Ferritin: an expanded role in metabolic regulation.

Synthesis of ferritin, a constitutive protein, is increased by iron. This protein is well recognized as a protein which detoxifies, stores and transports iron. The 24 subunits of ferritin assemble to form a protomer of Mr 480,000. This protein shell can sequester up to 4500 g atoms of iron as ferrichydroxyphosphate. Ferritin in vitro and in vivo binds other metal ions such as Cu, Zn, Cd, Pb, Be and Al. Next to Fe it binds large quantities of Be. Therefore, in vitro ferritin protects against and reverses the inhibition by Be of enzymes susceptible to this metal ion. Also, rats pretreated with Fe survive otherwise toxic levels of either pulmonary or intravenous exposure of Be. Liver ferritin from rats injected with Zn contains some of the injected metal ion. Incubation of such ferritin-zinc complex with zinc-requiring apoenzymes restores their activity. Fe(III) of ferritin is released only after its reduction to Fe(II) by a reductant. Incubation of phosphoglucomutase, a phosphoserine containing enzyme with ferritin and a reductant causes irreversible inactivation of the enzyme and removes 70% of its phosphate. Some other phosphoproteins are similarly inactivated but without the loss of the bound phosphate. Thus, uncontrolled release of iron from ferritin, in the presence of a reductant and oxygen can modify several biomolecules and can affect metabolic processes. A subclass of ferritin, acidic isoferritins, have been implicated in leukemia-associated inhibitory activity and has been suggested to inhibit production of Ia+ macrophage progenitors.