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PubMed1987

Transferrin binding of Al3+ and Fe3+.

An understanding of Al3+-induced diseases requires identification of the blood carrier of Al3+ to the tissues where Al3+ exerts a toxic action. Quantitative studies demonstrate that the protein transferrin (iron-free) is the strongest Al3+ binder in blood plasma. Under plasma conditions of pH 7.4 and [HCO3-]27 mmol/L, the successive stability constant values for Al3+ binding to transferrin are log K1 = 12.9 and log K2 = 12.3. When the concentration of total Al3+ in plasma is 1 mumol/L, the free Al3+ concentration permitted by transferrin is 10(-14.6) mol/L, less than that allowed by insoluble Al(OH)3, by Al(OH)2H2PO4, or by complexing with citrate. Thus transferrin is the ultimate carrier of Al3+ in the blood. We also used intensity changes produced by metal ion binding to determine the stability constants for Fe3+ binding to transferrin: log K1 = 22.7 and log K2 = 22.1. These constants agree closely with a revision of the reported values obtained by equilibrium dialysis. By comparison with Fe3+ binding, the Al3+ stability constants are weaker than expected; this suggests that the significantly smaller Al3+ ions cannot coordinate to all the transferrin donor atoms available to Fe3+.

Aluminum↗
PubMed1986

Iron-induced L1210 cell growth: evidence of a transferrin-independent iron transport.

L1210 leukemic cells can be cultured continuously in serum-free medium supplemented merely with either transferrin or iron salts. No transferrin or transferrin-like molecules were detected in the conditioned medium from cells established in serum-free medium plus iron. In these cells, iron uptake was found to occur through a saturable transport system exhibiting the properties of an allosteric regulatory protein. This transferrin-independent iron transport coexisted with transferrin-mediated iron uptake. When the iron concentration in the medium is less than 0.1 microM, transferrin must be present in the culture medium in order to observe cell growth. Under these culture conditions, a 16- to 18-h treatment with a 1 mM concentration of the iron chelator desferrioxamine resulted in less than 20% DNA synthesis compared to control cultures. DNA synthesis was reinitiated without a lag time after addition of 1 mM ferric citrate to the culture medium. No heme synthesis was needed to observe this DNA synthesis. However, in the presence of the antioxidant propyl gallate the reinitiation of DNA synthesis was abolished. Ferricyanide could not replace ferric citrate as a stimulant of DNA synthesis. Cytofluorometric analysis has shown that nearly 10% of the cells treated by desferrioxamine were blocked in G2 + M phase of cell cycle, suggesting that, in addition to DNA synthesis, iron chelation also blocked other mechanisms critical for cell growth.

Animals↗
PubMed1985

Hybridoma cultivation in defined serum-free media: growth-supporting substances. I. Transferrin.

Effect of transferrin and effect of transferrin omission on growth of hybridomas in serum-free medium were studied. Transferrin at concentration 0.5 microgram/ml and higher concentrations stimulated growth of hybridoma cells. The optimum growth-supporting concentration of transferrin was 5 micrograms/ml. Omission of transferrin led to death of hybridoma T3-03 cells within 3-4 days. Omission of transferrin for a period of only 24 h damaged cells so severely, that they were unable to recover fully.

Animals↗
PubMed1985

Acquisition of iron from transferrin regulates reticulocyte heme synthesis.

Fe-salicylaldehyde isonicotinoylhydrazone (SIH), which can donate iron to reticulocytes without transferrin as a mediator, has been utilized to test the hypothesis that the rate of iron uptake from transferrin limits the rate of heme synthesis in erythroid cells. Reticulocytes take up 59Fe from [59Fe]SIH and incorporate it into heme to a much greater extent than from saturating concentrations of [59Fe]transferrin. Also, Fe-SIH stimulates [2-14C]glycine into heme when compared to the incorporation observed with saturating levels of Fe-transferrin. In addition, delta-aminolevulinic acid does not stimulate 59Fe incorporation into heme from either [59Fe]transferrin or [59Fe]SIH but does reverse the inhibition of 59Fe incorporation into heme caused by isoniazid, an inhibitor of delta-aminolevulinic acid synthase. Taken together, these results suggest the hypothesis that some step(s) in the pathway of iron from extracellular transferrin to intracellular protoporphyrin limits the overall rate of heme synthesis in reticulocytes.

Aminolevulinic Acid↗
PubMed1979

Distribution of iron between the binding sites of transferrin in serum: methods and results in normal human subjects.

When it is incompletely saturated with iron, transferrin may exist in four molecular forms: apotransferrin, monoferric (A) transferrin (with iron occupying only the A site of the protein), monoferric (B) transferrin, and diferric transferrin. By combining electrophoresis in urea-polyacrylamide gels with crossed immunoelectrophoresis using specific antihuman transferin antiserum, it is possible to display and estimate the concentration of each of these four forms in normal human serum. The distribution of iron between the binding sites of transferrin is neither random nor determined by the relative binding strengths of transferrin's two sites. Rather, the more weakly binding and acid-labile B site of the protein is predominantly occupied.

