Binding to cellular receptors results in increased iron release from transferrin at mildly acidic pH

Expression and intracellular localization of leptin receptor long isoform-GFP chimera

The leptin receptor (OBR) and its ligand leptin (OB) are key players in the regulation of body weight. The OBR is a member of the class I cytokine receptor family and is alternatively spliced into at least six different isoforms. The multiple forms are identical in their extracellular and transmembrane regions but differ in lengths. The two predominant isoforms include a long form (OBR(l)) with an intracellular domain of 303 amino acids and a shorter form (OBR(s)) with an intracellular domain of 34 amino acids. We have constructed a recombinant OBR(l) chimera with the green fluorescent protein (GFP) by fusing GFP to the C-terminus of the OBR(l). The OBR(l)-GFP chimera was transiently transfected and expressed in SHSY5Y and HEK293 cells. In a STAT-Luciferase assay we show that the GFP moiety in this chimera did not affect the signalling capacity of OBR(l)-GFP. In both SHSY5Y and HEK293 cells transfected with OBR(l)-GFP, a predominant intracellular green OBR(l)-GFP fluorescence was detected in vesicles also positive for internalized fluorophore conjugated leptin. We also found that treatment with the lysosomotropic reagent monensin did not relocalize OBR(l)-GFP together with the human transferrin receptor in recycling endosomes, indicating OBR(l)-GFP not to participate in this pathway. In biotinylation-streptavidin pulse chase experiments, using antibodies raised against GFP and OBR, we observed that the rate of early appearance of OBR(s) at the cell surface, upon leptin stimulation, was faster than that found for OBR(l)-GFP. Taken together, our results provide novel data concerning the intracellular trafficking of the two different isoforms of the leptin receptor.

Crystal structure of the hereditary haemochromatosis protein HFE complexed with transferrin receptor

HFE is related to major histocompatibility complex (MHC) class I proteins and is mutated in the iron-overload disease hereditary haemochromatosis. HFE binds to the transferrin receptor (TfR), a receptor by which cells acquire iron-loaded transferrin. The 2.8 A crystal structure of a complex between the extracellular portions of HFE and TfR shows two HFE molecules which grasp each side of a twofold symmetric TfR dimer. On a cell membrane containing both proteins, HFE would 'lie down' parallel to the membrane, such that the HFE helices that delineate the counterpart of the MHC peptide-binding groove make extensive contacts with helices in the TfR dimerization domain. The structures of TfR alone and complexed with HFE differ in their domain arrangement and dimer interfaces, providing a mechanism for communicating binding events between TfR chains. The HFE-TfR complex suggests a binding site for transferrin on TfR and sheds light upon the function of HFE in regulating iron homeostasis.

Crystal structure of the ectodomain of human transferrin receptor

The transferrin receptor (TfR) undergoes multiple rounds of clathrin-mediated endocytosis and reemergence at the cell surface, importing iron-loaded transferrin (Tf) and recycling apotransferrin after discharge of iron in the endosome. The crystal structure of the dimeric ectodomain of the human TfR, determined here to 3.2 angstroms resolution, reveals a three-domain subunit. One domain closely resembles carboxy- and aminopeptidases, and features of membrane glutamate carboxypeptidase can be deduced from the TfR structure. A model is proposed for Tf binding to the receptor.

