Intestinal Absorption of Minerals II

1. Intestinal Absorption of Minerals II Iron and other micro-metals

2. Handling the metal internally Expelling the metal from the enterocyte Ferritin (Fe) Metallothionein (Zn, Cu, Cd) Serosal Side Efflux system for export

3. A metal ion upon entering the enterocytes is either delayed in transit or released quickly from antiluminal (basolateral) surface into the system Binding to Internal Factors Binding to internal stationary factors such as proteins and vesicles allows the metal to be sequestered for safety purposes and for regulating the flow of that metal into the system. The above scenario plays out for Fe, Zn and Cu, where storage and transport proteins have been identified. Ferritin: Binds Fe exclusively Metallothionein: Binds Cu, Zn, and Cd. Will not bind Fe

4. Ferritin Gold are gated pores that control iron release from the inner core

5. To store iron, the Fe2+ must first be oxidized to Fe3+ 24 subunits, with 2 distinct isoforms: H ferritin and L ferritin To release iron, the Fe3+ must first be reduced to Fe2+. H: predominates in heart L: predominates in liver

6. Ferrihydrite

7. Iron Export from Mucosal Cells

8. B2-microglobulin Fe2+ Iron Absorption (heme and non-heme) Duodenal Lumen Duodenal Mucosa Plasma Heme- Protein Heme + Polypeptides Biliverdin Bilirubin Bilirubin HFE CO CO Heme Oxygenase Heme Ferroportin Fe2+ Fe3+ Ferritin paraferrin Haephestin FR Fe3+ Fe3+ DCT-1 Fe2+ Fe3+ Mobilferrin (vesicles) Fe3+ Fe3+ B3 integrin Fe3+ Mucin (gastroferrin) Transferrin

9. Ferroportin, the only way iron can escape from the cell. Adriana Donovan and Nancy Andrews were the first to show the importance of ferroportin

10. Knockout mouse cells lacking the gene for ferroportin are unable to release iron (blue)

11. Ferroportin appears to be the portal for releasing iron from the cell. Also called IREG1 and MPT1 Ferroportin FP Hc Hepcidin (Hc) downregulates the surface concentration of ferroportin thereby controlling the concentration of the iron exported from the cell. Hc Considered the master regulator of cellular iron export

12. Hephaestin (a Cu protein): Iron exported from the enterocyte at the basal surface is primarily Fe2+. In order to transfer Fe2+ to transferrin it must be oxidized to Fe3+. Oxidation of Fe2+ to Fe3+ on the serosal side of the intestine is catalyzed by a copper protein, haephestin. Haephestin (name after the Greek god of metals) was isolated as the gene product of a genetic defect in mice called Sex-Linked Anemia (SLA). Mice with the defect were iron deficient causing a pronounced anemia

13. Regulation of Iron Uptake Exogenous dietary factors Role of IRPs (Iron regulatory proteins)

14. Dietary Factors that Influence Iron Absorption Facilitate Amino Acids Organic Acids Sugars Cysteine Glycine Histidine Lysine Methionine Ascorbate Citrate Lactate Malate tartaric Fructose Sorbitol Inhibit Polyphenolics Others Fibers Bran Hemicellulose Cellulose Pectin Guar gum Flavonoids Anthrocyanins Isoflavonoids Oxalates Carbonate Phosvitin (iron-binding protein in egg yolk)

15. Iron, a “one-way” metal Excretion of iron from the body is not regulated Iron status of the system is controlled only at the absorption stage

16. Intestinal iron transport is influenced by the amount of iron in the diet. Sequestering by ferritin when iron is in abundant supply is one mechanism. Another is regulating the surface density of the importing and exporting factors. DMT-1 and ferroprotin 1(FPN1), respectively. Both are subject to mobilizing effects in response to iron. When iron is low DMT1 is rich on the cell surface primed to input more iron. FPN1 in low iron is localize to the cytosol. When iron is enriched DMT1 is down-regulated, meaning it shifts to the cytosol. The opposite occurs with FPN1 where movement in response to iron is to the membrane preparing the cell to release more iron.

17. How do you interpret these observations? Iron storage protein (iron-dependent enzyme in the cytosol) No iron, no ferritin

18. Regulation at the level of transferrin receptor and ferritin mRNA Iron response elements Iron regulatory protein (IRP) Iron response elements Transferrin receptor is believed to be the factor that tunes iron status of the body to iron absorption in the intestine Iron regulatory protein (IRP)

19. Addendum IRPs control the expression of DMT-1 (DCT-1, Nramp2) mRNA and protein and are critically important to secure physiological levels of ferroportin, the iron export protein. IRPs thus coordinate the synthesis of key iron metabolism proteins in the duodenum. Galy et al, 2008

20. Vesicle Transport of Metals Zn Cu

21. Metallothionein A small metal binding protein with an unusually high amount of cysteine residues. One third of the residues are cysteine Binding sites for Zn2+ and Cu+ in different locations of the protein Primary role was considered to be detoxification, not nutrition

22. Metallothionein Cu Cysteine 6 copper atoms inside bound to cysteines

23. Storage and Release of Metals from Metallothionein Reduced glutathione controls the storage of copper and zinc by glutathione; oxidized controls the release Reduced

24. Absorption Serosa Mucosa NSBP Zn++-Albumin CRIP Zn++ Zn++ Saturable = Bound to form transport ligand CRIP-Zn Albumin MTI-Zn Zn++-Albumin MTI Zn++ Zn++ Non-saturable = Passive Diffusion CRIP=cysteine-rich intestinal protein; MTI=metallothionine; NSBP, non-specfic binding protein

26. A 13-year-old girl presented with a history of red scaly plaques involving the chest, arms and legs beginning in infancy. Punch biopsy revealed psoriasiform hyperplasia and pallor of the epidermis. The patient's serum zinc level was 36 μg/dl [nl. 66-144 μg/dl]. A diagnosis of acrodermatitis enteropathica was established and the patient responded well to zinc replacement therapy. Acrodermatitis enteropathica is a rare autosomal recessive disorder caused by mutations in SLC39A4, which encodes the tissue-specific zinc transporter ZIP4

27. Proteins Involved in the Absorption and Transport of Macro-metals Sodium Potassium Calcium Magnesium Process

28. Proteins Involved in the Absorption and Transport of Micro-metals Process Iron Zinc Copper