Binding Sites↗
PubMed1977

Transferrin promotion of 67Ga and 59Fe uptake by cultured mouse myeloma cells.

Radiotracer 67Ga-citrate is used as a tumor-seeking agent in clinical imaging investigations although fundamental reasons for its high uptake in certain malignant lesions remain unexplained. The mechanism by which 67Ga becomes concentrated in tumor cells has been investigated by comparing 67Ga and 59Fe uptake by cultured mouse myeloma cells with particular reference to uptake stimulation by transferrin. Concentrations of human transferrin down to 2 microgram/ml greatly stimulated cellular uptake of both tracers, whereas bovine transferrin proved relatively inactive. The rates of stimulated uptake of both tracers were similar as was their high degree of retention by cells, but their quantitative dependencies on transferrin concentration showed characteristic differences. Pretreatment of human transferrin with saturating amounts of nonradioactive Fe3+ canceled its ability to promote 59Fe uptake, but it had little effect on its promotion of 67Ga uptake. Further increase in the amount of added Fe3+ did cause a progressive depression of 67Ga uptake, but this effect probably relates to the iron distribution in the whole-cell culture system including the fetal calf serum component of cell growth medium. The results suggest that 67Ga and 59Fe reveal different aspects of the interaction of transferrin with cells.

Animals↗
PubMed1980

Demonstration of transferrin receptors on human placental trophoblast.

It has been postulated that the transplacental passage of maternal iron to the developing fetus requires binding of maternal transferrin to the trophoblast. We have therefore examined the ability of the human placenta to bind transferrin in vitro. Transferrin was demonstrated on trophoblast of human chorionic villi by immunohistologic methods. Moreover, after removal of transferrin bound in vivo by treatment of tissue with chaotropic solution or phosphate-buffered saline, freshly added transferrin was shown to bind in vitro in the same characteristic distribution. These findings suggest that placental iron transport is initiated by uptake of maternal transferrin iron to specific trophoblast binding sites.

Albumins↗
PubMed1981

[The antigen structure of human serum transferrin].

By means of enzymatic fragmentation the transferrin molecule was definitely divided up into fragments which were investigated immunochemically after their column-chromatographic isolation. Papain, pepsin, ficin, and bromelin were used as proteolytic enzymes. Immunoelectrophoresis and various procedures of immunodiffusion were used for characterizing the antigen structure of transferrin fragments in an immunochemical way. The antisera required were obtained by immunizing animals (horses, sheep, goats, rabbits) according to different procedures. It was possible to divide up transferrin into 4 defined fragments, with the antigen structure being divided into three separate groups (A, B, C). The antigen structure of human transferrin consists of at least 6 different antigen determinants designated as a1, b1, b2, b3, b4, and c1 in accordance with their being included in the corresponding antigen determinant group. An interior antigen determinant (bi) could be proved in the bromelin transferrin fragment II. An attempt was made to develop a model of transferrin molecule on the basis of the findings achieved.

Epitopes↗
PubMed1984

The role of transferrin in lymphocyte transformation.

The ability of mouse lymphocytes to transform in a serum-free medium in response to Concanavalin A was found to depend upon the presence of transferrin. Iron chelates were ineffective, as also were monoferric transferrin fragments, but both homologous and heterologous transferrins promoted transformation. Optimal transformation occurred with 10-50 micrograms/ml of transferrin, and at 30-70 per cent iron saturation. Transferrin was bound by the transforming cells, and subsequently released without degradation following the delivery of iron. Iron uptake preceded DNA synthesis. Iron chelates, despite their inability to promote transformation, could also deliver iron to lymphocytes. It is postulated that transferrin may fulfil functions additional to that of supplying iron.

Animals↗
PubMed1983

Lactoferrin, transferrin and acidic isoferritins: regulatory molecules with potential therapeutic value in leukemia.