The transferrin receptor: role in health and disease

The transferrin receptor is a membrane glycoprotein whose only clearly defined function is to mediate cellular uptake of iron from a plasma glycoprotein, transferrin. Iron uptake from transferrin involves the binding of transferrin to the transferrin receptor, internalization of transferrin within an endocytic vesicle by receptor-mediated endocytosis and the release of iron from the protein by a decrease in endosomal pH. With the exception of highly differentiated cells, transferrin receptors are probably expressed on all cells but their levels vary greatly. Transferrin receptors are highly expressed on immature erythroid cells, placental tissue, and rapidly dividing cells, both normal and malignant. In proliferating nonerythroid cells the expression of transferrin receptors is negatively regulated post-transcriptionally by intracellular iron through iron responsive elements (IREs) in the 3' untranslated region of transferrin receptor mRNA. IREs are recognized by specific cytoplasmic proteins (IRPs; iron regulatory proteins) that, in the absence of iron in the labile pool, bind to the IREs of transferrin receptor mRNA, preventing its degradation. On the other hand, the expansion of the labile iron pool leads to a rapid degradation of transferrin receptor mRNA that is not protected since IRPs are not bound to it. However, some cells and tissues with specific requirements for iron probably evolved mechanisms that can override the IRE/IRP-dependent control of transferrin receptor expression. Erythroid cells, which are the most avid consumers of iron in the organism, use a transcriptional mechanism to maintain very high transferrin receptor levels. Transcriptional regulation is also involved in the receptor expression during T and B lymphocyte activation. Macrophages are another example of a cell type that shows 'unorthodox' responses in terms of IRE/IRP paradigm since in these cells elevated iron levels increase (rather than decrease) transferrin receptor mRNA and protein levels. Erythroid cells contain the highest mass of the total organismal transferrin receptors which are released from reticulocytes during their maturation to erythrocytes. Hence, plasma contains small amounts of transferrin receptors which represent a soluble fragment of the extracellular receptor domain. Measurements of serum transferrin receptor concentrations are clinically useful since their levels correlate with the total mass of immature erythroid cells.

The hereditary hemochromatosis protein, HFE, specifically regulates transferrin-mediated iron uptake in HeLa cells

HFE is the protein product of the gene mutated in the autosomal recessive disease hereditary hemochromatosis (Feder, J. N., Gnirke, A., Thomas, W., Tsuchihashi, Z., Ruddy, D. A., Basava, A., Dormishian, F., Domingo, R. J., Ellis, M. C., Fullan, A., Hinton, L. M., Jones, N. L., Kimmel, B. E., Kronmal, G. S., Lauer, P., Lee, V. K., Loeb, D. B., Mapa, F. A., McClelland, E., Meyer, N. C., Mintier, G. A., Moeller, N., Moore, T., Morikang, E., Prasss, C. E., Quintana, L., Starnes, S. M., Schatzman, R. C., Brunke, K. J., Drayna, D. T., Risch, N. J., Bacon, B. R., and Wolff, R. R. (1996) Nat. Genet. 13, 399-408). At the cell surface, HFE complexes with transferrin receptor (TfR), increasing the dissociation constant of transferrin (Tf) for its receptor 10-fold (Gross, C. N., Irrinki, A., Feder, J. N., and Enns, C. A. (1998) J. Biol. Chem. 273, 22068-22074; Feder, J. N., Penny, D. M., Irrinki, A., Lee, V. K., Lebron, J. A., Watson, N. , Tsuchihashi, Z., Sigal, E., Bjorkman, P. J., and Schatzman, R. C. (1998) Proc. Natl. Acad. Sci. U S A 95, 1472-1477). HFE does not remain at the cell surface, but traffics with TfR to Tf-positive internal compartments (Gross et al., 1998). Using a HeLa cell line in which the expression of HFE is controlled by tetracycline, we show that the expression of HFE reduces 55Fe uptake from Tf by 33% but does not affect the endocytic or exocytic rates of TfR cycling. Therefore, HFE appears to reduce cellular acquisition of iron from Tf within endocytic compartments. HFE specifically reduces iron uptake from Tf, as non-Tf-mediated iron uptake from Fe-nitrilotriacetic acid is not altered. These results explain the decreased ferritin levels seen in our HeLa cell system and demonstrate the specific control of HFE over the Tf-mediated pathway of iron uptake. These results also have implications for the understanding of cellular iron homeostasis in organs such as the liver, pancreas, heart, and spleen that are iron loaded in hereditary hemochromatotic individuals lacking functional HFE.

