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PubMed1983

Endocytosis and exocytosis of transferrin by isolated capillary endothelium.

Vesicular transport of solutes across capillary walls may be regulated by specific solute-endothelial interactions. Little data is available on the vesicular transport of serum proteins which may transit the capillary wall in situ. Capillaries were isolated from epididymal fat and incubated in fluorescent-labeled transferrin and radiolabeled sucrose. Endocytosis and exocytosis of these tracers were quantitated on a picomolar basis over timed intervals and standardized against the amount of endothelial DNA present in the isolate. The rate of vesicular endocytosis of transferrin was 6-7 times greater than that of sucrose indicating a mechanism of selection for transferrin. Endocytosis as a function of external concentration exhibited complex kinetics for transferrin that was consistent with an adsorptive component and a fluid component. Sucrose uptake appeared to be simple fluid endocytosis but with a rate-limiting concentration at 500-600 microM. Vesicular exocytosis of both solutes from preloaded capillaries appeared to occur more rapidly than their endocytosis. This was probably not due to different rates of filling and emptying of attached vesicles nor to an intrinsic difference in rates of vesicle interiorization and refusing with the plasma membrane. Different rates of endocytosis and exocytosis may only be apparent since exocytosis of marker before the capillaries reach ingestion equilibrium would reduce the measured uptake rate.

Animals↗
PubMed1985

Transmembrane transport of iron from extracellular transferrin by lymphoma cells.

Transferrin is essential for the entry of iron into cells, but whether the entire iron-transferrin complex or only the iron enters is not known. Separation of the cellular from the interparticulate radioactivity is a common problem with such studies. By pelleting cells under oil, we have made precise measurements of the uptake capacity for 59Fe of mouse lymphoma RI cells. At 37 degrees C an 18-fold concentration of iron was observed within 30 min; at 0 degrees C this value was about 3-fold. At 37 degrees C a maximum of 18,000 molecules of 125I-labelled Fe-transferrin were bound to each cell; this was reduced by about half at 0 degrees C. At 37 degrees C about 10,000-12,000 binding sites for 125I-apotransferrin were detected on each cell. From our data we conclude that although essential for the transport of iron, it is unlikely that transferrin enters RI cells. It is possible that iron is actively transported and that the receptor population is heterogeneous.

Animals↗
PubMed1986

Study of activated T cells in man. II. Interleukin 2 receptor and transferrin receptor expression on T cells and production of interleukin 2 in patients with acquired immune deficiency syndrome (AIDS) and AIDS-related complex.

Because the expression of interleukin 2 (IL-2) receptor and transferrin receptor is essential for the proliferation of T cells to mitogens and antigens, we examined the expression of monoclonal antibody defined IL-2 receptor (Tac antigen) and transferrin receptor on unstimulated as well as on phytohemagglutinin (PHA)-activated highly enriched T cells from patients with acquired immune deficiency syndrome (AIDS) and AIDS-related complex (ARC). A trend of increased proportion of unstimulated T cells with Tac antigen and transferrin receptor was observed in patients with AIDS and ARC when compared to healthy heterosexual controls, but the differences were not significantly (P greater than 0.1). The proportions of Tac+ PHA-activated T cells were, however, significantly decreased in AIDS (P less than 0.001). ARC (P less than 0.001), and asymptomatic homosexuals (P less than 0.01) when compared to healthy heterosexuals. The proportions of transferrin receptor positive PHA-activated T cells were not significantly different among various groups. A significantly (P less than 0.01) decreased production of IL-2 was observed in AIDS. This study suggests that the poor proliferative responses of T cells may be due to several defects in lymphocyte-cytokine cascade and the deficiency of Tac antigen expression and of the production of IL-2 could be a few of several abnormalities contributing to poor T-cell functions in AIDS.

Acquired Immunodeficiency Syndrome↗
PubMed1991

Alcohol intoxication and sialic acid in erythrocyte membrane and in serum transferrin.

Microheterogeneity of serum transferrin as well as erythrocyte membrane sialic acid content were examined in alcoholic patients and healthy controls. Both the sialic acid content of erythrocyte membranes and of the circulating transferrin were significantly lower in alcoholic patients than in controls. A moderate daily ethanol intake (less than 80 g) allowed to observe a proportional relationship between alcohol intake and the carbohydrate deficient forms of transferrin, and also a correlation between alcohol intake and the membrane sialic acid content. This supports the hypothesis of ubiquitary alterations of glycosylations in connection to ethanol intoxication. Additional disturbances could explain the absence of correlations between membrane sialic acid, pattern of abnormal forms of serum transferrin, and alcohol intake in heavy alcoholic patients.

