Iron: Can’t live without enough of it Can’t live with too much of it

Camp Sunshine, July 15th 2015

Adapted from DBA Day Iron Overload by Dr. Lawrence Wolfe

Oxygen solubility

Plasma 2.3 ml/L

Whole Blood 200 ml/L

Hemoglobin and Myoglobin Reduce oxygen’s reactivity

O2 X

H2O OxX

Oxygen Transport Proteins – Hemoglobin/Myoglobin

Protected environment provided by Mb and Hb

The Heme Prosthetic Group

The heme iron has two oxidation states: Fe2+, ferrous; Fe3+, ferric

Ferrous iron can form up to 6 bonds

Ferric iron doesn’t bind oxygen

O2 Fe2+

H2O Fe3+

protected environment of globin chains

pathogenic variants

protoporphyrin IX

When bound to proteins both oxygen and iron are in protected states and bad things don’t happen

Compartment Iron Content (mg) Total Body Iron (%)

hemoglobin iron 1500(W)-2000(M) 67

storage iron (ferritin, hemosiderin) 1000 27

myoglobin iron 130 3.5

other tissue iron (cytochromes, etc.) 8 0.2

transport iron (transferrin) 3 0.08

labile pool 80 2.2

Iron Metabolism – Distribution in the Human Body

Iron is an essential, but also potentially highly toxic nutrient. Its uptake, transport, and storage in the body are highly regulated

Iron Distribution in Humans

Fe2+ + H2O2 Fe3+ + OH- + OH

proteins

nucleic acids

lipidsFenton Reaction

mutation

macrophage bone marrow

reactive oxygen species (ROS)

chain reaction more ROS production

excess

AKA: nontransferrin bound iron (NTBI)

ferrihydrite

Fe3+

OH/PO4

Adapted from Casiday and Frey, Washington University St. Louis

Fe3+ Fe2+

Iron Storage - Ferritin

Ferritin enters serum by an unknown mechanism under normal conditions (values proportional to cellular content) and is used as a non-invasive measure of iron stores. Measurements of serum ferritin can be used in the diagnosis of disorders of iron metabolism or tissue damage. Normal values: men 12-300 ng/ml; women 10-150 ng/ml. Ferritin can also be released to serum by damage to cells of the liver, spleen, or bone marrow and other pathogenic states

L subunits (iron binding)

H subunits (ferrioxidase activity)

(hemosiderin)

Iron Transport - Transferrin

Fe2+ Fe2+

Fe3+

ceruloplasmin

+

transferrin

Fe3+-transferrin-Fe3+

transferrin receptor

internalization

33%

67%

enterocytes liver

macrophagestransferrin saturation

Iron Uptake from Diet

ingested iron

Fe3+ Fe2+

R DMT1

Fe2+

ferriportin

ferritin

not absorbed

Fe2+Fe3+

transferrin

enterocyte

daily requirement

men 10 mg/1mg

menstruating women 20mg/2mg

vitamin C, ethanol

poor bioavailablity

GUT

CIRCULATION

macrophages play an important role in regulating circulating

iron using transporters similar or identical to those found on

enterocytes

DMT1 circulating iron

ferriportin circulating ironceruloplasmin

macrophages/ferriportin

macrophages/DMT1Replace iron lost by sloughing of intestinal and

skin cells and by bleeding

ingested iron

Fe3+ Fe2+

R DMT1

Fe2+

ferriportin

ferritin internalization, degradation

not absorbed

hepcidin

HFE TfR2HJVFe2+

enterocyte

loss with cellular slough

circulatory system

Regulation of Iron Absorption

ingested iron

Fe3+ Fe2+

R DMT1

Fe2+

ferriportin

ferritin internalization, degradation

not absorbed

hepcidin

HFE TfR2HJVFe2+

enterocyte

loss with cellular slough

circulatory system

Regulation of Iron Absorption

gene frequency hepcidin severity clinical findings

classic HFE Heterozygous frequency: 1/10 North Americans

↓ ++ symptoms start after 4th decade: chronic fatigue, hepatic fibrosis and cirrhosis, cardiomyopathy, diabetes mellitus, infertility, joint pain

