatoxin. Removal of the denaturant and reducing agent restored both the biological activity and the CD spectrum of C3a. Heat treatment or reduction and alkylation of human C3a produced biologically inactive and conformationally modified anaphylatoxin. In contrast, C3a inactivated by enzymatic removal of the COOH-terminal arginyl residue was structurally unchanged as judged by CD measurements. Consequently, it is proposed that in addition to the essential COOH-terminal arginyl residue, a highly ordered conformation is required for biological functionality of the C3a molecule.

Animals↗