Internal iron exchange Iron uptake by erythroid cells

The fate of iron bound to plasma transferrin has been studied by injecting a trace amount of radioactive 59Fe bound to transferrin. About 85% of the 59Fe normally enters developing red cells for incorporation into haemoglobin. This tissue distribution of transferrin-bound iron reflects the expression of transferrin receptors, which are present in high concentration on cells with a high iron requirement. The latter includes any rapidly dividing cells but is normally dominated by the cells of the erythron.

The expression of surface transferrin receptors is increased if cells have inadequate iron, and reduced if they are iron replete (p. 29). Cell transferrin receptors have the highest affinity for diferric transferrin. The transferrin-receptor complex is taken up by a process of receptor-mediated endocytosis (Figure 3.4). The iron is released at the low pH of the endosome, before the apotransferrin and receptor are recycled to the plasma and the

Transferrin receptor ^

Diferric transferrin

Apotransferrin

Transferrin receptor

Figure 3.4 Incorporation of iron from plasma transferrin into haemoglobin in developing red cells. Uptake of transferrin iron is by receptor-mediated endocytosis.

Transferrin receptor ^

Diferric transferrin

Apotransferrin

Transferrin receptor

Figure 3.4 Incorporation of iron from plasma transferrin into haemoglobin in developing red cells. Uptake of transferrin iron is by receptor-mediated endocytosis.

cell membrane respectively. Iron release from the endosome is via DMT1 (Table 3.1) and the iron enters the mitochondria or ferritin. Direct transfer of storage iron from macrophages to erythroblasts (rhopheocytosis) is now thought to be of little physiological significance. A soluble, truncated form of the receptor derived from these cell surfaces is detectable in serum. It is bound to transferrin and increased concentrations provide an early indication of an impaired iron supply to the tissues (p. 36). In the absence of iron deficiency, the serum transferrin receptor concentrations reflect erythroid activity.

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