Is ferritin the primary intracellular iron storage protein that limits redox-active free iron?

Ferritin serves as the principal intracellular macromolecule for iron storage, maintaining cellular homeostasis by safely sequestering iron.

Intracellular storage and buffering mechanisms

• Nanocage structure: Ferritin is structured as a 24-subunit nanocage of heavy (H) and light (L) chains. The H subunit contains a ferroxidase catalytic center that rapidly oxidizes reactive cytosolic ferrous iron ($\text{Fe}^{2+}$) into ferric iron ($\text{Fe}^{3+}$), while the L subunit assists in mineral nucleation and storage within a stable, non-toxic crystalline core.
• Labile iron pool regulation: This sequestration directly buffers the cytosolic labile iron pool (LIP). When cellular iron demands change, NCOA4-mediated ferritinophagy targets ferritin for lysosomal degradation, releasing stored iron back into the cytosol and increasing the LIP of redox-active free iron.

Prevention of oxidative damage and ferroptosis

• Inhibition of Fenton chemistry: By keeping the LIP minimized, ferritin prevents free, redox-active $\text{Fe}^{2+}$ from driving Fenton and Haber-Weiss reactions. These reactions utilize free iron to convert peroxides into highly destructive hydroxyl radicals and other reactive oxygen species (ROS).
• Cellular protection: Restricting redox-active free iron prevents oxidative stress, lipid peroxidation, DNA damage, and ferroptosis (iron-dependent cell death). While ferritin upregulation suppresses ROS-mediated injury, its depletion expands the LIP and accelerates oxidative cellular damage.

Bottom line

• Ferritin is the primary intracellular buffer that sequesters reactive $\text{Fe}^{2+}$ and stores it as stable $\text{Fe}^{3+}$, directly limiting the labile iron pool to prevent toxic Fenton chemistry, ROS generation, and ferroptosis.