Fluorescence recovery after photobleaching (FRAP) of a fluorescent transferrin internalized in the late transferrin endocytic compartment of living A431 cells: experiments

In this work, we verified that transferrin fluorescently labelled with lissamine rhodamine sulfochloride (Tf-LRSC) is internalized in epidermoid A431 carcinoma cells through the specific endocytic pathway of transferrin. The FRAP of this fluorescent marker internalized in the late compartment of transferrin endocytosis (LCT) was measured in living A431 cells. These experiments showed the presence of an active intracellular transport of Tf-LRSC which can be interpreted by a mechanism involving carrier vesicles budding from stationary vacuoles, saltating along microtubules and fusing with other stationary vacuoles, according to previous video-microscopy observations of a membranous traffic dynamics in these cells, revealed by a gold complex of an Anti-Transferrin Receptor (ATR) (M. De Brabander, R. Nuygens, H. Geerst, C.R. Hopkins, Cell. Motil. Cystoskel. 9 (1988) 30). When the A431 cells were treated with nocodazole or metabolic inhibitors, there remained a residual FRAP which was ascribed to the spontaneous reactivation of the bleached molecules. According to a theoretical result obtained in the companion paper (P. Wahl, F. Azizi, Biochim. Biophys. Acta 1327 (1997) 69-74), we derived the fractional FRAP characterizing the transport process of Tf-LRSC by subtracting the fractional FRAP of the nocodazole-treated cells from the fractional FRAP of the non-treated cells. This FRAP of transport was fitted to a formula derived in that companion paper and based on the mechanism outlined above. From the time constant value determined by this fit, the number of vesicles which fused with a unit of vacuole surface was calculated to be 0.15 microm(-2) s(-1). The rate value of the fusion of vesicles with vacuoles was divided by two in cells treated by AlF4-, and increased to 20% in cells treated with Brefeldin A. These results correspond to an homotypic fusion process regulated by an heterotrimeric G-protein. Our work suggests that FRAP can be used to bring information on the transport of membrane components in living eukaryotic cells.

Does an acidic pH explain why low density lipoprotein is oxidised in atherosclerotic lesions?

The oxidation of low density lipoprotein (LDL) within atherosclerotic lesions may be involved in atherogenesis. LDL oxidation by cells in the presence of iron is faster at acidic pH. In addition, LDL oxidation by iron alone or iron cysteine in the absence of cells is much faster at acidic pH, even at mildly acidic pH (pH 6.5). The effect of pH on LDL oxidation by copper ions is more complex, in that acidity slows down the initial oxidation, as measured by conjugated dienes, hydroperoxides and thiobarbituric acid-reactive substances, but can increase the later stages of LDL oxidation as measured by increased macrophage uptake. Extensive LDL oxidation by cells in atherosclerotic lesions probably requires a source of iron or copper as catalysts for the oxidation. Iron in plasma is carried by the protein transferrin. Lowering the pH releases some of the iron from transferrin so that it can catalyse LDL oxidation. Copper is carried in plasma on caeruloplasmin and becomes more effective in catalysing LDL oxidation when the caeruloplasmin is preincubated at acidic pH, or even at pH 7.0. These effects can be seen with concentrations of caeruloplasmin and transferrin below those present in plasma. By analogy to other inflammatory and ischaemic sites, atherosclerotic lesions may well have an acidic extracellular pH, particularly within clusters of macrophages where the oxidative stress may also be high. This localised acidic pH may help to explain why atherosclerotic lesions are one of the few sites in the body where extensive LDL oxidation occurs.

gamma-Glutamyl transpeptidase-dependent lipid peroxidation in isolated hepatocytes and HepG2 hepatoma cells

