Ferritin structure and function. Ferritin is a major intracellular iron storage protein in all organisms, and its structural properties are largely conserved through species. Apoferritin refers to the iron-free form of the protein; the iron-containing form is termed holoferritin or simply ferritin. Each apoferritin shell comprises 24 subunits of two kinds: a H-subunit and a L-subunit. Depending on the tissue type and physiologic status of the cell, the ratio of H- to L-subunits in ferritin can vary widely. Ferritin H- and L-subunits are mapped on chromosomes 11q23 and 19q13.3, respectively, and both have multiple pseudogenes . H-ferritin plays a major role in the rapid detoxification of iron, while the L-subunit is involved in nucleation, mineralization and long-term storage of iron .