تجاوز إلى المحتوى الرئيسي
User Image

Mohammad A. Alfuhaily, PhD د. محمد عبدالمحسن الفحيلي

Associate Professor

College of Applied Medical Sciences, Department of Clinical Laboratory Sciences

كلية العلوم الطبية التطبيقية
Building 24, Office 2299
مدونة

Chemistry of Hematological Disorders I

IRON

      

Functions

1. Oxygen transport and storage (Fe2+ in hemoglobin and myoglobin)
2. Energy production (cytochromes, flavoproteins)
3. Enzyme activity (cofactor)

      

Distribution

*70% in hemoglobin (as a binding site for O2 and CO2)
*25% in the RES (liver, spleen, bone marrow)
*< 0.1% in blood plasma (mainly bound to transferrin, but also to albumin and free hemoglobin)

      

Metabolism

*Ingested as ferric iron (Fe3+), and reduced by HCl in the stomach to the ferrous state (Fe2+).
*10% of Fe2+ is absorbed in the duodenum and jejunum (absorption increases with dietary vitamin C)
*The unabsorbed amount (90%) is excreted in the stool
*In the intestines, it’s bound to apoferritin, oxidized by ceruloplasmin to ferric and absorbed into blood on transferrin. The rest is stored in ferritin and hemosiderin. This is because free iron catalyzes the formation of free radicals that are cytotoxic.

      

Pathology

§ 

Iron Deficiency

-Affects 15% of the worldwide population especially children, adolescents, pregnant women and those of productive years.
-Causes: Blood loss, decreased release from ferritin, and low dietary intake.

§ 

Iron Overload (Hemochromatosis)

-Primary: Hereditary Hemochromatosis (HH)

*Increased iron absorption and accumulation in liver and heart.
*Complicated by DM, heart disease, hepatocellular carcinoma, arthritis, hypothyroidism, and impotence.

-Secondary: Dietary, therapeutic, or repeated transfusion intake.

      

Laboratory Evaluation


-Non-hemolyzed serum or heparin plasma (oxalate, citrate, and EDTA bind iron).

(1) Serum Iron

-Ferric iron bound to transferrin.
-Not sensitive or specific.
-Not used alone to assess iron status because it shows considerable physiological fluctuation.
-High in iron overload (increased absorption), necrotic hepatitis (increased release from ferritin), pernicious anemia (inefficient iron storage), and lead poisoning (less iron utilization).
-Low in blood loss and iron-deficiency anemia.
-Shows diurnal variation – peaks in the morning.
-Lower levels in pregnancy.
-Method: Enzymatic.

(2) Transferrin (TIBC)

-Iron protein carrier – 1 transferrin molecule binds 2 iron ions (affinity decreases with pH)
-NOT the actual transferrin concentration – expressed as total iron-binding capacity (TIBC). This is the total amount of iron that can potentially be bound to transferrin when all binding sites have been saturated.
-Only 20-30% of transferrin is occupied with iron, the additional iron that can be bound is termed unbound iron-binding capacity (UIBC); Hence:
TIBC = Iron + UIBC
TIBC (umol/L) = 25 x Transferrin (g/L)
-When iron is low, more transferrin is produced "in the hopes of snarfing up more iron", and so the capacity of the serum to bind iron increases.
-Exception: Anemia of chronic disease – interleukin leads to less transferrin production (to keep iron away from pathogens).
-Low in malnutrition, iron overload (ferritin deficiency), and kidney disease (increased loss), and intestinal disease (low absorption).
-Transferrin is primarily monitored as an early indicator of nutritional status.
-Negative acute-phase protein.
-Estrogen increases production.
-Percent Transferrin Saturation:
*Percent of iron-binding sites on transferrin occupied with iron.
*[Iron/TIBC] x 100 = %
*Screen for hemochromatosis (characteristic feature).
-Method: Immunoassays.