In the process of evaluating roles for purified preparations of lactoferrin, transferrin and acidic isoferritins in the regulation of myelopoiesis, it was found that: (1) values reported for lactoferrin in the serum and plasma of normal donors are in most cases an over-estimation, (2) lactoferrin suppresses the production/release of granulocyte-macrophage colony stimulatory factors (GM-CSF) from monocytes in the absence of T-lymphocytes and also suppresses the production/release of acidic isoferritin-inhibitory activity from monocytes, (3) lactoferrin, transferrin and acidic isoferritins act on their specific target cells which express Ia-like antigens, (4) lactoferrin and transferrin act in vivo to suppress rebound myelopoiesis in mice recovering from sublethal dosages of Cytoxan, with preliminary observations suggesting that lactoferrin has a greater apparent effect on the bone marrow and transferrin has a greater apparent effect on the spleen, (5) active lactoferrin derives from Fc receptor positive subpopulations of PMN from patients with CML as well as from normal donors, but the percentage of Fc receptor containing PMN is lower in CML, as is the amount of active lactoferrin found in their PMN, and (6) lactoferrin, transferrin and acidic isoferritins suppress the colony formation of U937 clonogenic cells, with lactoferrin and transferrin decreasing the release of growth factors from U937 cells which are needed to stimulate U937 colony formation, and lactoferrin and acidic isoferritins suppress the colony formation of WEHI-3 cells, with lactoferrin decreasing the release of growth factors from WEHI-3 cells which are needed to stimulate WEHI-3 colony formation. Speculation on the potential usefulness of these iron binding glycoproteins to control of disease progression is given in the discussion.

Animals↗
PubMed1983

Transferrin endocytosis and iron uptake during erythroid cell development.

Electron microscope autoradiography was used to quantitate the uptake of transferrin and transferrin-bound iron by rat erythroid cells at the various stages of development. The capacity of the cells to take up iron closely paralleled their ability to bind transferrin, suggesting that the level of transferrin receptors is the major factor which regulates the rate at which immature erythroid cells can accumulate iron. Maximal transferrin and iron uptake occurred at the basophilic normoblast stage and then decreased progressively during maturation to the reticulocyte. During development, immature rat erythroid cells acquire about twice as much iron from transferrin as is present in the haemoglobin of mature erythrocytes.

Animals↗
PubMed1980

Sertoli cells synthesize and secrete transferrin-like protein.

One of the major proteins secreted by Sertoli cells (Band 3) has a molecular weight on sodium dodecyl sulfate-polyacrylamide gels which corresponds to rat serum transferrin. The preparation of Sertoli cell-secreted proteins contains an iron-binding protein with electrophoretic properties on native polyacrylamide gels similar to serum transferrin. Antibodies against Sertoli cell-secreted proteins contain a component which will immunoprecipitate serum transferrin. Antibodies to serum transferrin cross-react with and immunoprecipitate a major component (Band 3) of [35S]-methionine-labeled Sertoli cell-secreted proteins. We propose that testicular transferrin is a major secretory product of Sertoli cells in culture. In addition to transferrin, antiserum to rat serum proteins will immunoprecipitate one other polypeptide from preparations of Sertoli cell-secreted proteins.

Animals↗
PubMed1982

Transferrin and gonadal dysfunction in man.

Transferrin concentrations were quantitated in the seminal fluid of normal, oligozoospermic, and azoospermic patients and related to other known parameters of testicular function. Transferrin concentration in the semen of patients 2 months after vasectomy (13.2 +/- 1.8 micrograms/ml) was significantly less than that obtained from pregnancy-proven donors (65.6 +/- 10.1 micrograms/ml). This indicates that approximately 80% of the seminal fluid transferrin is derived from the testes. The concentration of transferrin in semen from patients with azoospermia (14.4 +/- 1.8 micrograms/ml), severe oligozoospermia (17.5 +/- 1.7 micrograms/ml), and moderate oligozoospermia (21.8 +/- 4.3 micrograms/ml) was significantly lower than normospermic groups. Serum follicle-stimulating hormone (FSH) was measured in a group of infertile patients; those having an elevated FSH had a significantly lower concentration of semen transferrin, 14.1 +/- 1.6 micrograms/ml, compared with patients who had FSH levels within the normal range (33.7 +/- 5.3 micrograms/ml). It is possible that the underlying cause in decreased spermatogenesis associated with both an elevated FSH and a decreased transferrin concentration is impaired Sertoli cell function.

Follicle Stimulating Hormone↗
PubMed1982

The exchange of Fe3+ between acetohydroxamic acid and transferrin. Spectrophotometric evidence for a mixed ligand complex.

Transferrin, the serum iron transport protein, provides an excellent model for studying biological metal ion exchange reactions. A curious problem is that while a mixed ligand species of chelate-Fe3+-protein is anticipated from theoretical considerations and supported by kinetic results, no clear spectrophotometric evidence for such an intermediate has heretofore been obtained. In this study of the exchange of Fe3+ between acetohydroxamic acid and transferrin such evidence has been found. The reaction of Fe2+-acetohydroxamic acid with apotransferrin-CO3(2-) is distinctly biphasic when examined by stopped flow spectrophotometry. The first phase is complete within approximately 4 s and results in the formation of a transient species with a distinct spectral maximum at 432 nm. The second phase requires approximately 2 min and results in the formation of Fe3+-transferrin-CO3(2-). We suggest that the transient species is a mixed ligand complex. The reaction rate-concentration relationship for the formation of the intermediate is linear for Fe3+-acetohydroxamic acid and hyperbolic for apotransferrin-CO3(2-). This suggests a rate-limiting labilization of Fe3+-(acetohydroxamic acid)3 preceding attack by the apotransferrin-CO3(2-). The reverse reaction, the removal of Fe3+ from the Fe3+-transferrin-CO3(2-) by acetohydroxamic acid, does not provide spectral evidence for the intermediate. The velocity-concentration relationship shows a hyperbolic dependence on acetohydroxamic acid concentration and a linear dependence of Fe3+-transferrin-CO3(2-), suggesting a rate-limiting labilization of the Fe3+ of Fe3+-transferrin-CO3(2-) resulting from a conformational change.

