Intracellular labile iron

Cells maintain organellar pools of “labile iron” (LI), despite its propensity for catalyzing the formation of reactive oxygen species. These pools are identifiable by iron-chelating probes and accessible to pharmacological agents. Cytosolic LI has been assumed to have a dual function: providing a rapidly adjustable source of iron for immediate metabolic utilization, and for sensing by iron-regulatory proteins (IRPs) that regulate iron uptake and compartmentalization via transferrin receptors and ferritin. However, it now appears that IRPs may respond both to fluctuations in LI per se and to secondary signals associated with redox-active species. Recent information also indicates that iron can be delivered to mitochondria via pathways that circumvent cytosolic LI, suggesting possible alternative mechanisms of cell iron mobilization and trafficking. We discuss the changing views of intracellular LI pools in relation to iron homeostasis and cellular distribution in physiological and pathological states.