Ceruloplasmin Ferroxidase Activity Stimulates Cellular Iron Uptake by a Trivalent Cation-specific Transport Mechanism

Abstract

The balance required to maintain appropriate cellular and tissue iron levels has led to the evolution of multiple mechanisms to precisely regulate iron uptake from transferrin and low molecular weight iron chelates. A role for ceruloplasmin (Cp) in vertebrate iron metabolism is suggested by its potent ferroxidase activity catalyzing conversion of Fe2+ to Fe3+, by identification of yeast copper oxidases homologous to Cp that facilitate high affinity iron uptake, and by studies of “aceruloplasminemic” patients who have extensive iron deposits in multiple tissues. We have recently shown that Cp increases iron uptake by cultured HepG2 cells. In this report, we investigated the mechanism by which Cp stimulates cellular iron uptake. Cp stimulated the rate of non-transferrin 55Fe uptake by iron-deficient K562 cells by 2–3-fold, using a transferrin receptor-independent pathway. Induction of Cp-stimulated iron uptake by iron deficiency was blocked by actinomycin D and cycloheximide, consistent with a transcriptionally induced or regulated transporter. Cp-stimulated iron uptake was completely blocked by unlabeled Fe3+ and by other trivalent cations including Al3+, Ga3+, and Cr3+, but not by divalent cations. These results indicate that Cp utilizes a trivalent cation-specific transporter. Cp ferroxidase activity was required for iron uptake as shown by the ineffectiveness of two ferroxidase-deficient Cp preparations, copper-deficient Cp and thiomolybdate-treated Cp. We propose a model in which iron reduction and subsequent re-oxidation by Cp are essential for an iron uptake pathway with high ion specificity. The balance required to maintain appropriate cellular and tissue iron levels has led to the evolution of multiple mechanisms to precisely regulate iron uptake from transferrin and low molecular weight iron chelates. A role for ceruloplasmin (Cp) in vertebrate iron metabolism is suggested by its potent ferroxidase activity catalyzing conversion of Fe2+ to Fe3+, by identification of yeast copper oxidases homologous to Cp that facilitate high affinity iron uptake, and by studies of “aceruloplasminemic” patients who have extensive iron deposits in multiple tissues. We have recently shown that Cp increases iron uptake by cultured HepG2 cells. In this report, we investigated the mechanism by which Cp stimulates cellular iron uptake. Cp stimulated the rate of non-transferrin 55Fe uptake by iron-deficient K562 cells by 2–3-fold, using a transferrin receptor-independent pathway. Induction of Cp-stimulated iron uptake by iron deficiency was blocked by actinomycin D and cycloheximide, consistent with a transcriptionally induced or regulated transporter. Cp-stimulated iron uptake was completely blocked by unlabeled Fe3+ and by other trivalent cations including Al3+, Ga3+, and Cr3+, but not by divalent cations. These results indicate that Cp utilizes a trivalent cation-specific transporter. Cp ferroxidase activity was required for iron uptake as shown by the ineffectiveness of two ferroxidase-deficient Cp preparations, copper-deficient Cp and thiomolybdate-treated Cp. We propose a model in which iron reduction and subsequent re-oxidation by Cp are essential for an iron uptake pathway with high ion specificity. Iron is important in many biological processes because it is an ideal O2 carrier and because it can function as a protein-bound redox element. Iron deficiency is common worldwide and in infants can cause severe neurological deficit (1DeMaeyer E. Adiels-Tegman M. World Health Stat. Quart. 