Adult↗
PubMedDate not supplied

Immunoreactive transferrin receptor in sera of pregnant women.

Immunoreactive transferrin receptors in sera of 90 pregnant women of various gestational periods were investigated. The mean concentrations of the serum transferrin receptor in normal males and females were 251 +/- 94 ng/ml and 256 +/- 99 ng/ml, respectively. Serum transferrin receptor levels in pregnant women did not show a significant increase in the early stage of gestation. However, a rapid elevation of the mean concentration of transferrin receptor was observed after 20 weeks of pregnancy. The apparent increase with gestation period suggests that this immunoreactive receptor in the sera of pregnant women is derived from the placental syncytiotrophoblast and reflects the activity of maternofetal iron transport.

Female↗
PubMed1988

The bicarbonate-dependence of zinc(II)-transferrin binding.

The binding of zinc(II) to human serum transferrin has been studied as a function of the solution concentration of sodium bicarbonate in 100 mM, pH 7.4 hepes buffer at 25 degrees C. The apparent molar absorptivity of the zinc-transferrin complex has been determined from the initial slopes of titration curves of delta epsilon versus the ratio of [Zn]/[Tf]. This absorptivity represents the difference between the positive absorbance of the ternary Zn-HCO3-Tf species in the sample cuvette and the negative absorbance of binary HCO3-Tf species in the reference cuvette. Higher concentrations of bicarbonate increase the degree of saturation of apo-Tf with bicarbonate and thus increase the apparent absorptivity of the zinc-Tf complex. Titrations of apo- and monoferric transferrins with bicarbonate indicate that there is little, if any, difference in the bicarbonate binding constants of the two specific transferrin binding sites. An equilibrium constant of log K = 2.49 has been used to calculate the degree of saturation of the C-terminal binding site with bicarbonate. The zinc-binding affinity of this site depends linearly on this degree of saturation. The scatter in the zinc-binding constants of the weaker N-terminal site precludes a similar analysis of the bicarbonate-dependence of binding at this site. The results strongly support the previous proposal that binding of the synergistic bicarbonate anion is responsible for the uv absorption observed upon addition of bicarbonate to apoTf.

Bicarbonates↗
PubMed1987

Replacement of transferrin in serum-free cultures of mitogen-stimulated mouse lymphocytes by a lipophilic iron chelator.

Proliferation of mouse lymph node lymphocytes in response to concanavalin A in serum-free medium is normally dependent upon the presence of transferrin. In the absence of transferrin, little proliferation occurred, but the response was restored by addition of the iron complex of pyridoxal isonicotinoyl hydrazone (FePIH), a lipophilic iron chelator. Since cellular acquisition of PIH-bound iron is known not to involve the transferrin receptor, these results indicate that transferrin promotes lymphocyte proliferation solely because of its iron-donating properties, and does not provide any additional signalling event for proliferation.

Animals↗
PubMed1984

Heme inhibits transferrin endocytosis in immature erythroid cells.

The inhibitory effect of heme on iron uptake from transferrin by rat and rabbit reticulocytes and erythroid cells from the fetal rat liver was studied in vitro. Addition of hemin was shown to cause a decrease in the rate of transferrin endocytosis, the degree of inhibition being proportional to the reduction in iron uptake. The heme synthesis inhibitors, isoniazid and succinylacetone, stimulated the rate of transferrin endocytosis by 15-30% and caused a proportional increase in the rate of iron uptake, possibly by reducing the intracellular free heme concentration. It is concluded from these results that heme affects iron uptake by influencing the rate of transferrin endocytosis and recycling.

Animals↗
PubMed1984

[Effect of pH and the binding anion on In3+-transferrin interaction: spectroscopic study of the perturbed angular correlation of 172-245 keV gamma rays of indium 111].

