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At least 19 recordsLinked to original sources
PubMed1976

Micropinocytosis of transferrin by developing red cells: an electron-microscopic study utilizing ferritin-conjugated transferrin and ferritin-conjugated antibodies to transferrin.

Electron-microscopic examination of rat reticulocytes and normoblasts incubated with transferrin conjugated to ferritin or ferritin-labeled antitransferrin revealed binding of ferritin conjugates to the surface membrane, and uptake of ferritin conjugates in micropinocytotic vesicles. No binding or endocytosis of ferritin was visualized when rat reticulocytes or normoblasts were incubated with ferritin alone or ferritin conjugated to nonspecific rabbit IgG. These observations support the concept that transferrin binds to a surface membrane receptor and is subsequently internalized by the developing red cell. Time course and temperature dependence studies suggest the endocytosis of transferrin may be an important mechanism in delivery of iron to the developing red cell.

Animals↗
PubMed1979

Trophoblast transferrin and transferrin receptors in the host--parasite relationship of human pregnancy.

Transferrin and specific transferrin receptors are demonstrated on the microvillous surface of syncytiotrophoblast in human immature and term placentae by immuno histological techniques with the use of light and electron microscopy. That the distribution of transferrin is limited to the materno-foetal interface supports the hypothesis that binding of maternal transferrin to trophoblast receptors is involved in the process of iron transport to the foetus. Parallel studies with baboon placentae demonstrate the presence of trophoblast receptors which bind both baboon and human transferrin, thereby putting forward an experimental model which might be used to test the biological significance of placental transferrin receptors in primates. In addition, investigation of a large number of human cell lines shows that many transformed cells, but no normal cells (such as blood lymphocytes) or cells from primary culture (such as neonatal foreskin fibroblasts), possess the ability to bind transferrin to their membranes. These findings suggest that transferrin receptors may play important biological roles in addition to that of iron transport from mother to foetus. One such role could be the limitation of iron in intervillous spaces, thus depriving iron-requiring microorganisms of iron, hence serving as a non-specific factor of resistance for placentae. Another role for foetal transferrin receptors on trophoblasts could be to bind maternal transferrin at the materno-foetal interface, thus frustrating maternal immunosurveillance. This is similar to a mechahism used by schistosomes in the host-parasite relation where host proteins are bound by the parasite to escape immunological recognition. The presence of transferrin receptors on transformed cells suggests that this mechanism might also be employed by tumour cells. Finally, in view of previous studies which show that transferrin is required by stimulated lymphocytes to pass from the G1 to the S phase of cellular replication, it is proposed that trophoblast transferrin receptors could limit the amount of transferrin in intervillous spaces and thus impede the proliferation and possible cytotoxicity of maternal activated lymphocytes at the materno-foetal interface.

Female↗
PubMed1979

Ferrocyanide staining of transferrin and ferritin-conjugated antibody to transferrin.

To evaluate the ultrastructural distribution of transferrin on the surface of L1210 ascites tumor cells, we used ferrocyanide to stain ferric iron (Prussian blue reaction) in transferrin, as well as in ferritin conjugated to antibody that was immunologically attached to the transferrin. Small deposits averaging 5 nm in diameter identified transferrin iron, whereas large cuboidal deposits averaging 50 nm in diameter stained ferritin conjugated-antibody that was bound to both transferrin and apotransferrin on the cell surface. The ability of transferrin to deliver iron to ascites tumor cells was confirmed by kinetic studies of transferrin labeled with 59Fe and 125I. These preliminary results are consistent with release of transferrin iron at the cell surface and demonstrate additional uses for ferrocyanide in ultrastructural cytochemical techniques.

Animals↗
PubMed1979

The rate limiting step in the reticulocyte uptake of transferrin and transferrin iron. Effects of some incubation variables.

Reticulocytes incubated in an isotonic NaCl saline medium containing glucose, glutamine and amino acids, were able to detach both iron atoms from all the transferrin incorporated by them. In the absence of these metabolites, although transferrin uptake was the same, the reticulocytes failed to remove completely the iron from the transferrin which they incorporated. It has been shown before that there is unspecific as well as specific binding of transferrin to the reticulocyte. By incubating the cells in the presence of a high concentration of bovine serum albumin, we have been able to prevent the unsepcific attachment of transferrin. At least 94% of the iodinated transferrin was capable of donating its iron to the reticulocytes.

