Transferrin (TFN)

Transferrin is a free peptide (apo transferrin) that undergoes a conformation change after binding with iron. Iron circulates in the plasma until it attaches to a transferrin receptor on a target cell. A carbonate (CO) has to be present to help attract iron to transferrin by creating opposing repulsive charges. Transferrin can bind to two atoms of ferric iron (Fe3+) with high affinity. The carbonate needed also serves as a ligand to stabilize iron in the binding site of transferrin. Clathrin/receptor-mediated endocytosis mediates the uptake of iron by transferrin receptors [6]. An acidic environment of Ph5.6 reduces iron-transferrin affinity, which encourages the release of iron from its binding site and endocytosed into a cell [1].

Transferrin is the main protein in the blood that binds to iron and transports it throughout the body. It binds oxygen in the lungs and releases oxygen as blood circulates to other parts of the body. Normally, iron is transported throughout the body by transferrin, which is produced by the liver. In healthy people, most iron is incorporated into the hemoglobin within RBCs. The remainder is stored in the tissues as ferritin or hemosiderin, with additional small amounts used for other purposes (e.g., to produce other proteins such as myoglobin and some enzymes). Besides oxygen transport, iron, and thus transferrin, is essential for electron transfer and DNA synthesis, cellular growth and proliferation.

Transferrin value is often interpreted together with other parameters of the blood. When hemoglobin and hematocrit on a complete blood count (CBC) are low or high, iron deficiency or iron overload are suspected. Therefore, besides CBC, to diagnose or monitor such states, the test for transferrin is required, as well as serum iron test, total iron-binding capacity (TIBC), unsaturated iron-binding capacity (UIBC), transferrin saturation and ferritin.

Function

Functions of transferrin include:

Free Fe3+ is insoluble at a neutral pH; when iron binds to transferrin, it becomes soluble.

Deliver and transfer iron to all the various biological tissues between sites of absorption, utilization, and storage [9].

Prevent the formation of reactive oxygen species.

Chelate-free toxic iron and acts as a protective scavenger.

Deliver WBC macrophages to all tissues [10]

Transferrin is a part of the innate immune system; the binding of transferrin to iron impedes bacterial survival.

Transferrin acts as a marker for inflammation; the level of transferrin decreases during inflammation.

Testing

The laboratory's reference range for transferrin is 204-360 mg/dL. Transferrin can be used to assess the iron level in the body along with other markers in the body. Transferrin level testing is used to determine the cause of anaemia, examine iron metabolism and determine the iron-carrying capacity of the blood. Transferrin saturation levels cannot be interpreted alone. They are used in conjunction with other laboratory tests, such as serum ferritin and TIBC. Ferritin is the first marker to become low, therefore, more sensitive than transferrin in diagnosing Iron deficiency anaemia [12]. Total or transferrin iron-binding capacity (TIBC) is a test that measures the blood's capacity to bind iron with transferrin. Low transferrin saturation is seen in iron deficiency. 

Transferrin Concentration for TIBC Measurement

Since transferrin is the main transport protein for iron, levels of transferrin can also be used to calculate TIBC. Each mole of transferrin, with an approximate molecular weight of 79.5 kDa is capable of binding to two moles of iron, molecular weight of 55.8 kDa. In other words, transferrin concentration of 1 g/L, equivalent to 12.57 μmol/L, should be able to carry 25.1 μmol/L of iron. Therefore, TIBC can be estimated by multiplying the calculated transferrin concentration by a converting factor (usually approximate to 25.1 for TIBC in μmol/L or 1.4 for TIBC in μg/L).

Total iron – binding capacity TIBC = Transferrin × conversion factor

Transferrin saturation TSAT = Serum iron / (Transferrin × conversion factor) × 100

Transferrin can be measured by several immunochemical assays. It can be measured by immunochemical turbidimetry, nephelometry, and radial immune diffusion methods.

Using transferrin as a measure of TIBC is not without potential limitations. Some of which are attributed to differences in the used value of the conversion factor, as this is dependent on the chosen value of transferrin molecular weight (usually ranges between 79 and 80 kDa). Also, the method does not take into account other iron-binding proteins. Furthermore, beside the variations between different transferrin immunochemistry assays, other factors may contribute to discrepancies within the same method such as the use of different antibodies that may vary in their affinity to transferrin epitopes.

