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Bryan Mackenzie, PhD
Associate Professor

Iron deficiency is the most prevalent micronutrient deficiency worldwide. Meanwhile iron overload associated with conditions like hereditary hemochromatosis or thalassemia poses a serious threat to many other individuals. Divalent metal-ion transporter-1 (DMT1) is indispensable for iron homeostasis. It is the front-line, primary route of iron uptake in the intestine. DMT1 is also responsible for mobilization of iron from the endosome to cytosol, a crucial step in the transferrin-associated uptake of iron in erythroid precursor cells. Export of iron from enterocytes and from macrophages (recycling iron from senescent red blood cells) is thought to be mediated by ferroportin (Fpn). Whereas Fpn was identified a decade ago, very little is known about how this transporter works.

We are investigating the molecular mechanisms, substrate selectivity and structure–function relationships of DMT1 and Fpn, and their working associations with other components of the iron-absorptive machinery. Recent work has reveaed that both DMT1 and Fpn are iron-preferring transporters that are reactive also with certain other transition metals [1,2]. Calcium is a low-affinity noncompetitive inhibitor—but not a transported substrate—of DMT1, explaining in part the effect of high dietary calcium on iron bioavailability [5]. We have also characterized the Zrt-, Irt-like protein-14 (Zip14) and found it to be a broad-scope metal-ion transporter that could participate in uptake of nontransferrin-bound iron (NTBI) characteristic of iron-overload disorders [4]. We hope to gain a better understanding of how these transporters function in diverse environments and how they may contribute to the etiology of iron-overload disorders. We expect these studies will help drive the development of novel approaches to improve metal-ion nutrition, and treat iron overload or heavy-metal intoxication.

The approaches we use include the voltage clamp, radiotracer assays, and fluorescence-based assays in RNA-injected Xenopus oocytes, together with the use of genetically-modified animal models (including the intestine-specific DMT1 knockout mouse, and intestinal Na+/H+ exchanger nulls).

DMT1-mediated Fe2+ transport is energized by the H+ electrochemical gradient, placing DMT1 in a large and important class of membrane proteins we call cotransporters. Mammalian cotransporters use Na+ or H+ electrochemical gradients as the energy source to drive uphill (concentrative) transport of a broad range of nutrients or solutes. Conventional cotransporter models are deterministic—they comprise a series of ligand-induced conformational changes that result in alternating access to a substrate-binding region, and ion-coupling stoichiometries are fixed. Experimental data for DMT1, however, are difficult to reconcile with such a model. When we expressed wildtype mammalian DMT1 or mutant proteins in Xenopus oocytes, we observed significant uncoupled fluxes ('slippage') both of H+ and Fe2+ and variable stoichiometry [3]. Among our present goals, the development of a new model that can account for such slippage will better direct experimental investigation into the molecular mechanisms of DMT1.

Selected Publications:
  • [1] Colin J Mitchell, Ali Shawki, Tomas Ganz, Elizabeta Nemeth, and Bryan Mackenzie (2014) Functional properties of human ferroportin, a cellular iron exporter reactive also with cobalt and zinc. Am J Physiol Cell Physiol, in press.
    View original publication at AJP:Cell Online
  • [2] Anthony C Illing, Ali Shawki, Christopher L Cunningham, and Bryan Mackenzie (2012) Substrate profile and metal-ion selectivity of human divalent metal-ion transporter-1. J Biol Chem, 287, 30485–30496.
    View original publication at JBC.org
  • [3] Ali Shawki, Patrick B Knight, Bryan D Maliken, Eric J Niespodzany, and Bryan Mackenzie (2012) H+-coupled divalent metal-ion transporter-1: Functional properties, physiological roles and therapeutics. Curr Top Membr 70, 169–214. [Review]
    View original publication at ScienceDirect.com
  • [4] Jorge J Pinilla-Tenas, Brian K Sparkman, Ali Shawki, Anthony C Illing, Colin J Mitchell, Ningning Zhao, Juan P Liuzzi, Robert J Cousins, Mitchell D Knutson, and Bryan Mackenzie (2011) Zip14 is a complex broad-scope metal-ion transporter whose functional properties support roles in the cellular uptake of zinc and nontransferrin-bound iron. Am J Physiol Cell Physiol 301, C862–C871.
    View original publication at AJP:Cell Online
  • [5] Ali Shawki and Bryan Mackenzie (2010) Interaction of calcium with the human divalent metal-ion transporter-1. Biochem Biophys Res Comm 393, 471–475.
    View original publication at ScienceDirect
Recent Abstracts Relating to Work-in-Progress:
  • Ali Shawki, Sarah R Anthony, Yasuhiro Nose, Tomasa Barrientos De Renshaw, Dennis J Thiele, and Bryan Mackenzie (2012) Intestinal divalent metal-ion transporter-1 is critical for iron homeostasis but is not required for maintenance of Cu or Zn. FASEB J 26, 1112.2. [Abstract]
    View abstract at FASEB Journal Online
  • Ali Shawki, Robert Kim, Sarah R Anthony, Patrick B Knight, Rusty Baik, Emily M Bradford, Gary E Shull, and Bryan Mackenzie (2012) Intestinal brush-border Na+/H+ exchanger NHE3 is required for iron homeostasis in the mouse. Genes Nutr 6, Suppl 1, S46. [Abstract]
    View abstract at Springer Online
Publications, Complete List at PubMed

Publications and Citations at Google Scholar

Other Publications, Not Listed in PubMed:
  • Bryan Mackenzie and Matthias A Hediger (2004) Molecular physiology of the H+-coupled iron transporter DMT1, in: The Nramp Family, Mathieu Cellier and Philippe Gros (Eds), Landes Bioscience / Eurekah.com / Kluwer Academic / Plenum Publishers, Georgetown, pp 73–81. [Review]
  • Bryan Mackenzie (1999) Selected techniques in membrane transport, ch 11 in: Biomembrane Transport, Lon J Van Winkle (Ed), Academic Press, San Diego, pp 327–342. [Review]
    View pdf version of this article © 1999 Academic Press
  • Bryan Mackenzie, Aamir Ahmed and Michael J Rennie (1992) Muscle amino acid metabolism and transport, in: Mammalian Amino Acid Transport: Mechanisms and Control, Michael S Kilberg and Dieter Häussinger (Eds), Plenum Press, New York, pp 195–231. [Review]
    Buy the book at Amazon.com

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