Amino Acids↗
PubMed1975

In vivo evidence for the functional heterogeneity of transferrin-bound iron. III. Studies of transferrin at high and low iron saturation.

The functional heterogeneity of the transferrin iron pool of rats was studied by means of selective radioiron labeling of transferrin at high and low iron saturations. A sample of iron-poor plasma transferrin brought to 90 per cent iron saturation by the addition of 59Fe-nitrilotriacetate was mixed with a similarly labeled plasma sample of 55Fe-transferrin at 10 per cent iron saturation. The mixture was injected intravenously into groups of normal rats which were killed after 30 minutes, 3, and 24 hours for measurement of the distribution of the 59Fe and 55Fe in various tissues. 59Fe from diferric transferrin disappeared more rapidly from plasma and was preferentially removed by red blood cells, bone marrow, liver, and spleen. This phenomenon was most apparent at 30 minutes and 3 hours with little difference in the distribution of 59Fe and 55Fe at 24 hours. These studies add further support for the Fletcher-Huehns hypothesis of the functional heterogeneity of the transferrin iron pool in the rat.

Animals↗
PubMed1977

In vivo evidence for the functional heterogeneity of transferrin-bound iron. IV. Selective uptake by erythroid precursors of radioiron from portal vein plasma transferrin during intestinal iron absorption.

In addition to the previously demonstrated selective tissue uptake of iron from the two binding sites of transferrin, the Fletcher-Huehns hypothesis predicts that iron absorbed by the intestine is delivered selectively to the erythroblast-oriented iron-binding site of transferrin in portal plasma. We have tested this prediction in rats by measuring in vitro the rate and amount of radioiron taken up by reticulocytes and bone marrow erythroblasts from selectively labeled portal plasma and randomly labeled peripheral plasma. Portal plasma transferrin was significantly more effective than peripheral plasma in delivering radioiron to both reticulocytes and marrow erythroblasts; on a per-cell basis the erythroblasts took up about five times more radioiron. Iron-deficient reticulocytes were more avid but less discriminating than iron-replete reticulocytes in uptake of iron from the two plasma sources. When injected into normal test rats in vivo, radioiron from portal plasma was preferentially removed by red cell precursors and preferentially incorporated into heme extracted from marrow and spleen. These results support the concept of selective release of iron to erythroblast-oriented binding sites of portal plasma transferrin by intestinal cells during absorption. Combined with previously demonstrated selective tissue uptake of iron from transferrin, these experiments offer strong support for the active role of transferrin in the internal iron exchange of the rat.

Anemia↗
PubMed1976

Resonance Raman spectra of iron(III)-, copper(II)-, cobalt(III)-, and manganese(III)-transferrins and of bis(2,4,6-trichlorophenolato)diimidazolecopper(II) monohydrate, a possible model for copper(II) binding to transferrins.

Fe(III), Cu(II), Co(III), and Mn(III) complexes of ovo- and human serum transferrins show resonance enhanced Raman bands near 1600, 1500, 1270, and 1170 cm-1 upon excitation with laser frequencies which fall within the visible absorption bands of those metalloproteins. Comparison of the visible absorption and resonance Raman spectra of the Cu(II)-transferrin complexes with those for the Cu(II) model compound, bis(2,4,6-trichlorophenolato)diimidazolecopper(II) monohydrate, indicates that the resonance Raman bands are due to enhancement of phenolic vibrational modes. For the model (Cu(II) compound, a normal coordinate analysis was used to aid our assignment of the observed resonance bands at 1562, 1463, 1311, and 1122 cm-1 to A1 vibrational modes of the 2,4,6-trichlorophenolato moiety. These assignments are consistent with those made for Cu(II)-transferrins. The latter assignments were based upon calculated A1 frequencies for p-methylphenol (Cummings, D.L., and Wood, J.L. (1974), J. Mol. Struct. 20, 1). The wavelength shifts in the resonance bands for the model compound from those for Cu(II)-transferrins are due to the influence of the chloro substituents on the planar vibrations of phenol. These results clearly identify tyrosine as a ligand in copper binding to transferrins.