1985; 38: 302-316PubMed Google Scholar). In contrast, iron in excess of cellular needs is toxic and dietary overload or hereditary hemochromatosis (HH) 1The abbreviations used are: HH, hereditary hemochromatosis; Cp, ceruloplasmin; GSH, reduced glutathione; Nramp, natural resistance-associated macrophage protein; NTA, nitrilotriacetic acid; PAGE, polyacrylamdie gel electrophoresis; MOPS, 4-morpholinepropanesulfonic acid. leads to tissue iron deposition and injury (2Britton R.S. Tavill A.S. Bacon B.R. Brock J.H. Halliday J.W. Pippard M.J. Powell L.W. Iron Metabolism in Health & Disease. W. B. Saunders, London1994: 311-351Google Scholar, 3Bacon B.R. Brittenham G.M. Park C.H. Tavill A.S. Ann. N. Y. Acad. Sci. 1988; 526: 155-163Crossref PubMed Scopus (11) Google Scholar). Alterations of iron pools have been implicated in diverse human diseases, including neurodegenerative disease, aging, atherosclerosis, cancer, and microbial infection (4Brock J.H. Brock J.H. Halliday J.W. Pippard M.J. Powell L.W. Iron Metabolism in Health & Disease. W. B. Saunders, London1994: 353-389Google Scholar). The precise balance required to maintain appropriate intracellular and tissue iron levels has led to the evolution of several independent cellular iron transport pathways. The transferrin-mediated iron uptake pathway has been extensively studied and is an important model for understanding the recycling, receptor-mediated endocytosis pathway. In this pathway, diferric transferrin is endocytosed by a cell surface transferrin receptor and iron is delivered to the cell by an endosomal pathway accompanied by recycling of transferrin and its receptor (5De Jong G. Van Dijk J.P. Van Eijk H.G. Clin. Chim. Acta. 1990; 190: 1-46Crossref PubMed Scopus (340) Google Scholar). The physiological significance of the transferrin receptor is suggested by its presence in virtually all cultured cells and by the fact that hypotransferrinemic animals develop severe hemochromatosis (6Craven C.M. Alexander J. Eldridge M. Kushner J.P. Bernstein S. Kaplan J. Proc. Natl. Acad. Sci. U. S. A. 1987; 84: 3457-3461Crossref PubMed Scopus (224) Google Scholar). The central importance of the transferrin pathway is shown by recent studies of HH, the most common inherited disease in people of northern European descent. Positional cloning studies of Feder et al.(7Feder J.N. Gnirke A. Thomas W. Tsuchihashi Z. Ruddy D.A. Basava A. Dormishian F. Domingo Jr., R. 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. Prass C.E. Quintana L. Starnes S.M. Schatzman R.C. Brunke K.J. Drayna D.T. Risch N.J. Bacon B.R. Wolff R.K. Nat. Genet. 1996; 13: 399-408Crossref PubMed Scopus (3366) Google Scholar) identified HFE as a candidate gene for HH, and two HFE mutations account for about 80–85% of all HH. The normal HFE gene product, by binding to the transferrin receptor and lowering its affinity for transferrin, down-regulates intestinal iron absorption (8Feder J.N. Penny D.M. Irrinki A. Lee V.K. Lebrón J.A. Watson N. Tsuchihashi Z. Sigal E. Bjorkman P.J. Schatzman R.C. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 1472-1477Crossref PubMed Scopus (731) Google Scholar). Mutant HFE does not bind the transferrin receptor and lacks this suppressor activity. Transferrin-independent cellular iron uptake has been described in many cultured cells including HepG2 (9Randell E.W. Parkes J.G. Olivieri N.F. Templeton D.M. J. Biol. Chem. 1994; 269: 16046-16053Abstract Full Text PDF PubMed Google Scholar) and K562 (10Inman R.S. Wessling-Resnick M. J. Biol. Chem. 1993; 268: 8521-8528Abstract Full Text PDF PubMed Google Scholar) cells. The function of the transferrin-independent system is unknown, but Kaplan et al. (11Kaplan J. Jordan I. Sturrock A. J. Biol. Chem. 1991; 266: 2997-3004Abstract Full Text PDF PubMed Google Scholar) have proposed that it clears potentially toxic, low molecular weight iron chelates (e.g. non-transferrin bound iron). Such chelates accumulate to a very large extent in HH patient plasma (12Grootveld M. Bell J.D. Halliwell B. Aruoma O.I. Bomford A. Sadler P.J. J. Biol. Chem. 1989; 264: 4417-4422Abstract Full Text PDF PubMed Google Scholar). Transferrin-independent uptake mechanisms are not well understood; however, two putative transporters, DCT1 and SFT, have been recently identified in mammalian cells (13Gutierrez J.A. Yu J. Rivera S. Wessling-Resnick M. J. Cell Biol. 1997; 139: 895-905Crossref PubMed Scopus (79) Google Scholar, 14Gunshin H. Mackenzie B. Berger U.V. Gunshin Y. Romero M.F. Boron W.F. Nussberger S. Gollan J.L. Hediger M.A. Nature. 1997; 388: 482-488Crossref PubMed Scopus (2679) Google Scholar). A cell surface ferrireductase is also active and may be required for transferrin-independent iron uptake (15Inman R.S. Coughlan M.M. Wessling-Resnick M. Biochemistry. 