Soluble transferrin receptor conventionally refers to the cleaved extracellular portion of the transferrin receptor 1 that is released into serum. This receptor is a dimer protein of two identical subunits, linked together by two pairs of disulfide bonds. Its molecular mass 190,000 Dalton.
The measurement of the soluble transferrin receptor (sTfR) is used as a measure of blood iron levels and the investigation of anemia. The sTfR is regarded as a more stable marker of iron levels in an inflammatory state. This is in contrast to ferritin, a routine investigation for anemia. As ferritin is an acute-phase reactant, it may be elevated in inflammatory states. To date, the conventionally identified soluble transferrin receptor has not been itself implicated in intracellular delivery of transferrin and associated iron.
In a patient with a preexisting inflammatory state, an investigating physician may test for sTfR to identify whether an anaemia is an iron deficiency anemia, in which sTfR is elevated, or not. This is particularly useful in distinguishing between the anemia of chronic disease and iron-deficiency anaemias.
A soluble receptor for any ligand could also refer to a molecule present is solution (for example a secreted protein) which would bind with the target ligand and then effect cellular delivery. In this context the multifunctional glycolytic enzyme glyceraldehye-3-phosphate dehydrogenase (GAPDH) has been reported as a soluble receptor for transferrin. It has been demonstrated to deliver more transferrin as compared to the receptors anchored on the cells surface in numerous cell types.