The formation of ternary complexes, transferrin-anion-In111 has been investigated by means of gamma-gamma coincidence spectrometry of the 172-245 keV rays. The angular correlation between the two gamma-rays emitted in cascade depends on the magnetic and electric fields gradients, consequently the chemical structure of metal holder. Any modification of this structure causes the variation of angular correlation. The study of G22 (infinity) as function of pH (G22(infinity): integrated perturbed angular correlation coefficient) has been performed to turn out the hydrolysis of In111 in aqueous solution, metal complex formation in presence of chelating agents (citric acid and sodium bicarbonate) and the formation of protein-metal complexes. The presence of complexing agents limits the domain of In111 colloid existence and allows fast transfer of ionised indium on the transferrin. Two types of metal-protein interactions has been turn out. The first in the weakly acidic range of pH is characterized by an affinity constant near to this of citric acid. The second lying in neutral and basic range of pH, where the formation rate of transferrin-In111 complex is fast (t less than 500 s). In citrate medium, for pH 6-7,5 the rate of metal transfer on the protein, studied by means of G22 (infinity) = f(t), is function of pH. The binding anion appears as an indispensable element for the formation of protein-metal complexes. The In111 previously chelated by 8-Hydroxyquinoline is fixed by the protein if only exits a binding anion in the solution. This mays bring in the formation of an intermediate active state, indispensable step for the ternary complex formation transferrin-anion-In111.

Anions↗
PubMed1988

Structure and spatial conformation of the iron-binding sites of transferrins.

Transferrins are iron-binding glycoproteins involved in iron metabolism and antibacterial defense mechanisms. Since the discovery of transferrins, many studies have attempted to characterize the iron ligands and to establish the conformation of the iron-binding sites. From chemical and spectroscopic studies, it was generally accepted that iron was hexacoordinated to Tyr and His residues, to a water molecule and to a (bi)carbonate ion, electrostatically linked to an Arg residue. On the basis of these studies, on the one hand, and on the basis of the homologies between the amino acid sequences of transferrins, on the other hand, predicted data have been provided about the number and location of the iron ligands. Recent X-ray crystallography studies of human lactotransferrin have partially confirmed the above-mentioned predicted data and have brought invaluable information about the nature of the ligands and the conformation of the iron-binding site. On the basis of the obtained results, a scheme has been proposed in which the iron is coordinated to 2 Tyr, 1 His and 1 Asp residues, to a (bi)carbonate linked to an Arg residue and probably to a water molecule. The iron-binding site is located at the interface between the two domains which constitute each lobe of the transferrins.

Amino Acid Sequence↗
PubMed1976

Functional heterogeneity and pH-dependent dissociation properties of human transferrin.

Human diferric transferrin was partially labeled with 59Fe at low or neutral pH (chemically labeled) and by replacement of diferric iron previously donated to rabbit reticulocytes (biologically labeled). Reticulocyte 59Fe uptake experiments with chemically labeled preparations indicated that iron bound at near neutral pH was more readily incorporated by reticulocytes than iron bound at low pH. The pH-dependent iron dissociation studies of biologically labeled transferrin solutions indicated that Fe3+, bound at the site from which the metal was initially utilized by the cells, dissociated between pH 5.8 and 7.4. In contrast, lower pH (5.2--5.8) was required to effect dissociation of iron that has remained bound to the protein after incubation with reticulocytes. These findings suggest that each human transferrin iron-binding site has different acid-base iron-binding properties which could be related to the observed heterogenic rabbit reticulocyte iron-donating properties of human transferrin and identifies that the near neutral iron-binding site initially surrenders its iron to these cells.

Animals↗
PubMed1991

Effect of metabolic inhibitors on uptake of non-transferrin-bound iron by reticulocytes.

The relationship between transferrin-free iron uptake and cellular metabolism was investigated using rabbit reticulocytes in which energy metabolism was altered by incubation with metabolic inhibitors (antimycin A, 2,4-dinitrophenol, NaCN, NaN3 and rotenone) or substrates. Measurements were made of cellular ATP concentration and the rate of uptake of Fe(II) from a sucrose solution buffered at pH 6.5. There was a highly significant correlation between the rate of iron uptake into cytosolic and stromal fractions of the cells and ATP levels. Iron transport into the cytosol showed saturation kinetics. The metabolic inhibitors all reduced the Vmax but had no effect on the Km values for this process. It is concluded that the uptake of transferrin-free iron by reticulocytes is dependent on the cellular concentration of ATP and that it crosses the cell membrane by an active, carrier-mediated transport process. Additional studies were performed using transferrin-bound iron. The metabolic inhibitors also reduced the uptake of this form of iron but the inhibition could be accounted for entirely by reduction in the rate of transferrin endocytosis.