Binding Sites↗
PubMed1977

Horizontal polyacrylamide gradient gel electrophoresis for the simultaneous phenotyping of transferrin, post-transferrin, albumin and post-albumin in the blood plasma of cattle.

A simple method of horizontal polyacrylamide gel electrophoresis was described for the simultaneous phenotyping of transferrin, post-transferrin, albumin and post-albumin in the blood plasma of cattle. A step gradient gel of 8, 4, 12 and 14% acrylamide concentration was used. The method enabled the detection of a new protein polymorphism in the post-transferrin region. Two alleles were observed. The transferrin phenotypes involving D1 and D2 alleles were clearly separated. The resolution of the post-albumin fractions was also better than described by earlier methods.

Alleles↗
PubMed1975

Transferrin, iron, and dermatophytes. I. Serum dematophyte inhibitory component definitively identified as unsaturated transferrin.

The factor present in normal human serum which inhibits growth of dematophytic fungi is characterized and identified. Serum inhibitory factor (SIF) is nondialyzable, heat stable at 56 degrees C. for 4 hours, and fungistatic. SIF was found to be an inhibitor of the dermatophyte genera Trichophyton, Microsporium, and Epidermophyton as well as the dimorphic yeast Candida albicans. SIF activity directly correlated with a serum's unbound iron-binding capacity (UIBC) in that lower UIBC'S were less inhibitory. Addition of iron to serum neutralized the inhibitory activity and this neutrilization was shown to be specific for iron since zinc, magnesium, managnese, and copper failed to alter serum inhibitory activity. Furthermore, addition of purified iron-free transferrin to a neutralized serum restored the SIF activity in parallel with the UIBC. Removal of transferrin from serum by affinity chromatography was confirmed by polyacrylamide gel electrophoresis and iron-binding assays. Transferrin-free serum produced by this procedure was shown to lack inhibitory activity. These data indicate that SIF is unsaturated transferrin and strongly suggests that it inhibits dermatophytes by binding iron which many organisms need for growth.

Arthrodermataceae↗
PubMed1975

Study on sex-specific transferrin polymorphism and on the identification of transferrins by radioactive labelling.

Sex-specific differences with regard to the intensity of transferrin bands were observed in a noninbred adult mouse population after separation of the serum proteins by polyacrylamide gel electrophoresis. Amongst the female animals, an additional protein fraction was found just above the position of the transferrin bands. By means of a tracer method, using 59Fe-labelling, it could be shown that the additional fraction is not a part of the transferrin bands.

Age Factors↗
PubMed1979

Serum iron and transferrin saturation in women with special reference to women with low transferrin saturation. The population study of women in Göteborg 1968-1969.

Serum iron and total iron binding capacity (TIBC) were determined in a population sample of 1462 women in age strata between 38 and 60. Serum iron and TIBC values were similar in the various ages studied but with a slight trend towards higher serum iron and lower TIBC values in the upper ages. Transferrin saturation was used to divide the material arbitarily into women with and without iron deficiency. The dividing point chosen was 16%. The women thus defined as iron deficient had lower mean haemoglobin values than women in the total population sample and were more often anaemic. They had also lower MCV, MCH and MCHC indices than women in the total population sample. Of these indices, MCH seemed to discriminate the state of iron deficiency better than MCV and MCHC. Except for an increased mean menstrual blood loss no obvious cause of iron deficiency could be found in these women with low transferrin saturation.

Adult↗
PubMed1979

Three types of human asialo-transferrin and their interactions with the rat liver.