Gamma-glutamyltranspeptidase (GGT), a plasma membrane-bound enzyme, provides the only activity capable to effect the hydrolysis of extracellular glutathione (GSH), thus favoring the cellular utilization of its constituent amino acids. Recent studies have shown however that in the presence of chelated iron prooxidant species can be originated during GGT-mediated metabolism of GSH, and that a process of lipid peroxidation can be started eventually in suitable lipid substrates. The present study was undertaken to verify if a GGT-dependent lipid peroxidation process can be induced in the lipids of biological membranes, including living cells, and if this effect can be sustained by the GGT highly expressed at the surface of HepG2 human hepatoma cells. In rat liver microsomes (chosen as model membrane lipid substrate) exposed to GSH and ADP-chelated iron, the addition of GGT caused a marked stimulation of lipid peroxidation, which was further enhanced by the addition of the GGT co-substrate glycyl-glycine. The same was observed in primary cultures of isolated rat hepatocytes, where the lipid peroxidation process did not induce acute toxic effects. GGT-stimulation of lipid peroxidation was dependent both on the concentration of GSH and of ADP-chelated iron. In GGT-rich HepG2 human hepatoma cells, the exposure to GSH, glycyl-glycine, and ADP-chelated iron resulted in a nontoxic lipid peroxidation process, which could be prevented by means of GGT inhibitors such as acivicin and the serine-boric acid complex. In addition, by co-incubation of HepG2 cells with rat liver microsomes, it was observed that the GGT owned by HepG2 cells can act extracellularly, as a stimulant on the GSH- and iron-dependent lipid peroxidation of microsomes. The data reported indicate that the lipid peroxidation of liver microsomes and of living cells can be stimulated by the GGT-mediated metabolism of GSH. Due to the well established interactions of lipid peroxidation products with cell proliferation, the phenomenon may bear particular significance in the carcinogenic process, where a relationship between the expression of GGT and tumor progression has been envisaged.

Modulation of human T lymphocyte proliferation by 4-hydroxynonenal, the bioactive product of neutrophil-dependent lipid peroxidation

The proliferative capacity of immune cells is known to be sensitive to conditions of oxidative stress and lipid peroxidation. We tested the hypothesis that activated neutrophils can induce peroxidation in extracellular lipid substrates, and evaluated the effects of 4-hydroxy-2,3-trans-nonenal (4-HNE)--the most reactive aldehydic product of lipid peroxidation--on mitogen-induced proliferation of human T lymphocytes. Neutrophils activated in the presence of extracellular lipid substrates (liposomes, cellular membranes) induced lipid peroxidation. By means of cytoimmunofluorescent labeling and confocal microscopy, the binding of 4-HNE to surface and cytoplasmic proteins of activated neutrophils was observed. Short (20 min) pre-treatment of cells with low concentrations of 4-HNE were able to dose-dependently decrease the proliferation of human peripheral blood lymphocytes challenged with PHA or anti-CD3 monoclonal antibody OKT3, as well as the proliferation of a tetanus specific human T-cell line challenged with tetanus toxoid. In these conditions, the binding of 4-HNE to surface and cytoplasmic proteins of lymphocytes was also observed. When the proliferative capacity of peripheral blood lymphocytes was monitored over several days after 4-HNE treatment and PHA challenge, a recovery and a rebound in cell proliferation was observed. Data reported indicate that the lipid peroxidation promoted by activated neutrophils can exert modulatory effects on the responsivity of human T cells, through the action of its most reactive product, 4-HNE.

The ferritins: molecular properties, iron storage function and cellular regulation

The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are found which do not arise directly from ferritin breakdown. Understanding the numerous inter-relationships between the various intracellular iron complexes presents a major challenge.

Effects of overexpression of the transferrin receptor on the rates of transferrin recycling and uptake of non-transferrin-bound iron

The possibilities that the recycling of the transferrin receptor is a rate-limiting step in the efflux of endocytosed transferrin, and that the receptor functions as a trans-membrane Fe transporter were investigated in untransfected Ltk- cells and in cells transfected with different levels of DNA for wild-type, mutant and chimeric human transferrin receptors. The uptake of transferrin-bound Fe and non-transferrin-bound Fe(II), and the surface binding, endocytosis and recycling of transferrin were measured. In cells that expressed increasing numbers of surface transferrin receptors, the rate of Fe uptake increased at a slower rate than the number of receptors. By measurement of the rates of endocytosis and recycling of transferrin it was shown that this effect was not due to a deficiency of endocytosis, but to a slower rate of recycling as the receptor numbers increased. Hence, a restricted recycling rate of the transferrin receptor appeared to be responsible for the slower rate of Fe uptake by cells with high receptor numbers, presumably because one or more cytosolic components required for recycling were in limited supply. The rate of uptake of non-transferrin-bound Fe(II) was not influenced by the number of transferrin receptors present on the surface of the cells even though this varied more than 20-fold between the different cell lines. Hence, this investigation does not support the hypothesis that the receptors play a direct role in the transport of Fe(II) across cell membranes, as has been proposed previously [Singer, S. J. (1989) Biol. Cell 65, 1-5].

