Intestinal Absorption of Minerals II

Iron and other micro-metals

Handling the metal internally

Expelling the metal from the enterocyte

Serosal Side

Ferritin (Fe)

Metallothionein(Zn, Cu, Cd)

Efflux system for export

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 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.

Metallothionein: Binds Cu, Zn, and Cd. Will not bind Fe

Ferritin: Binds Fe exclusively

Binding to Internal Factors

Ferritin

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

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 heartL: predominates in liver

To store iron, the Fe2+ must first be oxidized to Fe3+

b.

                                                                                                                                                                         

           

Iron is stored in ferritin as Fe(III) in the mineral [FeO(OH)]8[FeO(H2PO4)]. This mineral

can be represented by ferrihydrite, FeO(OH) (shown above). Note: the name "ferrihydrite" is used for both [FeO(OH)]8[FeO(H2PO4)] and FeO(OH). Note: Iron (III)

ions are shown in brown, and oxygen (II) ions are shown in red.

Ferrihydrite

Iron Export from Mucosal Cells

Duodenal Lumen Duodenal Mucosa Plasma

Heme-Protein

Heme+

Polypeptides

Mucin(gastroferrin)

Fe3+

Fe3+

Fe3+

B2-microglobulin

HFE Biliverdin Bilirubin Bilirubin

CO COHeme

Oxygenase

Heme

FeFe2+2+

FeFe33

++

DCT-1

B3 integrin

paraferrin

Mobilferrin (vesicles)

FeFe2+2+ FeFe2+2+

FeFe33

++

Iron Absorption (heme and non-heme)

Ferroportin

Ferritin

FeFe3+3+

FR FeFe3+3+Haephestin

Transferrin

Ferroportin, the only way iron can escape from the cell.

Adriana Donovan and Nancy Andrews were the first to show the importance of ferroportin

Knockout mouse cells lacking the gene for ferroportin are unable to release iron

(blue)

Ferroportin appears to be the portal for releasing iron from the cell. Also called IREG1 and MPT1

FP

Ferroportin

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

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

Regulation of Iron Uptake

Exogenous dietary factors

Role of IRPs (Iron regulatory proteins)

Dietary Factors that Influence Iron Absorption

Amino Acids Organic Acids Sugars

CysteineGlycineHistidineLysineMethionine

AscorbateCitrateLactateMalatetartaric

FructoseSorbitol

Facilitate

Inhibit

BranHemicelluloseCellulosePectinGuar gum

Fibers PolyphenolicsFlavonoidsAnthrocyaninsIsoflavonoids

Others

OxalatesCarbonatePhosvitin (iron-binding protein in egg yolk)

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

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.

How do you interpret these observations?

(iron-dependent enzyme in the cytosol)

Iron storage protein

No iron, no ferritin

Regulation at the level of transferrin receptor and ferritin mRNA

Iron regulatory protein (IRP)

Iron regulatory protein (IRP)

Iron response elements

Iron response elements

Transferrin receptor is believed to be the factor that tunes iron status of the body

to iron absorption in the intestine

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

Vesicle Transport of Metals

Zn Cu

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

Metallothionein

6 copper atoms inside bound to cysteines

Cu

Cysteine

Storage and Release of Metals from Metallothionein

Reduced glutathione controls the storage of copper and zinc by glutathione; oxidized controls the release

Reduced

Absorption

Mucosa Serosa

Zn++ Zn++

NSBP

MTI

MTI-Zn

CRIP

CRIP-Zn

Zn++ Zn++

Zn++-Albumin

Albumin

CRIP=cysteine-rich intestinal protein; MTI=metallothionine; NSBP, non-specfic binding protein

Non-saturable = Passive Diffusion

Saturable =Bound to

form transportligand Zn++-Albumin

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

Cellular access

protein

Glucose, amino acid transporters

K+ channel protein, Na/K ATPase

CaT1 DCT-1 (DMT-1)?

Cytosol storage protein

None None Vesicles None

Cytosol

trafficking protein

None None Calbindin None

Basal export protein

Na/K ATPase None Ca-ATPase None

Plasma trafficking protein

None None Albumin Albumin

Sodium Potassium Calcium Magnesium

Proteins Involved in the Absorption and Transport of Macro-metals

Process

Cellular access

protein

Mobilferrin

DCT-1 (DMT-1)

Zip4 Ctr1

Cytosol storage protein

Ferritin Metallothionein Metallothionein

Cytosol

trafficking protein

Mobilferrin Zip4-containing vesicles, CRIP

Atox1

Basal export protein

Ferroportin ZnT1 ATP7A

Plasma trafficking protein

Transferrin 2- macroglobulin albumin

Albumin,

transcuprein, ceruloplasmin

Process Iron Zinc Copper

Proteins Involved in the Absorption and Transport of Micro-metals