Study links cysteine toxicity to iron overload in mitochondria

A study has identified the mechanism that makes excess cysteine lethal to cells: the amino acid can extract iron from ferritin and drive that metal into mitochondria, where it disrupts energy production. The findings, reported by Rockefeller University and published in Nature Metabolism, help explain why cells keep free cysteine levels unusually low.

Cysteine is one of the 20 amino acids used to assemble proteins. It is also involved in maintaining the cell’s chemical balance, producing iron-sulfur clusters and making glutathione, a major cellular antioxidant. Yet unlike the other protein-building amino acids, cysteine becomes acutely harmful when it accumulates.

The new work suggests that the danger comes not simply from cysteine’s chemical reactivity, but from its effect on iron storage and transport. By freeing iron that is normally contained inside ferritin, cysteine causes the metal to build up in mitochondria. The resulting damage impairs the organelles that supply energy to the cell and eventually causes cell death.

A necessary molecule with a narrow safe range

Cells need cysteine, but they generally avoid allowing much of it to remain in its free form. Instead, they rapidly use it to make glutathione, which can be present at far higher concentrations without producing the same toxic effect.

That difference has long posed a biological puzzle. Cysteine and glutathione both contain a reactive sulfur group, so their contrasting effects could not be explained simply by the presence of that chemical feature. Researchers led by Kivanç Birsoy at Rockefeller University set out to determine why cells appear to favor glutathione as their principal antioxidant while tightly restricting free cysteine.

The question also has practical relevance. Some research has suggested that cysteine supplements may have benefits, but the study’s findings underline the potential hazards of exposing cells to high concentrations of the amino acid. Too little cysteine can reduce glutathione production and contribute to cell death; too much can trigger a separate, iron-related form of damage.

The team used a genome-wide CRISPR screen to disable genes across cells and identify changes that allowed them to survive cysteine exposure. Two clues stood out. One was SLC25A28, which produces a transporter that moves iron into mitochondria. The other involved proteins that help dismantle ferritin, the complex responsible for storing iron in a relatively safe form.

Those results directed the researchers toward a connection between cysteine and ferritin. Follow-up experiments showed that cysteine reacts with ferritin-held iron and converts it into a state that can leave the storage complex. Glutathione, despite having a similar sulfur-containing group, did not produce the same release of iron.

Iron turns against the cell’s power system

Once liberated, the iron was transported into mitochondria. There it damaged iron-sulfur proteins, which are essential to several processes that support mitochondrial energy generation. As those systems failed, energy production collapsed and the cells died.

The researchers also tested whether interrupting the pathway could prevent the damage. Blocking iron’s release from ferritin, or stopping its movement into mitochondria, protected cells from otherwise toxic cysteine concentrations. These experiments connected cysteine exposure to cell death through a specific chain of events rather than through general chemical stress alone.

The results provide an evolutionary explanation for the cellular preference for glutathione. Packaging cysteine into that antioxidant may require energy, but it prevents the amino acid from interacting with stored iron and threatening mitochondria. In this account, maintaining a small free-cysteine pool is a protective strategy for safeguarding the cell’s energy-producing machinery.

Possible implications for cancer research

The discovery may eventually inform studies of cancer metabolism. Some cancer cells alter their nutrient use and take up unusually large amounts of an oxidized form of cysteine. That behavior could create a potentially dangerous cysteine burden, meaning such cells may need adaptations that prevent iron from accumulating in their mitochondria.

If those protective adaptations can be identified, researchers may be able to interfere with them and expose a weakness specific to cancer cells. The study does not establish a treatment, but it points to iron handling and mitochondrial protection as possible areas for future investigation.

For now, the work is primarily a basic biology advance. Birsoy’s laboratory is continuing to examine how cells detect and control chemically reactive molecules, including how those systems might be altered in cancer and other diseases. The study’s central contribution is to show why keeping cysteine scarce can be essential: uncontrolled cysteine can unlock iron, and that iron can disable the machinery that keeps cells alive.