2,4-Dinitrophenol↗
PubMed1992

Iron-transferrin binding to isolated guinea pig enterocytes and the regional localisation of intestinal iron transfer during ontogeny.

Neonatal animals are iron replete but in comparison with adults they display increased intestinal iron absorption. In order to examine possible mechanisms for this developmental adaptation we have measured the appearance of iron in peripheral blood following 30 min exposure of duodenal and ileal segments of adult and neonatal guinea pigs in vivo to 59Fe-ascorbate. Parallel experiments have determined the kinetics of 125I-labelled diferric transferrin binding to villus enterocytes isolated from duodenum and ileum. In adult animals the rate of appearance of 59Fe in peripheral blood was 11-fold greater following duodenal, compared to ileal exposure to the radioligand. No such regional difference was detected in the neonate. Isolated cells showed saturable binding of [125I]transferrin which was maximal between 30 and 60 min. The kinetics of specific transferrin binding by adult duodenal and ileal enterocytes were similar and were also not significantly different to respective values in neonatal duodenal and ileal cells. Thus, it is likely that increased iron absorption in the neonate is due in part to enhanced ileal iron transfer. The interaction of transferrin with its receptor, however, is not involved in this developmental change in uptake.

Animals↗
PubMed1985

Partial characterization of transferrins of catfish (Silurus glanis L.) and pike (Esox lucius L.).

Basic composition and properties of isolated transferrins of Silurus glanis and Esox lucius have been compared. In transferrin of S. glanis carbohydrate is absent, but it is present in transferrin of E. lucius (2.5%). The N-terminal amino acid is alanine in both species. Mol. wts are 68,400 (S. glanis) and 86,800 (E. lucius). Transferrins of the two species are heterogeneous, but genetic polymorphism was not observed.

Amino Acids↗
PubMed1991

Electrophoretic characterization of human, equine and bovine transferrins.

1. Human, bovine and equine transferrins have been characterized with respect to mol. wt, and behavior on urea-polyacrylamide gels, and isoelectric focussing gels. 2. As shown by SDS-polyacrylamide gel electrophoresis human transferrin has one major polypeptide whereas both bovine and equine transferrins have two polypeptides. 3. The transferrins show multiple banded patterns on urea-polyacrylamide and isoelectric focussing gels, particularly when iron saturated. The various forms are not resolved by neuraminidase treatment.

Animals↗
PubMed1987

Externalization of transferrin receptor in established human cell lines.

The externalization of transferrin receptors was found in established human tumor cell lines at the rate of 10-35 ng/hour/10(6) cells, when they were incubated with transferrin at 37 degrees C. This externalization is inhibited by lowering the incubation temperature to 4 degrees C or eliminating the ligand from the culture medium. Metabolic inhibitors such as sodium azide, colchicine, cytochalasin B and chloroquine also decreased the rate of externalization. Almost 95% of released transferrin receptors were precipitated by centrifugation at 100,000 x g for 30 min, suggesting that transferrin receptor is externalized into the medium as a vesicular form.

Biomarkers, Tumor↗
PubMed1988

Studies of the slow bidirectional transport of iron and transferrin across the blood-brain barrier.

Although iron is involved in brain function, very little is known about the regulation of its concentrations in the central nervous system. We quantitatively measured the entry and exit rates of iron, transferrin (its major transport protein), and albumin in mice. The blood to brain transport of iron greater than transferrin greater than albumin and the brain to blood transport of transferrin greater than albumin greater than iron. The results suggest that iron and transferrin have slow, bidirectional, probably saturable, and to some degree independent transport systems, although iron introduced directly into the brain is not readily available for brain to blood transport.

Animals↗
PubMed1991

Acquisition of iron from transferrin by Bordetella pertussis.

It has been demonstrated that under iron-restricted conditions Bordetella pertussis can take up iron from human transferrin within 30 min of exposure. B. pertussis utilizes two mechanisms for acquiring iron from human transferrin, a direct contact method and a siderophore mediated system. Both systems are shown to result in bacterial internalization of iron from transferrin. However, direct contact between B. pertussis and transferrin provides far more effective iron uptake than siderophore activity alone.

Biological Transport↗