Iron Hemostasis Porphyrin Metabolism

Question 1

How is Iron Deficiency Anemia diagnosed?

Answer 1

Low Serum Iron

High Total Iron Binding Capacity (TIBC)

Question 2

What is the normal iron intake in the Western diet?

Who has higher than normal requirements?

Answer 2

10-20 mg per day (Adequate)

Require Higher than Normal: Children and Menstruating/Pregnant Women

Question 3

What two forms does iron exist in in the body?

Which form is most iron found in?

Answer 3

Functional and Storage forms

Most iron is found in the Functional form

Question 4

Functional Form of Iron

1. Where is it found?

Answer 4

***Actively Participating in Metabolic Processes***

1. About 80% is found in hemoglobin
   - Smaller in myoglobin, cytochromes, or Fe-containing enzymes

Also, another small pool is bound to Transferrin for transport in the blood

Question 5

Storage Form of Iron

1. Where is it found?

Answer 5

***Sequestered in particles and biologically inactive***

Two modes exist:

1. Ferritin-bound: dynamic molecule, from which iron can be released according to demand
2. Hemosiderin-bound: degenerated iron-protein complex that cannot be mobilized
   - Hemosiderin granules in tissues is a sign of iron overload

Question 6

Describe the uptake and storage of Iron

Answer 6

1. Iron is taken up by enterocytes and is bound in the plasma by transferrin
2. Plasma iron is taken up by erythroid precursor cells in the bone marrow and incorporated into RBCs
3. After phagocytosis of RBCs by splenic macrophages, iron is stored as ferritin
4. Macrophage ferritin can be released into the plasma, where it is bound to transferrin

***Plasma iron (bound to transferrin) is exchanged with liver iron stores (bound to ferritin)***

Question 7

What are the main two ways that iron is lost?

Answer 7

Shedding of epithelial cells** (particularly enterocytes) and **blood loss

***About 1-2 mg***

Question 8

How is iron hemostasis regulated?

Answer 8

Iron hemostasis can be regulated only** by adjusting the **uptake from the diet (i.e. the absorption by the epithelial cells in the proximal duodenum)

***Only way iron leaves the body is by sloughing off of duodenal/other epithelal cells

Question 9

Uptake of Heme vs. Non-Heme iron

Answer 9

Heme iron uptake: taken up efficiently (~25%) through Heme Carrier Protein HCP1

Non-Heme iron uptake: non heme (Fe³⁺) is first reduced to Fe²⁺ by duodenal cytochrome p450 then is taken up by the Divalent Metal Transporter DMT1 (Less efficient ~5%)

Question 10

What happens to the action of enterocytes when iron stores are full?

Answer 10

Uptake into enterocytes is still active, as it is not regulated by body iron requirements

Question 11

What are the two options that enterocytes have after taking up as much dietary iron as possible?

Answer 11

1. Fe2+ can be stored in the enterocyte as mucosal ferritin; this oute leads to EXCRETION of iron, as enterocytes are quickly turned over and sloughed off in the intestine
2. Fe2+ can be released into the plasma through ferroportin

Question 12

What is the key regulatory protein in the enterocyte?

Answer 12

Hepcidin: regulates the export of iron from the enterocyte by inhibiting Fe2+ release through ferroportin on the basal surface of enterocytes

***This prevents the dissemination of iron through the body***

(Once released, Fe2+ is oxidized to Fe3+ and binds to transferrin in the plasma)

Question 13

What effect does iron overload have on enterocyte function?

Answer 13

NO EFFECT

• Enterocytes (lifespan 5-6 days) do not function as an iron reservoir
• Iron deposits are merely a ***short-term buffer***

Question 14

What is the function of Transferrin?

Answer 14

Transferring binds iron –> iron-Transferrin binds Transferrin Receptor and is taken up in clathrin-coated pits –> taken into endosome where ATPase acidifies the environment (now a lysosome) –> Fe3+ is released from transferrin and is reduced in the acidic environment to Fe2+ –> DMT1 transports Fe2+ into the cytoplasm

Question 15

Where is iron stored in the cytoplasm?

Answer 15

In Ferritin (Soluble and mobilized easily)

Hemosiderin forms when ferritin particles accumulate and denature (Insoluble and does not release iron as easily)

Question 16

Where are the highest concentrations of Ferritin-bound iron found?

Answer 16

Liver (to synthesize cytochrome__s)

Spleen (from phagocytosis of RBCs)

Bone Marrow (because of RBC synthesis)

Question 17

What is one of the first signs of iron overload?

Answer 17

Appearance of hemosiderin inside macrophages

Question 18

How does hepcidin function and what are the two physiological consequences of this?

Answer 18

Function: Hepcidin inhibits iron transport protein Ferroportin

Physiological Consequences:

1. Enterocytes do not release iron into blood; instead stored and lost during epithelial cell turnover
2. Macrophages do not release iron into blood; instead, macrophages accumulate iron stores in the form of ferritin** and **hemosiderin

Question 19

What regulates iron uptake and what do high concentrations of this molecule do?

Answer 19

Hepcidin

High concentrations reduce iron uptake

Question 20

Describe the post transcriptional regulation of Ferritin

Answer 20

1. The 5’ UTR of ferritin contains an Iron Response Element (IRE) that forms a stemloop strucutre
2. Iron regulatory proteins (IRPs) normally bind the IRE and block translation; however, free iron in the cytoplasm will bind the IRP and displace it, allowing for translation of Ferritin to begin quickly

Question 21

How is post-transcriptional regulation of D-ALA synthase done?

Answer 21

(Same as Ferritin)

Free iron increases** D-ALA synthase translation and **increases the synthesis of Heme

Question 22

How is Transferrin Receptor post-transcriptionally regulated?