(3) Ferritin

-The form of iron stored in cells.
-The best test to estimate iron stores (abnormal before anemia or other changes develop). Low levels are diagnostic of iron depletion.
-High levels are NOT diagnostic of iron overload.
-Acute-phase reactant (CRP can be measured).
-Soluble Transferrin Receptor (TfR):
           *Expressed on cells surface when iron is needed.
           *Interpreted as TIBC.
           *Better specificity.
-Method: Immunoassays.

Condition

Iron

TIBC

%TS

Ferritin

Iron-deficiency Anemia

Low

High

Low

Low (near 0)

Anemia of Chronic Disease

Low

Low

Low

High

Sideroblastic Anemia

High

Low

High

High

Hemochromatosis

High

Low

High

High

 

(4) Folic Acid & Vitamin B12

 

                 Folic Acid

B12

Source

             Diet (veggies) + gut bacteria

Diet (animal products)

Metabolism

Absorbed in jejunum, then reduced to methyltetrahydrofolate (MTHF) in the intestines (active form). Excess out in urine and feces

Stomach acid separates B12 from protein, IF from parietal cells binds it and the complex is absorbed in the ileum. In blood, it’s carried on transcobalamin II to body cells including the liver for storage. Excess out in urine

Role

Coenzymes for RBC, WBC, & DNA synthesis; cell division & repair; and nerve health

Deficiency

 Megaloblastic anemia (impaired DNA synthesis and enlarged, short-lived RBC precursors).
Neural tube defects (needed more during pregnancy and especially lactation as milk binds FA)

Megaloblastic anemia
Neurologic abnormalities

Causes of Deficnecy

              
               Low intake
               Alcoholism

 

Malabsorption:
-↓IF
-gastrectomy
-celiac/crohn’s disease
-D. latum/bacterial overgrowth)

       

 -Pernicious anemia is a specific type of megaloblastic anemia caused by malabsorption of B12 due to loss of intrinsic factor as a result of parietal cell atrophy or Ab to IF.

-Diagnosis:

*Dietary assessment.
*Lab workup:

1.    

Although serum FA is sensitive, RBC FA is the best marker of
deficiency (reflects stores). RBC folate is low in either deficiency because B12 is required for its storage. Measure both.

2.    

Serum and urine homocysteine is high in FA deficiency.

3.    

Formiminoglutamic Acid (FIGLU) Excretion: The formimino group of histidine-derived FIGLU is transferred to tetrahydrofolic acid. In deficiency, FIGLU is lost in urine.

4.    

Methylmalonic Acid (MMA) Excretion: The deoxyadenosylcobalamin active form of B12 is required to convert methylmalonyl-CoA to succinyl-CoA. In deficiency, the accumulated methylmalonyl-CoA is converted to MMA excreted in urine.

5.    

B12-Binding Capacity: Detects deficiency of transcobalamin(s).

6.  Malabsorption investigation:

i.    

Celiac screen: TTG; endomysial Ab; gliadin Ab.

ii.    

Parietal cell Ab; intrinsic factor Ab.


-Method: Immunoassays – Oral contraceptives cause low FA levels.

(5) G6PD:

§ 

Physiology

-Glucose-6-phosphate dehydrogenase oxidizes glucose-6-phosphate to 6-phosphoglucono-δ-lactone in the pentose phosphate pathway.
-This pathway is the only source for RBC’s to have NADPH which, as a H+ donor, will keep a reduced state of glutathione to remove Hgb-oxidizing agents such as H2O2, some anitmalarial drugs, and fava beans. Vitamin E is a powerful antioxidant.
-Otherwise, Hgb would be oxidized into the non-functional methaemoglobin that builds up in the cell and precipitates out as Heinz bodies, leading to membrane weakening and hemolysis, and making RBCs a target for macrophages.

§ 

Pathology

-Sex-linked recessive mutations in the G6PD gene lead to decreased activity and G6PD-deficiency anemia. It is the most common enzyme defect (400 million cases).
-Diagnosis:

*EDTA or heparin whole blood.
*Beutler fluorescence test: No fluorescence is a +ve test.
*Reticulocytosis: False normal/high G6PD.
*Measured best 2-3 weeks after recovery.