## 3 Brief Overview of Iron Metabolism, the Labile Iron Pool and How It Is Altered in Cancer  
### Cellular Iron Homeostasis  
At physiological pH and oxygen concentration, iron(II) in the body is readily oxidised to iron(III); as a consequence of this, iron is chaperoned to prevent it from partaking in undesirable chemical reactions such as the Haber-Wiess reaction [44]. Once iron has been taken in by the digestive tract, iron is then transported in the plasma by transferrin (Tf). Transferrin binds two ferric ions (Fe\({}^{3+}\)) and has a high affinity for iron. The iron-transferrin complex binds to transferrin receptor 1 (TfR1) on the surface of a cell that expresses it. The di-ferric transferrin enters the cell by receptor-mediated endocytosis. Once released, iron is reduced by ferrireductase before transportation across the endosomal membrane by the divalent metal transporter1 (DMT1) [45]. Once iron is in the cytoplasm, it is stored in a complex with ferritin (Ft) or used to form Fe-S clusters or heme proteins or contribute to the labile iron pool [46]. Iron can also be transported out of the cell by ferroportin (FPN1). Iron which is neither stored nor transported out of the cell exists in several organelles as the labile iron pool. The labile iron pool is in a dynamic equilibrium, shifting and changing its constituents depending on the availability of iron, resulting from the influx and efflux of iron from the cell or the stored forms of iron. The iron within this labile iron pool can either be Fe\({}^{2+}\) or Fe\({}^{3+}\) but is generally accepted to contain more Fe\({}^{2+}\) due to the great quantity of water and reductants in cells [47]. Iron within the labile iron pool is bound to intracellular proteins to minimise potentially toxic Fenton chemistry. One of the main intracellular proteins is iron(II)glutathione conjugates [47; 48]. The labile iron pool is generally regulated by storing excess iron in a complex with ferritin or exporting it outside the cell with FPN-1. One of the main ways of protecting normal cells from excess labile iron and Fenton chemistry is storing iron in ferritin. For exporting, Fe\({}^{2+}\) iron is converted back to Fe\({}^{3+}\) by ferroidase [49]. The molecules Ft and FPN-1 have an iron response element: hairpin loop structures in the 5\({}^{\prime}\) direction on their untranslated region (UTR) of mRNA [50]. These interact with mRNA binding molecules called iron-regulating protein one and iron-regulating protein two (IRP1 and IRP2). When iron conditions are low, IRP1 binds to an iron-responsive element (IRE) in the 5\({}^{\prime}\)UTR on the mRNAs of Ft or FPN-1, which blocks, through steric hindrance, the recruitment of ribosomes and obstructs translation [51]. IRP1 binds to the 3\({}^{\prime}\) UTR on TfR1, and DMT1 masks the mRNA to endonuclease digestion, stabilising it. The consequence of the IRP1s actions on iron is to reduce export and increase import, increasing iron levels in the cell. This occurs as IRP1 is an enzyme known as cytosolic aconitase and contains a full [4 Fe-4S] cluster in the presence of high iron conditions. However, when intracellular iron levels are low, there is not enough iron for Fe-S biogenesis resulting in a [3 Fe-4S] cluster on the enzyme [52]. Therefore, aconitase cannot function enzymatically, resulting in its IRP1 activation. IRP1 therefore utilises a unique iron-sulphhur cluster switch to sense iron levels. Simultaneously, low iron conditions result in the ubiquitination of IRP2, resulting in more iron efflux [50]. Through these mechanisms, in normal conditions, the labile iron pool size results from increases or decreases in iron influx and efflux, which are regulated in correspondence to the cells' needs. Further to IRP1, iron homeostasis can also be maintained by hepcidin through several mechanisms: it is a hormone that induces FPN-1 degradation, can inhibit recycled iron release from macrophages, absorption in the duodenum and lastly, through iron stored in hepatocytes [53].