Pumping iron in the '90s

The role o f iron in cell division, cell death and human disease has recently gained increased attention. The best studied process for iron uptake into mammalian cells involves traps ferrin and its receptor. This review discusses evidence supporting the existence of other routes by which iron can enter mammalian cells. Specifically, iron uptake by the cell-surface GPI-linked traps ferrin homologue, melanotransferrin or p97, is described and possible functions of this traps ferrin-independent pathway are proposed.

Functional analysis of human/chicken transferrin receptor chimeras indicates that the carboxy-terminal region is important for ligand binding

Chimeric human/chicken transferrin receptors have been constructed using the polymerase chain reaction. Different regions of the 671-residue external domain of the human transferrin receptor were replaced by the corresponding sequences from the chicken transferrin receptor. As chicken transferrin receptors do not bind human transferrin, functional analysis of such chimeric receptors provides an approach to define the ligand-binding site of the human transferrin receptor. Four of 16 chimeric human/chicken transferrin receptors expressed in chick embryo fibroblasts were efficiently transported to the plasma membrane and displayed on the cell surface. Studies of the four chimeric receptors indicated that binding of human transferrin was abolished if the carboxy terminal 192 amino acids of the human transferrin receptor (residues 569-760) were replaced with the corresponding region from the chicken transferrin receptor. Further, a chimeric receptor in which the carboxy-terminal 72 residues were derived from the chicken transferrin receptor exhibited a 16-fold decrease in binding affinity for human transferrin. In contrast, analysis of the other two chimeric receptors showed that 340 amino acids of the human transferrin receptor external domain more proximal to the transmembrane region (residues 151-490) could be replaced with the corresponding region from the chicken transferrin receptor without loss of high-affinity ligand binding. In contrast, two mAbs against the human transferrin receptor external domain, B3/25 and D65.3, that do not compete with transferrin binding, do not bind the chimeric transferrin receptors in which the membrane proximal part is replaced by chicken sequences, while they do bind the two other chimeric transferrin receptors with high affinity. These data indicate that sequence differences in the carboxy-terminal region of human and chicken transferrin receptor external domains are important for the species specificity of transferrin binding and imply that this portion of the human transferrin receptor is critical for ligand binding.

Transport of iron and other transition metals into cells as revealed by a fluorescent probe

Transport of nontransferrin-bound iron into cells is thought to be mediated by a facilitated mechanism involving either the trivalent form Fe(III) or the divalent form Fe(II) following reduction of Fe(III) at the cell surface. We have made use of the probe calcein, whose fluorescence is rapidly and stoichiometrically quenched by divalent metals such as Fe(II), Cu(II), Co(II), and Ni(II) and is minimally affected by variations in ionic strength, Ca(II) and Mg(II). Addition of Fe(II) salts to calcein-loaded human erythroleukemia K-562 cells elicited a slow quenching response that was markedly accelerated by the ionophore A-23187 and was reversed by membrane-permeant but not by impermeant chelators. These observations were confirmed by fluorescence imaging of cells. Other divalent metals such as Co(II), Ni(II), and Mn(II) permeated into cells at roughly similar rates, and their uptake, like that of Fe(II), was blocked by trifluoperazine, bepridil, and impermeant sulfhydryl-reactive organomercurials, indicating the operation of a common transport mechanism. This method could provide a versatile tool for studying the transport of iron and other transition metals into cells.

