title: Functional properties of human ferroportin, a cellular iron exporter reactive also with cobalt and zinc.
abstract: Iron homeostasis is achieved by regulating the intestinal absorption of the metal and its recycling by macrophages. Iron export from enterocytes or macrophages to blood plasma is thought to be mediated by ferroportin under the control of hepcidin. Although ferroportin was identified over a decade ago, little is understood about how it works. We expressed in Xenopus oocytes a human ferroportin-enhanced green fluorescent protein fusion protein and observed using confocal microscopy its exclusive plasma-membrane localization. As a first step in its characterization, we established an assay to detect functional expression of ferroportin by microinjecting oocytes with (55)Fe and measuring efflux. Ferroportin expression increased the first-order rate constants describing (55)Fe efflux up to 300-fold over control. Ferroportin-mediated (55)Fe efflux was saturable, temperature-dependent (activation energy, Ea ≈ 17 kcal/mol), maximal at extracellular pH ≈ 7.5, and inactivated at extracellular pH < 6.0. We estimated that ferroportin reacts with iron at its intracellular aspect with apparent affinity constant < 10(-7) M. Ferroportin expression also stimulated efflux of (65)Zn and (57)Co but not of (64)Cu, (109)Cd, or (54)Mn. Hepcidin treatment of oocytes inhibited efflux of (55)Fe, (65)Zn, and (57)Co. Whereas hepcidin administration in mice resulted in a marked hypoferremia within 4 h, we observed no effect on serum zinc levels in those same animals. We conclude that ferroportin is an iron-preferring cellular metal-efflux transporter with a narrow substrate profile that includes cobalt and zinc. Whereas hepcidin strongly regulated serum iron levels in the mouse, we found no evidence that ferroportin plays an important role in zinc homeostasis.
authors.affiliation: Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Cincinnati, Ohio;
authors.forename: Colin J
authors.initials: CJ
authors.lastname: Mitchell
authors.forename: Ali
authors.initials: A
authors.lastname: Shawki
authors.forename: Tomas
authors.initials: T
authors.lastname: Ganz
authors.forename: Elizabeta
authors.initials: E
authors.lastname: Nemeth
authors.forename: Bryan
authors.initials: B
authors.lastname: Mackenzie
mesh_terms: D000818:Animals; D001692:Biological Transport; D027682:Cation Transport Proteins / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism*; D003035:Cobalt / Q000378:metabolism*; D017930:Genes, Reporter; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D064451:Hepcidins / Q000008:administration & dosage / Q000378:metabolism; D006706:Homeostasis; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000097:blood / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018613:Microscopy, Confocal; D011485:Protein Binding; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013696:Temperature; D014982:Xenopus laevis; D015032:Zinc / Q000097:blood / Q000378:metabolism*; D000098742:Ferroportin
chemical_list: D027682:Cation Transport Proteins; D064451:Hepcidins; D011993:Recombinant Fusion Proteins; C485184:enhanced green fluorescent protein; D000098742:Ferroportin; D049452:Green Fluorescent Proteins; D003035:Cobalt; D007501:Iron; D015032:Zinc
keywords: Xenopus laevis oocyte; cobalt transport; enterocytes; iron overload; iron transport; macrophages; membrane transport; trace metal nutrition; zinc transport