Excess cysteine triggers cell death by releasing stored iron
Researchers have discovered that high concentrations of cysteine can strip iron from ferritin, causing iron to accumulate in mitochondria and leading to cell death. The study, published in Nature Metabolism, explains why cells normally convert cysteine into glutathione to keep free cysteine levels low. This finding resolves a long-standing question about the evolutionary advantage of using glutathione as the primary antioxidant.
The study identifies a precise toxic mechanism: cysteine molecules chemically strip iron atoms from ferritin, the storage protein that normally sequesters the metal safely. Once liberated, this iron migrates into mitochondria, where accumulation triggers cell death. This explains why cysteine is uniquely dangerous among the twenty protein-building amino acids—it is the only one that becomes acutely toxic when present in excess, despite being essential for protein synthesis and iron-sulfur cluster formation.
The findings also address a long-standing evolutionary question about glutathione's dominance as the cell's primary antioxidant. Both cysteine and glutathione contain reactive sulfur groups, yet cells tolerate high glutathione concentrations while tightly limiting free cysteine. The Birsoy lab's earlier work on glutathione transport and organelle regulation provided groundwork for this discovery, suggesting that glutathione's evolutionary success stems from its safety profile—it delivers antioxidant benefits without the iron-releasing toxicity that would threaten mitochondrial function.
This discovery could influence how cysteine supplements are marketed and consumed, as the amino acid is sometimes promoted for health benefits despite its cellular toxicity at high levels. It may also inform research into iron-related disorders and mitochondrial diseases, potentially guiding future therapeutic approaches. Scientists studying cell death pathways could gain new investigative tools, while clinicians might reconsider cysteine levels in patients with metabolic conditions. The findings underscore how fundamental biochemical choices carry significant consequences for cellular health and organismal well-being.