Studies of the effect of sulphydryl and other inhibitors on reticulocyte uptake of doubly-labelled transferrin

Transfer of iron from Ferastral and other organic complexes to transferrin as measured by reticulocyte uptake

The transfer of iron from the iron carbohydrate complexes, Ferastral, Imferon, and Jectofer, and from ferric chloride has been studied by the effect of such transfer in reducing reticulocyte uptake of 59Fe from labelled transferrin. There are plasma factors which augment the transfer of iron from complex to transferrin. The pattern of transfer from Ferastral and from Imferon are similar: at concentration of 5000 microgram/100 ml and 1250 microgram/100 ml in plasma these complexes transfer about 0.8% and 1.7%, respectively. Jectofer transfers about four times these amounts under similar conditions. In the case of Ferastral there is evidence of an equilibrium between transferrin-bound and Ferastral-bound iron. The characteristics of Ferastral assessed in this way suggest that it may prove suitable for therapeutic use as a total dose infusion.

Studies on the role of transferrin and endocytosis in the uptake of Fe3+ from Fe-nitrilotriacetate by mouse duodenum

Addition of iron-binding proteins (human serum transferrin, mouse serum transferrin, human lactoferrin) to the luminal fluid in tied-off segments of mouse intestine in vivo led to reduced 59Fe3+ absorption from 59Fe3+-nitrilotriacetate when compared to 59Fe3+-nitrilotriacetate alone. Assay of transferrin in luminal fluid from tied segments revealed only trace amounts of immunoreactivity. The levels of luminal transferrin are unaltered in chronic hypoxia where iron absorption is significantly enhanced. Studies in vitro revealed that NH4Cl, dansylcadavarine, para-chloromercuribenzoate and trinitrobenzenesulphonate have no effect on initial 59Fe3+ uptake rates from 59Fe3+-nitrilotriacetate, while N-ethylmaleimide (1 mM) caused a 40% inhibition. In vivo 59Fe3+ uptake was unaffected by preincubation of tied-off segments with colchicine (5 mM) for up to 2 h. These results suggest that receptor-mediated endocytosis of transferrin is not a significant mechanism in the uptake of luminal Fe3+ by mouse duodenum.

Internalization of iron-transferrin complex by murine L1210 leukemia cells and rat reticulocytes demonstrated by a minibead probe

To determine if the cellular uptake of iron is associated with internalization of iron-transferrin (TF) complex by the cell, we synthesized a visual probe in which TF is covalently bound to amide-modified latex minibead, submicrometer in size (0.345 micron). Incubation of the probe with L1210 leukemia cells and rat reticulocytes led to the binding of the probe to the cell surface visualized and semiquantified by scanning and transmission electron microscopy. The binding was inhibited by preincubation with nonderivatized iron-TF complex. Internalization of the probe occurred through clathrin-coated pits and vesicles. Minibeads derivatized by nontransport proteins or glycine as well as nonderivatized minibeads did not appreciably bind to the cells and were not internalized. Ethylamine, an inhibitor of receptor-mediated endocytosis abolished the internalization but not the binding of the probe which, then, accumulated on the cell surface. These findings provide direct evidence for internalization of TF during the iron uptake.

Ferritin as a cytosol iron transport intermediate in human reticulocytes

The major iron-bearing cytosol components of human reticulocytes identified after incubation with 59 Fe-125I-transferrin have been studied further. Component C previously found to behave consistently as an intermediate in the iron transport pathway to haem is shown to consist entirely of ferritin. After a short pulse of labelled transferrin incubation, chase experiments showed a fall of ferritin label with time and a corresponding increase in haemoglobin-iron incorporation. There was no loss of ferritin to the culture medium. Restriction of iron uptake by reticulocytes using both p-hydroxymercuribenzoate inhibition of uptake and incubation with progressively lower saturations of iron-transferrin gave linearly related incorporation of 59Fe into ferritin and haemoglobin at all levels of iron uptake, thus negating the concept of ferritin as an 'overspill' form of reticulocyte iron. The results suggest that cytosol ferritin is an obligatory intermediate in reticulocyte iron transport.

Binding sites of iron transferrin on rat reticulocytes. Inhibition by specific antibodies

1. In the process of iron uptake by precursors of the erythrocytes probably more than one membrane component is involved; besides the specific transferrin receptor, another membrane component with a high iron activity after incubation with 59Fe can be isolated. 2. A striking resemblance exists between rat and human reticulocyte components which are involved in the process of iron uptake. 3. Incubation of reticulocytes with Fab fragments of an antibody against the membrane receptor for transferrin causes a concentration-dependent decrease in transferrin binding and iron uptake. 4. The membrane receptor complex isolated is still heterogeneous; analytical ultracentrifugation studies suggest a molecular weight lower than 230,000. 5. Intact immature red cells are necessary for specific binding of transferrin with the receptor followed by iron uptake. This is the only mechanism for iron uptake. 6. Immunofluorescence studies showed that the receptor for transferrin is localized at the outside of the cell membrane.

The isolation and partial characterization of transferrin binding components of the rabbit reticulocyte plasma membrane

An affinity chromatograpy method utilising transferrin liganded agarose has been developed for the partial purification of transferrin binding components from Triton X-100 solubilised rabbit reticulocyte plasma membranes. A protein of molecular weight 30-35 000, shown to be located at the reticulocyte extra-cellular surface by lactoperoxidase 125I labelling, was isolated by the affinity method. The protein appeared to form a dimer of molecular weight 65-70 000 in Triton X-100 solution and was shown to associate with both 125I-labelled and unlabelled rabbit transferrin to form a high molecular weight complex in the same solution. N-[14C]Ethylmaleimide appeared to disrupt this association with transferrin and inhibit the formation of the dimer in Triton X-100 by binding to the protein. The protein appeared as a broad band of molecular weight 40 000 on sodium dodecyl sulphate polyacrylamide gel electrophoresis.

Transferrin and iron uptake by isolated rat liver mitochondria

Isolated rat liver mitochondria accumulate iron from the suspending medium when [59Fe]transferrin is used as a model compound. The accumulation proceeds by two different mechanisms, i.e. by an energy-independent and an energy-dependent (uncoupler sensitive) mechanism, which have different time, pH, and temperature dependencies. The energy-dependent accumulation, which is inhibited by ruthenium red and sulphydryl reagents, reaches a saturation level of approx. 30 pmoles iron/mg protein during 30 min incubation. The energy-independent accumulation of iron-transferrin reveals no saturation kinetics, it is inhibited neither by ruthenium red nor by N-ethylmaleimide, and it proceeds linearly for at least 90 min. With [125I]transferrin as a model compound, quantitatively the energy-independent accumulation is as reported for [59Fe]transferrin. There is, however, no energy-dependent accumulation of [125I]transferrin. The results indicate that the energy-dependent accumulation of [59Fe]transferrin represents a process by which mitochondria accumulate iron from transferrin.