Answer 22

(Same as Ferritin, except…)

The regulation is inverted

Binding of the IRPs stabilizes the mRNA and increases translation. Free iron decreases transferring receptor translation and reduces iron uptake

Question 23

Serum Iron Test

1. Utility

Answer 23

1. Used to diagnose iron poisoning/overload

***Little clinical value - does not inform about total iron stores in the body***

Question 24

Total Iron Binding Capacity (TIBC)

(i. e. Transferrin Saturation)
1. Normal vs. Pathologic Values

Answer 24

Normal: About 30% saturated with iron

Below 15% points to iron deficiency

Question 25

**Serum Ferritin** 1. Utility

Answer 25

Serum Ferritin is considered to be the **_best measure_** for **body iron stores**, as it measures Ferritin, which is both the **intracellular storage form** of iron and is present in the **serum**

Question 26

**Red Cell Protoporphyrin** 1. Utility

Answer 26

Protoporphyrin is the **iron-free** pre-cursor of heme 1. **Elevated value** indicates a **_shortage of iron_** to complete heme synthesis

Question 27

**Iron Deficiency Anemia** 1. Causes

Answer 27

**1. \*\*\*​Chronic Blood Loss**- always assume this is the cause until proven otherwise, as this is the **_most common_** (caused by **malignant growth** or **bleeding ulcers**) **2. Chronic Disease**- frequent cause in **_hospitalized patients_** ; inflammatory mediators (e.g. *IL-6*) stimulate the synthesis of **_hepcidin_**, which then **REDUCES** **iron release** from entrocytes **3. Poor Dietary Intake** **4. Intestinal _Parasites_** **5. Malabsorptive Disease (Celiac Disease)**

Question 28

What are the stages of development of **Iron Deficiency Anemia**? (3)

Answer 28

**1. Iron Depletion** (tissue iron stores become depleted and serum ferritin levels fall) **2. Deficient Erythropoiesis with _NORMAL_ hemoglobin concentration** (RBC production slows down due to slow synthesis of heme. Serum protoporphyrin levels rise. Increased production of transferrin leads to a drop in transferrin saturation) **3. Iron deficiency anemia** (Hemoglobin production is inadequate, leading to hypochromic, microcytic anemia)

Question 29

Treatment of Iron Deficiency Anemia

Answer 29

Establish cause - look for occult blood loss Supplement iron orally or intramuscular Iron deficiency can take **six months to reverse**

Question 30

What conditions lead to **Iron Overloa****d**? (4)

Answer 30

**1. Chronic Blood Transfusions** for treatment of hemolytic disorders. Iron stores accumulate in macrophages from the destruction of damaged RBCs **2. Inappropriate parenteral nutrition** **3. Ineffective hematopoiesis (_RENAL FAILURE_)** **4. Hereditary iron uptake disorder**

Question 31

**Hemochromatosis** 1. Cause 2. Mechanism of Disease

Answer 31

**Cause:** Common hereditary uptake disorder, caused by a **mutation** in the **_HFE gene_**, which leads to **repression** of **hepcidin** expression **MoD:** low concentrations of **hepcidin** lead to an increase in iron release from enterocytes \*\*\*In hereditary hemochromatosis, there is little deposition of iron in enterocytes because of increase iron efflux through ferroportin

Question 32

What is the **committed/regulated step** of **Heme Synthesis**? Where does it take place? What regulates this step, and thus, heme synthesis?

Answer 32

1. **Committed Step:** The ***_ALA SYNTHASE_*** *(Aminolevulini Acid Synthase)* reaction that synthesizes **aminolevulinic acid** from **_succinyl-CoA**_ and _**glycine_** 2. This reaction occurs in the **Mitochondria** 3. This reaction is regulated by **1)** I**ron Concentration** (The ALA synthase mRNA has an IRE and high concentrations of iron activate translation), and **2) Heme** (_Oxidized_ (Fe³⁺) _heme_, hemin, represses Ala synthase activity)

Question 33

What are **Porphyrias**? How do they normally present? Triggers?

Answer 33

The diseases of **heme synthesis** Normally present as **acute abdominal pain** **Triggers****:**medications,**_alcohol_**(due to synthesis of cytochromes for the p450 enzymes of the**microsomal ethanol oxidizing sytems (MEOS)** --\> accumulation of porphyrin precurses in porphyria patients)

Question 34

**Acute Intermittent Prophyria (AIP)** 1. Cause 2. Results of Disease 3. Symptoms

Answer 34

**Cause:** deficiency in **porphobilinogen deaminase** **Result of Disease:** ALA and PBG accumulate in circulation and **urine**, giving it a **_dark red color_** **Symptoms:** Diffuse _neurological_; **Confusion** and **Sharp Abdominal Pain**

Question 35

**Porphyria Cutanea Tarda (PCT)** 1. Cause 2. Results 3. Symptoms

Answer 35

**Cause:** Deficiency of **uroporphyrinogen decarboxylase** **Results:** Buildup of porphyrins (detected in the urine using **UV light** --\> emits a **_pink fluorescence_**) \*\*\*Porphyrins are able to **absorb _visible_ and _UV light_**\*\*\* **Symptoms:** _Photosensitivity_ of the skin, leading to **blistering** of exposed areas

Question 36

**Lead Poisoning** 1. Impact on Heme Synthesis 2. Symptoms

Answer 36

Inhibits **porphobilinogen synthase** and **ferrochelatase** **Impact on Heme Synthesis:** Leads to the accumulation of ALA and other heme precursors in the heme-producing tissues **Symptoms:** Similar to other **porphyrias** (e.g. confusion, abdominal pain)