1994; 33: 11850-11857Crossref PubMed Scopus (72) Google Scholar, 16Jordan I. Kaplan J. Biochem. J. 1994; 302: 875-879Crossref PubMed Scopus (82) Google Scholar). Important insights into eukaryotic non-transferrin iron metabolism have come from recent yeast genetic studies. Two iron transporters (or “permeases”), Ftr1p from Saccharomyces cerevisiae (17Stearman R. Yuan D.S. Yamaguchi-Iwai Y. Klausner R.D. Dancis A. Science. 1996; 271: 1552-1557Crossref PubMed Scopus (582) Google Scholar) and the homologous Fip1p from Schizosaccharomyces pombe(18Askwith C. Kaplan J. J. Biol. Chem. 1997; 272: 401-405Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar), facilitate high affinity iron uptake by iron-deficient yeast. A surface ferrireductase, Fre1p (or its homologue Fre2p), is also necessary for yeast iron uptake (19Dancis A. Klausner R.D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). A for copper in iron uptake has led to the cloning of in S. cerevisiae C. Van A. L. S. D.M. Kaplan J. 1994; Full Text PDF PubMed Scopus Google Scholar, D.M. Kaplan J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, D.S. R. Dancis A. T. T. Klausner R.D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar) and its homologue in S. C. Kaplan J. J. Biol. Chem. 1997; 272: 401-405Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar), that are required for iron uptake. These are oxidases which the cell with the transporters (17Stearman R. Yuan D.S. Yamaguchi-Iwai Y. Klausner R.D. Dancis A. Science. 1996; 271: 1552-1557Crossref PubMed Scopus (582) Google Scholar). has to human ceruloplasmin and Cp is a copper with a ferroxidase activity that conversion of to The and Cp a role for the in vertebrate cell iron uptake. We have shown recently that Cp increases 55Fe uptake by iron-deficient HepG2 cells by a transferrin receptor-independent pathway Science. 1998; PubMed Scopus Google Scholar). iron transcriptionally Cp by HepG2 cells Science. 1998; PubMed Scopus Google Scholar), and in yeast C. Van A. L. S. D.M. Kaplan J. 1994; Full Text PDF PubMed Scopus Google Scholar, D.M. Kaplan J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). of iron in other of iron deficiency may be an important physiological function of Cp. The recent that patients with Cp gene have iron deposits in and other an important role for Cp in iron in D.S. R. Dancis A. T. T. Klausner R.D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, J. Science. 1996; 271: PubMed Scopus Google Scholar, J. J. Clin. 1996; PubMed Scopus Google Scholar). In this we the molecular mechanism by which Cp increases iron uptake by K562 cells. results that Cp-stimulated iron uptake Cp ferroxidase activity and utilizes a trivalent cation-specific transport pathway. human Cp was from Cp was by an and by and that about 80–85% was as the of Cp. from was from transferrin receptor was from and was from from was from and human reduced cycloheximide, nitrilotriacetic and other from K562 cells in with The cells iron-deficient by in for The cells with and with and in the A of was by with a excess of in R. W. J. Clin. 1990; PubMed Scopus Google Scholar). iron uptake, K562 cells in of for was and to a to The was with of and was by The of 55Fe bound a was from to for K562 and iron-deficient as with in the presence or of Cp for The cells two by and in and by a and by for from the by for in the and and was by into of of and to and in the gel was with In a in the was by for with or with a with and was by of Cp copper by with as described by et al. G. S. L. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). Cp was by with was the reduced as shown by of the was and the for The and excess by and by an MOPS, was by an ferroxidase was from as described by al. G. S. L. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). A was A. M. G. J. Biol. Chem. 1990; Full Text PDF PubMed Google Scholar) by of copper to in A excess of the was to in for copper was by Cp was for with in as described by et G. E. J. Biochem. PubMed Scopus Google Scholar). was by the of was by Cp ferroxidase activity was by a of the of and M. Clin. Chim. Acta. 13: PubMed Scopus Google Scholar). In Cp or Cp was to a gel to an the was with in for by with in ion by of was by the of a which was for We have shown that Cp stimulates high affinity iron uptake by iron-deficient HepG2 cells Science. 