Carbonates↗
PubMed1981

The effect of iron and transferrin on the response of serum-free cultures of mouse lymphocytes to concanavalin A and lipopolysaccharide.

Incorporation of [14C]-thymidine into mouse lymph node cells stimulated with either Con-canavalin A or lipopolysaccharide in serum-free medium was markedly enhanced by the addition of transferrin. Thymidine incorporation was similar in transferrin-containing serum-free medium and in medium containing 5% foetal calf serum. Transferrins from both homologous and heterologous species were equally effective, but iron-binding half-molecules of transferrin, and low molecular weight iron chelates produced no enhancement. The optimal response was obtained with 10--50 micrograms/ml of transferrin, and with 30%--70% iron saturation. Although the major function of transferrin in lymphocyte cultures is probably to supply iron, it may also fulfil other functions.

Animals↗
PubMed1981

[Properties of rat liver polyribosomes synthesizing transferrin].

A radioimmunological study of rat liver polyribosomes synthesizing transferrin was carried out. The 125I-labelled monospecific antibodies against transferrin interact with membrane-bound liver polyribosomes to form a specific antigen-antibody complex. No binding of 125I-antibodies against transferrin to free liver polyribosomes was observed. The transferrin polyribosomes contain 11-13 monoribosomes per 1 molecule of mRNA. An injection of cycloheximide to the animals results in an increase in the size of the transferrin polyribosomes up to 16 monoribosomes per 1 mRNA molecule. During translation of the liver polyribosomal fractions in a cell-free system of wheat embryos only the membrane-bound polyribosomes were capable to synthesize immunoprecipitated transferrin.

Animals↗
PubMed1995

Dissociation between tissue iron concentrations and transferrin saturation among inbred mouse strains.

Excessive absorption of dietary iron results in pathologic hepatic iron accumulation in the genetic disorder hemochromatosis. Genetic factors have also been suggested to play a role in African iron overload that is induced by the intake of iron-rich fermented beer. We have used inbred strains of mice to investigate previously unrecognized genetic factors controlling iron metabolism. Among different strains of mice fed a basal iron diet, serum iron levels varied twofold. In contrast, serum transferrin levels were remarkably constant. This indicates that transferrin saturation with iron, but not the serum level of transferrin, is likely to be genetically determined in mice. Hepatic iron stores also varied twofold among the different mouse strains. Remarkably, hepatic iron stores failed to reflect transferrin saturation, suggesting that different genetic factors control transferrin saturation and hepatic iron stores. When mice were challenged with a high-iron diet, the concentration of carbonyl iron required to completely saturate transferrin with iron was 10 times greater for C57BL/6 and BALB/c mice than for DBA/2 and AKR mice. These results are reminiscent of the iron-loading locus proposed to interact with dietary iron in African iron overload. Our studies with inbred strains of mice have revealed a number of novel control points in iron metabolism that may be genetically determined. The mouse model may thus be useful for understanding the molecular basis of hemochromatosis and African iron overload.

Animals↗
PubMed1994

[Perilymph fistula: diagnosis by detection of perilymph in the middle ear by beta-2 transferrin immunofixation].

beta 2-transferrin is a specific protein found in the cerebrospinal fluid and in the perilymph. Detecting beta 2-transferrin in the middle ear is of great interest when a fistula of the perilymph is suspected. This protein can be detected on microsamples with an immunofixation technique. We searched for beta 2-transferrin in pure perilymph, cerebrospinal fluid and serum in 8 patients operated by translabyrinthine approach for acoustic tumor removal. Search for beta 2-transferrin was performed in liquid from the inner ear in 3 labyrinthectomies. Samples were taken on collagen sponges or with micro syringes. beta 2-transferrin was detected in the perilymph of patients operated for neurinoma and in 2 of the 3 labyrinthectomies. This protein was found in only one of the patients for which the diagnosis of perilymphatic fistula had been retained. Detection of beta 2-transferrin in the middle ear can be proposed as a specific diagnostic test for perilymphatic fistula when the clinical situation does not suggest a fistula involving cerebrospinal fluid.

Adolescent↗