Transcript Lecture 07 Absorption 2

Intestinal Absorption of Minerals II
Iron and other micro-metals
Handling the metal internally
Expelling the metal from the enterocyte
Ferritin (Fe)
Metallothionein
(Zn, Cu, Cd)
Serosal Side
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 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
Ferritin
Gold are gated pores that control iron release
from the inner core
To store iron, the Fe2+ must
first be oxidized to Fe3+
To release iron, the Fe3+ must first
be reduced to Fe2+.
24 subunits, with
2 distinct isoforms: H ferritin
and L ferritin
H: predominates in heart
L: predominates in liver
b.
Ferrihydrite
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.
Iron Export from Mucosal Cells
Iron Absorption (heme and non-heme)
Duodenal Mucosa
Duodenal Lumen
HemeProtein
Biliverdin
HFE
Heme
+
Polypeptides
Heme
FR
DCT-1
Fe3+
Bilirubin
Fe3+ Mucin
(gastroferrin)
B3 integrin
Fe3+
Fe2+
paraferrin
Fe
3+
Fe2+
Fe3+
Bilirubin
CO
B2-microglobulin
Ferritin
Fe3+
Plasma
Mobilferrin
CO
Fe
2+
Ferroportin
Haephestin
Fe3+
(vesicles)
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
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
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
Facilitate
Amino Acids
Organic Acids
Cysteine
Glycine
Histidine
Lysine
Methionine
Ascorbate
Citrate
Lactate
Malate
tartaric
Sugars
Fructose
Sorbitol
Inhibit
Fibers
Bran
Hemicellulose
Cellulose
Pectin
Guar gum
Polyphenolics
Flavonoids
Anthrocyanins
Isoflavonoids
Others
Oxalates
Carbonate
Phosvitin (ironbinding 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 storage protein
(iron-dependent enzyme
in the cytosol)
No iron, no ferritin
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
Iron regulatory protein
to iron absorption in the intestine
(IRP)
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
Cu
Cysteine
6 copper atoms inside bound to cysteines
Storage and Release of Metals from Metallothionein
Reduced glutathione
controls the storage
of copper and zinc
by glutathione;
oxidized controls the
release
Reduced
Absorption
Serosa
Mucosa
NSBP
Zn++
Saturable =
Bound to
form transport
ligand
CRIP
Zn++
CRIP-Zn
MTI-Zn
MTI
Zn++
Zn++-Albumin
Non-saturable = Passive Diffusion
Albumin
Zn++-Albumin
Zn++
CRIP=cysteine-rich intestinal protein; MTI=metallothionine;
NSBP, non-specfic binding protein
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
Proteins Involved in the Absorption and Transport of Macro-metals
Process
Sodium
Potassium
Calcium
Magnesium
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
Albumin
Albumin
None
Proteins Involved in the Absorption and Transport of Micro-metals
Process
Iron
Zinc
Copper
Cellular access Mobilferrin
protein
DCT-1 (DMT-1)
Zip4
Ctr1
Cytosol
Ferritin
storage protein
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