1998; PubMed Scopus Google Scholar). a of the mechanism of Cp-stimulated iron uptake, we used human K562 cells the and uptake are well (10Inman R.S. Wessling-Resnick M. J. Biol. Chem. 1993; 268: 8521-8528Abstract Full Text PDF PubMed Google Scholar). HepG2 cells which Cp in an to iron uptake Science. 1998; PubMed Scopus Google Scholar), K562 cells not Cp (or transferrin, J. Biol. Chem. Full Text PDF PubMed Google Scholar) precise of Cp uptake by iron-deficient K562 cells by about was about Cp and uptake was about a the human plasma of The of iron uptake by Cp was in iron-deficient cells but not in cells a consistent with studies of HepG2 cells Science. 1998; PubMed Scopus Google Scholar). Cp stimulated iron uptake by the of uptake from about to about the uptake rate that the of active transporters was not by Cp, but that the affinity of the for iron was or that the of was by Cp. to that Cp stimulated iron into the and the that Cp binding of iron to the cell surface (e.g. by conversion of Fe2+ to we iron uptake using a to of iron Van J. PubMed Scopus Google Scholar). to this cells are with a to Fe3+ to Fe2+ and with to results to with the normal Cp iron uptake by in iron-deficient K562 cells using the or the normal not iron intracellular and iron was K562 cells with the cells and and by In this Cp stimulated about a in in and all of this was for by in the or of the cell was bound to the intracellular of into was by and was not in the of D and the a that with of as was by with but not by a Cp into in from iron-deficient cells by about of the gel was shown by These results that the in iron with cells in the presence of Cp is to cellular uptake and into a active intracellular iron The that Cp not iron uptake by cells suggested that a required for the was induced or by iron this be K562 cells with actinomycin D or to iron Cp-stimulated iron uptake was completely blocked by of an iron (or a by iron deficiency Cp stimulates iron as well as K562 cells and Cp-stimulated iron was Cp not iron The of or to the as Fe3+ not this was consistent with in HepG2 cells Science. 1998; PubMed Scopus Google Scholar), but not with results of et al. M. S. 1997; PubMed Scopus Google Scholar) who a Cp-stimulated in iron from HepG2 cells. The may be to the of Cp the of et al. M. S. 1997; PubMed Scopus Google Scholar) may processes by Cp. The conversion of Fe2+ to Fe3+ by Cp ferroxidase activity is to facilitate the binding of iron to S. D.A. E. J. Biol. Chem. Full Text PDF PubMed Google Scholar). is that Cp stimulates cellular iron uptake by the of which Fe3+ by transferrin receptor-mediated K562 cells are not to transferrin J. Biol. Chem. Full Text PDF PubMed Google Scholar), this pathway was by of a transferrin to the The Cp-stimulated iron uptake but completely blocked 55Fe uptake by transferrin results using a transferrin receptor These results that Cp-stimulated iron uptake was transferrin but not the that other receptor-mediated processes endocytosis of the putative Cp J. 1988; M. M. 1985; 13: Google Scholar) or other The was used to receptor-mediated endocytosis R. M.C. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). uptake, but was a very of transferrin-mediated iron uptake results using R.D. Van J. G. C. A. J. Biol. Chem. Full Text PDF PubMed Google Scholar) to receptor-mediated endocytosis not The of Cp-stimulated cell transport was divalent including or and not a excess not for Cp-stimulated 55Fe uptake In the Fe2+ in the presence of Cp completely uptake. that Cp stimulates a with for a trivalent the is consistent with a for ferroxidase activity of Cp. The role of Cp ferroxidase activity was using Cp. We of the of to bind copper and Cp ferroxidase activity G. E. J. Biochem. PubMed Scopus Google Scholar). of Cp with completely blocked its iron uptake activity not iron uptake by of the cellular transport pathway the of Cp to cells with thiomolybdate-treated Cp iron uptake. The by was by an for ferroxidase activity The of was by and which a the role of Cp ferroxidase activity in iron was by of copper by with G. S. L. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). not into iron-deficient K562 cells The of ferroxidase activity was by an and the of the was shown by and the of it was to by with copper in the presence of as a The Cp about of its ferroxidase activity and about of its iron uptake activity D and The of two of Cp is that ferroxidase activity is necessary for Cp-stimulated iron uptake, and that the Fe3+ is into the cell by a trivalent transporter. The results also that Cp increases iron uptake by of not by of the the is or it multiple trivalent we the of Al3+, Cr3+, and to for iron uptake. trivalent cations Cp-stimulated iron cellular uptake in a about trivalent cations may Cp ferroxidase activity E. J. Biol. Chem. Full Text PDF PubMed Google Scholar), we investigated the of Al3+, Cr3+, and iron uptake to of ferroxidase activity by for a transporter. Al3+, a that iron uptake not Cp ferroxidase was ion and Ga3+, not ferroxidase activity that trivalent cations not Cp-stimulated iron uptake by of Cp ferroxidase activity. may iron uptake by with iron for a binding a trivalent cation-specific transporter. The of iron transport by multiple trivalent cations a trivalent transporter. the results not that cations are may with Fe3+ for the binding the subsequent into the this uptake by K562 cells was uptake was high and two consistent with uptake by the used for Cp-stimulated iron uptake uptake was in iron-deficient K562 cells in cells. uptake was by Fe3+ by and and by Fe2+ Cp, but not by Fe2+ Cp not uptake, that Cp does not the transporter. results that the Cp-stimulated Fe3+ uptake is a high affinity trivalent transporter. results that Cp stimulates cellular iron uptake by conversion of Fe2+ to Fe3+ and transport by a trivalent cation-specific transporter. uptake mechanism the iron uptake pathway in S. cerevisiae and S. in A J. Science. 1996; 271: PubMed Scopus Google Scholar). to this model the yeast ferrireductase the uptake pathway by reduction of Fe3+ to The reduced is to (or a Cp homologue with ferroxidase activity. Fe3+ is into the cell by Ftr1p (or a that with These yeast are all transcriptionally iron deficiency by of the Y. Dancis A. Klausner R.D. J. PubMed Scopus Google Scholar). we propose a model in which Cp is in the of an iron transport pathway for mammalian cells In this model iron is reduced by a surface ferrireductase, by Cp, and into the cell by a trivalent cation-specific transporter. The for the mammalian pathway and the and with yeast are The for a surface ferrireductase activity for transferrin-independent iron uptake has been shown for human I. Kaplan J. Biochem. J. 1994; 302: 875-879Crossref PubMed Scopus (82) Google Scholar), HepG2 (9Randell E.W. Parkes J.G. Olivieri N.F. Templeton D.M. J. Biol. Chem. 1994; 269: 16046-16053Abstract Full Text PDF PubMed Google Scholar), and K562 cells (15Inman R.S. Coughlan M.M. Wessling-Resnick M. Biochemistry. 1994; 33: 11850-11857Crossref PubMed Scopus (72) Google Scholar). studies have not the role of ferrireductase activity in Cp-stimulated iron however, the for the is consistent with an important role of a ferrireductase physiological in the of The of the mammalian ferrireductase is not but the may be Fre1p has to its (19Dancis A. Klausner R.D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). results that Cp stimulates 55Fe uptake by iron-deficient K562 cells by to for ferroxidase activity for Cp-stimulated iron uptake was shown using two ferroxidase-deficient thiomolybdate-treated Cp and of the pathway is to the yeast pathway a with to Cp, has been implicated in high affinity iron uptake C. Van A. L. S. D.M. Kaplan J. 1994; Full Text PDF PubMed Scopus Google Scholar). Cp, is a with ferroxidase activity D.M. Kaplan J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). A for ferroxidase activity in iron uptake has been suggested by the ferroxidase activity and iron uptake C. Van A. L. S. D.M. Kaplan J. 1994; Full Text PDF PubMed Scopus Google Scholar, D.S. R. Dancis A. T. T. Klausner R.D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). A recent has shown by that the ferroxidase activity of is required for high affinity iron uptake by yeast Kaplan J. J. Biol. Chem. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). A the yeast and mammalian is in the by iron deficiency of in S. cerevisiae C. Van A. L. S. D.M. Kaplan J. 1994; Full Text PDF PubMed Scopus Google Scholar) and Cp in HepG2 cells Science. 