IRON TRANSPORT

Iron is transported in plasma tightly (but reversibly) bound to transferrin, a glycoprotein of approximately the same size as albumin.
Total iron-binding capacity (TIBC) is an indirect measure of transferrin.

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Regulation of the synthesis of transferrin receptor and ferritin via IRE and IRP:

At low cellular iron content, IRP binds to IRE at ’-UTR on the mRNA for the transferrin receptor, and at the 5’-UTR on the mRNA for ferritin, leading to increased synthesis of the receptor and reduced synthesis of ferritin.

At high irin content, IRP dissiciates from IRE, leading to reduced receptor synthesis (reduced uptake capacity) and increased ferritin synthesis (increased storage capacity).

IRP: Iron-regulating protein
IRE: Iron-responsive element
UTR: Un-translated region of mRNA

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What is Transferrin saturation?

Transferrin saturation is the relationship (in %) between serum iron and total iron-binding capacity (TIBC):

100 x [ serum iron / TIBC ]

• Normal range: 15 - 45%
• This measurement is performed by measuring the amount of iron in serum (spectrophotometrically).
• Then iron is added until 100% saturation and the total iron-binding capacity (= amount of transferrin) is calculated. This analysis requires a serum sample without haemolysis and of some size.

• Now, transferrin is measured directly as protein.
• A conversion factor is needed when comparing the two methods of measurement: (TIBC µmol/L = 25.1 (µmol/g) x Tf (transferrin) g/L)

Figure Relationships of serum iron, unbound (or unsaturated) iron-binding capacity and total iron-binding capacity in various clinical conditions.
Note that in iron deficiency, in late pregnancy and in users of oral contraceptives TIBC (transferrin) is increased. With infection/inflammation transferrin saturation (%) may appear normal, and in primary haemochromatosis TIBC is reduced.

[Ref 1]