Regulation of heme biosynthesis: distinct regulatory features in erythroid cells

Our previous research has demonstrated that in hemoglobin-synthesizing cells, as compared with nonerythroid cells, a step in iron transport from transferrin localized between the transferrin receptor and ferrochelatase is rate-limiting for the synthesis of heme. In this communication we report our more recent studies on the mechanisms involved in the regulation of the transferrin receptors and ferrochelatase in differentiating erythroid cells. Our studies indicate that transferrin receptor gene expression is regulated differently in hemoglobin synthesizing as compared with uninduced murine erythroleukemia (MEL) cells: 1) With nuclear run-on assays our experiments showed increased transferrin receptor mRNA transcription cells of MEL following induction of erythroid differentiation with dimethylsulfoxide (DMSO). 2) DMSO treatment of MEL cells does not increase iron-responsive element binding protein (IRE-BP) activity which is, however, increased in uninduced MEL cells by Fe chelators. 3) Following induction of MEL cells there is an increase in the stability of transferrin receptor mRNA whose level is only slightly affected by iron excess. Using murine ferrochelatase cDNA as a probe, two ferrochelatase transcripts having lengths of 2.9 kb and 2.2 kb were found in extracts of mouse liver, kidney, brain, muscle and spleen, the 2.9 kb transcript being more abundant in nonerythroid tissues and the 2.2 more predominant in spleen. In MEL cells, the 2.9 ferrochelatase transcript is also more abundant; however, following induction of erythroid differentiation by DMSO there is a preferential increase in the 2.2 kb transcript which eventually predominates. With mouse reticulocytes, the purest immature erythroid cell population available, over 90% of the total ferrochelatase mRNA is present as the 2.2 kb transcript. Our further experiments indicate that the 2.2 kb transcript results from the utilization of the upstream polyadenylation signal and suggest that the preferential utilization of the upstream polyadenylation signal may be an erythroid-specific characteristic of ferrochelatase gene expression. These results provide further evidence for the idea that iron metabolism and heme synthesis are controlled by distinct mechanisms in erythroid versus nonerythroid cells.

Membrane biochemistry and chemical hepatocarcinogenesis

Biochemical membrane alterations appearing during the process of chemical carcinogenesis are described. Emphasis is put on membrane composition, structure, and biogenesis. In this presentation the knowledge gained from experimental studies of liver and skin in the process of cancer development is acknowledged. Important biochemical changes have been reported in lipid composition, fatty acid saturation, constitutional enzyme expression, receptor turnover and oligomerization. Functional consequences of the altered membrane structure is discussed within the concepts of regulation of cell proliferation, regulation of membrane receptor expression, redox control, signal transduction, drug metabolism, and multidrug resistance. Data from malignant tumours and normal tissue are addressed to evaluate the importance of the alterations for the process and for the eventual malignant transformation.

Receptor-modulated iron release from transferrin: differential effects on N- and C-terminal sites

Iron release to PPi from N- and C-terminal monoferric transferrins and their complexes with transferrin receptor has been studied at pH 7.4 and 5.6 in 0.05 M HEPES or MES/0.1 M NaCl/0.01 M CHAPS at 25 degrees C. The two sites exhibit kinetic heterogeneity in releasing iron. The N-terminal form is slightly less labile than its C-terminal counterpart at pH 7.4, but much more facile in releasing iron at pH 5.6. At pH 7.4, iron removal by 0.05 M pyrophosphate from each form of monoferric transferrin complexed to the receptor is considerably slower than from the corresponding free monoferric transferrin. However, at pH 5.6, complexation of transferrin to its receptor affects the two forms differently. The rate of iron release to 0.005 M pyrophosphate by the N-terminal species is substantially the same whether transferrin is free or bound to the receptor. In contrast, the C-terminal form releases iron much faster when complexed to the receptor than when free. Urea/PAGE analysis of iron removal from free and receptor-complexed diferric transferrin at pH 5.6 reveals that its C-terminal site is also more labile in the complex, but its N-terminal site is more labile in free diferric transferrin. Thus, the newly discovered role of transferrin receptor in modulating iron release from transferrin predominantly involves the C-terminal site. This observation helps explain the prevalence of circulating N-terminal monoferric transferrin in the human circulation.