1998; PubMed Scopus Google Scholar). K562 cells not Cp, A is in the cellular of the In is and with a D.M. Kaplan J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). In contrast, mammalian Cp has a and is may the of iron in mammalian that is in yeast. The Cp and the trivalent is not is that Cp with the to the of Fe3+ The identification of putative the surface of multiple cell M. M. 1985; 13: Google Scholar), which are by iron deficiency A. Acta. 1996; PubMed Scopus Google Scholar), is consistent with this Fe3+ by Cp may be delivered to the by The ion of the mammalian non-transferrin iron uptake is in to the cells and uptake used in that the Cp-stimulated iron in K562 cells is trivalent cation-specific the of by divalent the by trivalent the transport of and the for ferroxidase activity in iron have suggested that Fe2+ is the transport the for and by (9Randell E.W. Parkes J.G. Olivieri N.F. Templeton D.M. J. Biol. Chem. 1994; 269: 16046-16053Abstract Full Text PDF PubMed Google Scholar). in of recent studies the of reduced and iron in the transport pathway, not the ion for Fe2+ transport has been by studies that non-transferrin iron uptake is blocked by divalent cations A. Alexander J. J. C.M. Kaplan J. J. Biol. Chem. 1990; Full Text PDF PubMed Google Scholar). studies in K562 cells an of divalent cations to a in with (10Inman R.S. Wessling-Resnick M. J. Biol. Chem. 1993; 268: 8521-8528Abstract Full Text PDF PubMed Google Scholar). Two that and other trivalent cations regulate iron uptake PubMed Google Scholar, S. B.E. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar), but is for iron uptake a trivalent cation-specific pathway. the ion of the yeast iron transporters Ftr1p and have not been The of the Cp-stimulated iron uptake is not of the the two yeast transporters has not been homologous mammalian have not been mammalian iron transporters have recently been and are pathway iron is a of the gene H. Mackenzie B. Berger U.V. Gunshin Y. Romero M.F. Boron W.F. Nussberger S. Gollan J.L. Hediger M.A. Nature. 1997; 388: 482-488Crossref PubMed Scopus (2679) Google Scholar). divalent cations are by DCT1 it is not to Cp-stimulated A role for SFT, an iron from K562 is by the fact that the iron transport is not by most divalent cations. its in recycling and the stimulated uptake of iron and non-transferrin iron uptake, indicate a role in intracellular iron transport (13Gutierrez J.A. Yu J. Rivera S. Wessling-Resnick M. J. Cell Biol. 1997; 139: 895-905Crossref PubMed Scopus (79) Google Scholar). The presence of iron reduction and in a pathway is In of the yeast uptake pathway described an ferrireductase and an intracellular a C. Van A. L. S. D.M. Kaplan J. 1994; Full Text PDF PubMed Scopus Google Scholar, A. Yuan D.S. C. C. Kaplan J. Klausner R.D. 1994; Full Text PDF PubMed Scopus Google Scholar). model has been by and the ferroxidase of Cp, is D.M. Kaplan J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The function of the ferrireductase is most to Fe2+ from Fe3+ in non-transferrin bound iron in The function of the ferroxidase is to the Fe2+ to the Fe3+ that is by the trivalent transporter. The of a pathway of iron transport a pathway of Fe2+ transport a divalent cation-specific is is that the pathway specificity. a trivalent cation-specific because Fe3+ with the trivalent cations in tissues. In the of a divalent Fe2+ have a with as to this proposed the ferrireductase activity iron and the ferroxidase activity transport specificity. The precise role of Cp in iron in is The and that in plasma Cp or activity may iron and to human The of hemochromatosis in patients is as that the normal function of Cp is to iron from cells and S. A. H. S. S. N. N. Nat. Genet. PubMed Scopus Google Scholar, H. S. S. S. M. N. Ann. PubMed Scopus Google Scholar). studies may also be consistent with a role of Cp in cellular iron uptake. iron into cells by the pathway subsequent iron uptake iron in by other pathways. In this Cp may in as a of iron uptake, and in its the in iron may to of an iron to a that low affinity uptake by the tissue A understanding of the function of Cp in cellular iron in a to the role of Cp in iron in physiological and