Clinical Significance

Iron deficiency is recognized as the most prevalent nutritional deficit in the world.   The amount of transferrin in the blood indicates the amount of iron in the body.  High transferrin signifies low iron, which means there is less iron bound to transferrin, allowing for a high circulation of non-bound iron transferrin in the body, revealing a possible iron deficiency anaemia. The liver increases the production of transferrin as a form of homeostasis to enable transferrin to bind to iron and transport it to the cells. Upregulation of transferrin receptors occurs in iron deficiency anaemia [13].  Concerning the percentage of transferrin-iron complex, low iron-bound transferrin indicates low iron levels in the body, which affects haemoglobin and erythropoiesis.  The significance of transferrin is that it can detect iron deficiency and can be used to monitor erythropoiesis.

In anaemia of chronic disease, there is a decreased transferrin level.

Causes of low transferrin

Liver damage leading to reduced production of transferrin

Kidney insult or injury leads to loss of transferrin in urine.

Infection

Malignancy

Atransferrinemia: A genetic mutation resulting in the absence of transferrin, which leads to hemosiderosis in the heart and liver, which can lead to heart and liver failure. This condition is treated by plasma infusion.

Low transferrin in plasma indicates iron overload, which means the binding site of transferrin is highly saturated with iron.  Iron overload suggests hemochromatosis, which will lead to the deposition of iron on tissues.

Other associations with transferrin and its receptors include,

Diminishing tumour cells when the receptor is used to attract antibodies

High transferrin saturation increased the risk of cardiovascular mortality if patients have high transferrin saturation (>55%) and LDL levels [14]

High transferrin

High transferrin signifies low iron, which means there is less iron bound to transferrin, allowing for a high circulation of non-bound iron transferrin in the body, revealing a possible iron deficiency anaemia.

When the level of iron is insufficient to meet the body's needs, the level of iron in the blood drops and iron stores are depleted.

This may occur because:

an increased need for iron, for example during pregnancy or childhood, or due to a condition that causes chronic blood loss (e.g., peptic ulcer, colon cancer)

not enough iron is consumed (either foods or supplements)

iron is not absorbed from the foods eaten, e.g in celiac disease.

Insufficient levels of circulating and stored iron may eventually lead to iron-deficiency anaemia (decreased haemoglobin and haematocrit, smaller and paler red cells). The early stage of iron deficiency is the slow depletion of iron stores. This means there is still enough iron to make red cells, but the stores are being used up without adequate replacement. The serum iron level may be normal in this stage, but the ferritin level will be low. As iron deficiency continues, all the stored iron is used and the body tries to compensate that. The liver increases the production of transferrin as a form of homeostasis to enable transferrin to bind to iron and transport it to the cells. The serum iron level continues to decrease and transferrin and TIBC and UIBC increase.

A high TIBC, UIBC, or transferrin can be indicated in patients

with iron deficiency

with pregnancy

which use of oral contraceptives.

Low transferrin

Conversely, too much iron can be toxic to the body. Iron storage and ferritin levels increase when more iron is absorbed than the body needs. Absorbing too much iron over time can lead to the progressive build-up of iron compounds in organs and may eventually cause their dysfunction and failure, like for example, in patients with:

hemochromatosis – a condition in which the body absorbs and builds up too much iron, even on a normal diet. Additionally, iron overdose can occur when someone consumes more than the recommended amount of iron;

mutations in the HFE gene, which causes hereditary hemochromatosis;

hemosiderosis - atransferrinemia: a genetic mutation, resulting in the absence of transferrin, which leads to hemosiderosis in the heart and liver, which can lead to heart and liver failure;

repeated transfusions;

sickle cell anaemia;

thalassemia major or other forms of anaemia;

alcoholism and chronic liver disease;

malignancy

A low TIBC, UIBC, or transferrin may also occur if someone has

malnutrition;

inflammation;

Liver damage leading to reduced production of transferrin.

Kidney insult or injury leads to loss of transferrin in urine.