juvenile HJV Rare ↓↓ ++++ Symptoms start after first decade: abdominal pain, hypogonadism, heart failure, diabetes mellitus

juvenile HAMP very rare ↓↓ ++++ Symptoms similar to HJV-related HH

TfR2 very rare ↓ +++ Symptoms similar to HFE-related HH

SLC40A1 (ferriportin)

rare ↓ + Symptoms similar to HFE-related HH

Iron Overload: Hereditary Hemochromatosis

Transfusion therapy results in iron overload

• 1 blood unit contains 200 mg iron

• A 60 kg patient with thalassemia receiving 45 units of blood annually has transfusional iron intake of 9 g iron/year– 0.4 mg iron/kg body wt/day

• In addition, up to 4 mg/day may be absorbed from the gut– Up to 1.5 g iron/year

• Overload can occur after 10–20 transfusions

200–250 mg iron:Whole blood: 0.47 mg iron/mL ‘Pure’ red cells: 1.16 mg iron/mL

Porter JB. Br J Haematol 2001;115:239–252

Iron overload is an inevitable consequence of multiple blood

transfusions

Erythron2 g

Hershko C et al. Ann NY Acad Sci 1998;850:191–201

Normal distribution and turnover of body iron

Iron balance is achieved in the normal state

2–3 mg/day

Parenchyma0.3 g

Liver 1 g

20–30 mg/day

Reticuloendothelialmacrophages

0.6 g1–2 mg/day

20–30 mg/day

Gut

20–30 mg/day

Transferrin

ParenchymaReticuloendothelial

macrophages

Gut

NTBI

Erythron

Transferrin

Transfusions

20–40 mg/day

TransferrinReticuloendothelial

macrophagesParenchyma

NTBI, non-transferrin-bound iron

Hershko C et al. Ann NY Acad Sci 1998;850:191–201

Imbalance of distribution and turnover of body iron with transfusion therapy

Iron balance is disturbed by blood transfusion because the body cannot remove the excess iron

Iron overload leads to formation of NTBI

Uncontrolled iron loading of organs

Subsequent formation of NTBI in plasma

Fe

FeFe

FeFe

FeFe

100%

30%

Normal: no NTBI produced Iron overload

Transferrin saturation due to:• Frequent blood transfusions, or• Ineffective erythropoiesis leading to

increased iron absorption

Tra

nsfe

rrin

sat

ura

tion

Pituitary

ParathyroidThyroid

Heart

Liver Pancreas

Gonads

Transferrin ironControlled uptake

Non-transferriniron

Uncontrolled uptake

Organelle damage

Free-radical generation

Functionaliron

LabileIron

Storageiron

Uncontrolled uptake of labile iron leads to cell and organ damage

Porter JB. Am J Hematol 2007;82:1136–1139

Labile iron

Cell death Fibrosis

Organelle damage TGF-β1

Free radical generation

Lipid peroxidation

Lysosomal fragility

Enzyme leakageCollagen synthesis

TGF, transforming growth factor

Cohen AR and Porter JB. In Disorders of Hemoglobin: Genetics, Pathophysiology, and Clinical Management, Steinberg MH et al. (Eds); 2001:979–1027

Iron overload negatively affects organ function

Liver cirrhosis/ fibrosis/cancer

Diabetes mellitus

Endocrine disturbances→ growth failure

Cardiac failure Infertility

Excess iron is deposited in multiple organs, resulting in organ damage

Iron overloadCapacity of serum transferrin to bind

iron is exceeded

NTBI circulates in the plasma; some forms of NTBI (eg LPI) load tissues with excess iron

Excess iron promotes the generation of free hydroxyl radicals, propagators of oxygen-related

tissue damage

Insoluble iron complexes are deposited in body tissues and

end-organ toxicity occurs