Stores iron in a soluble, non-toxic, readily available form. Important for iron homeostasis. Has ferroxidase activity. Iron is taken up in the ferrous form and deposited as ferric hydroxides after oxidation. Also plays a role in delivery of iron to cells. Mediates iron uptake in capsule cells of the developing kidney. 4 Fe(2+) + 4 H(+) + O2 = 4 Fe(3+) + 2 H2O Oligomer of 24 subunits. There are two types of subunits: L (light) chain and H (heavy) chain. The major chain can be light or heavy, depending on the species and tissue type. The functional molecule forms a roughly spherical shell with a diameter of 12 nm and contains a central cavity into which the insoluble mineral iron core is deposited. At 9.5 dpc, detected at low levels in the developing heart and central nervous system. At later stages of development, widely expressed, predominantly in the heart and brown fat tissue. Homozygous mutant embryos die in utero between 3.5 and 9.5 dpc (PubMed:10652280). Heterozygous animals are healthy and fertile and do not present any apparent abnormalities. They show slightly elevated tissue light chain ferritin content and 7- to 10-fold more light chain ferritin in the serum than normal mice, but their serum iron remains unchanged. Belongs to the ferritin family.