Table1. A summary of the changes in iron tests seen in various diseases of iron status

   1)       Steere AN, Byrne SL, Chasteen ND, Mason AB. Kinetics of iron release from transferrin bound to the transferrin receptor at endosomal pH. Biochim Biophys Acta. 2012 Mar;1820(3):326-33. [PMC free article] [PubMed]

2)       Wally J, Buchanan SK. A structural comparison of human serum transferrin and human lactoferrin. Biometals. 2007 Jun;20(3-4):249-62. [PMC free article] [PubMed]

3)       Giansanti F, Panella G, Leboffe L, Antonini G. Lactoferrin from Milk: Nutraceutical and Pharmacological Properties. Pharmaceuticals (Basel). 2016 Sep 27;9(4) [PMC free article] [PubMed]

4)       Dautry-Varsat A. Receptor-mediated endocytosis: the intracellular journey of transferrin and its receptor. Biochimie. 1986 Mar;68(3):375-81. [PubMed]

5)       Anderson ER, Shah YM. Iron homeostasis in the liver. Compr Physiol. 2013 Jan;3(1):315-30. [PMC free article] [PubMed]

6)       Luck AN, Mason AB. Transferrin-mediated cellular iron delivery. Curr Top Membr. 2012;69:3-35. [PMC free article] [PubMed]

7)       Wally J, Halbrooks PJ, Vonrhein C, Rould MA, Everse SJ, Mason AB, Buchanan SK. The crystal structure of iron-free human serum transferrin provides insight into inter-lobe communication and receptor binding. J Biol Chem. 2006 Aug 25;281(34):24934-44. [PMC free article] [PubMed]

8)       Hall DR, Hadden JM, Leonard GA, Bailey S, Neu M, Winn M, Lindley PF. The crystal and molecular structures of diferric porcine and rabbit serum transferrins at resolutions of 2.15 and 2.60 A, respectively. Acta Crystallogr D Biol Crystallogr. 2002 Jan;58(Pt 1):70-80. [PubMed]

9)       Tandara L, Salamunic I. Iron metabolism: current facts and future directions. Biochem Med (Zagreb). 2012;22(3):311-28. [PMC free article] [PubMed]

10)   Gordon S, Plüddemann A. Tissue macrophages: heterogeneity and functions. BMC Biol. 2017 Jun 29;15(1):53. [PMC free article] [PubMed]

11)   Grant BD, Donaldson JG. Pathways and mechanisms of endocytic recycling. Nat Rev Mol Cell Biol. 2009 Sep;10(9):597-608. [PMC free article] [PubMed]

12)   Waldvogel-Abramowski S, Waeber G, Gassner C, Buser A, Frey BM, Favrat B, Tissot JD. Physiology of iron metabolism. Transfus Med Hemother. 2014 Jun;41(3):213-21. [PMC free article] [PubMed]

13)   Bermejo F, García-López S. A guide to diagnosis of iron deficiency and iron deficiency anemia in digestive diseases. World J Gastroenterol. 2009 Oct 07;15(37):4638-43. [PMC free article] [PubMed]

14)   Wells BJ, Mainous AG, King DE, Gill JM, Carek PJ, Geesey ME. The combined effect of transferrin saturation and low-density lipoprotein on mortality. Fam Med. 2004 May;36(5):324-9. [PubMed]

15)   https://www.ncbi.nlm.nih.gov/books/NBK532928/#:~:text=Transferrin%20is%20a%20blood%20plasma,%2C%20spleen%2C%20and%20bone%20marrow.

16)   https://www.sciencedirect.com/topics/medicine-and-dentistry/transferrin

17)   https://iris.who.int/bitstream/handle/10665/133707/WHO_NMH_NHD_EPG_14.6_eng.pdf?sequence=1

18)   Transferrin, Serum. Mayo Clinic Mayo Medical Laboratories. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/34623. Accessed on May 2017. (© 1995– 2017).

19)   Jamil Talukder. Chapter 60. Role of transferrin: an iron-binding protein in health and diseases in Nutraceuticals (Second Edition): Efficacy, Safety and Toxicity 2021, Pages 1011-1025 https://doi.org/10.1016/B978-0-12-821038-3.00060-4

20)   Aminat S. Ogun; Adebayo Adeyinka. Biochemistry, Transferrin. Treasure Island (FL): StatPearls Publishing; Last Update: August 11, 2020

21)   Transferrin and Iron-binding Capacity (TIBC, UIBC), last reviewed on July 22, 2017, Available online https://labtestsonline.org/tests/transferrin-and-iron-binding-capacity-tibc-uibc

22)   Iron test, last reviewed on July 22, 2017, Available online https://labtestsonline.org/tests/iron-tests