Three types of asialo-transferrin were obtained from immunologically pure human transferrin by chromatography on DEAE-cellulose, followed by desialylation and affinity chromatography on a column of the immobilized asialo-glycoprotein-binding hepatic lectin from rabbit liver. Of the asialo-transferrins, type 1 was derived from the principal DEAE-cellulose chromatographic component of transferrin, i.e. the one that contains two biantennary glycans. The two other asialo-transferrins (types 2 and 3) were derived from a minor DEAE-chromatographic transferrin component, which is assumed to possess one biantennary and one triantennary glycan. The three asialo-transferrin types were indistinguishable by electrophoretic mobility, but they were readily distinguished on the basis of their binding strengths to the hepatic lectin in intact rats. Glycan structures responsible for the difference in binding strengths between asialo-transferrin types 2 and 3 are not known. Metabolic studies in rats showed that none of the individual asialo-transferrin types was capable of generating a signal for endocytosis at low doses (<1mug/100g body wt.) and, consequently, most of the injected protein was recoverable with the plasma and the liver 35min after injection. However, endocytosis and catabolism of each asialo-transferrin type was readily induced by injecting a larger dose (50-250mug/100g body wt.) of unlabelled asialo-transferrin of the same type or of a different type a short interval after the labelled dose. These findings support the view that the dose-dependent uptake of human asialo-transferrin by the hepatocyte, as established in an earlier study with asialo-transferrin made from whole transferrin [Regoeczi, Taylor, Hatton, Wong & Koj (1978) Biochem. J.174, 171-178], also holds for these asialo-transferrin subfractions. Furthermore, the present studies indicate that asialo-transferrins of different carbohydrate compositions are capable of synergistically promoting endocytosis of each other.

Animals↗
PubMed1992

Transferrin: a potential source of iron for oxygen free radical-mediated endothelial cell injury.

The ability of transferrin to potentiate oxygen free radical-mediated endothelial cell injury was assessed. 51Cr-labeled endothelial cells derived from rat pulmonary arteries (RPAECs) were incubated with hydrogen peroxide (H2O2) in the presence and absence of holosaturated human transferrin, and the effect of transferrin on H2O2-mediated endothelial cell toxicity was determined. Addition of holosaturated transferrin potentiated H2O2-mediated RPAEC cytotoxicity at concentrations of H2O2 greater than 10 microM, suggesting that transferrin may provide a source of iron for free radical-mediated endothelial cell injury. Free radical-mediated injury is dependent on non-protein-bound iron. The ability of RPAECs to facilitate the release of iron from transferrin was assessed. We determined that RPAECs facilitate the release of transferrin-derived iron by reduction of transferrin-bound ferric iron (Fe3+) to ferrous iron (Fe2+). The reduction and release of transferrin-derived Fe2+ were inhibited by apotransferrin and chloroquine, indicating a dependence on receptor-specific binding of transferrin to the RPAEC cell surface, with subsequent endocytosis, acidification, and reduction of transferrin-bound Fe3+ to Fe2+. The release of transferrin-derived Fe2+ was potentiated by diethyldithiocarbamate, an inhibitor of intracellular superoxide dismutase (SOD). In contrast, exogenous SOD did not alter iron release, suggesting that intracellular superoxide anion (O2-) may play an important role in mediating the reduction and release of transferrin-derived iron. Results of this study suggest that transferrin may provide a source of iron for oxygen free radical-mediated endothelial cell injury and identify a novel mechanism by which endothelial cells may mediate the reduction and release of transferrin-derived iron.

Animals↗
PubMed1976

Transferrin uptake and release by reticulocytes treated with proteolytic enzymes and neuraminidase.

The mechanism of transferrin uptake by reticulocytes was investigated using rabbit transferrin labelled with 125I and 59Fe and rabbit reticulocytes which had been treated with trypsin, Pronase or neuraminidase. Low concentrations of the proteolytic enzymes produced a small increase in transferrin and iron uptake by the cells. However, higher concentrations or incubation of the cells with the enzymes for longer periods caused a marked fall in transferrin and iron uptake. This fall was associated with a reduction in the proportion of cellular transferrin which was bound to a cell membrane component solubilized with the non-ionic detergent, Teric 12A9. The effect of trypsin and Pronase on transferrin release from the cells was investigated in the absence and in the presence of N-ethylmaleimide which inhibits the normal process of transferrin release. It was found that only a small proportion of transferrin which had been taken up by reticulocytes at 37 degrees C but nearly all that taken up 4 degrees C was released when the cells were subsequently incubated with trypsin plus N-ethylmaleimide, despite the fact that about 80% of the 59Fe in the cells was released in both instances. Neuraminidase produced no change in transferrin and iron uptake by the cells. These experiments provide evidence that transferrin uptake by reticulocytes requires interaction with a receptor which is protein in nature and that following uptake at 37 degrees C, most of the transferrin is located at a site unavailable to the action of proteolytic enzymes. The results support the hypothesis that transferrin enters reticulocytes by endocytosis.

Animals↗