Glutathione is central to cellular redox chemistry. The majority of glutathione redox research has been based on the chemical analysis of whole-cell extracts, which unavoidably destroy subcellular compartment-specific information. Compartment-specific real-time measurements based on genetically encoded fluorescent probes now suggest that the cytosolic glutathione redox potential is about 100 mV more reducing than previously thought. Using these probes in yeast, we show that even during severe oxidative stress, the cytosolic glutathione disulfide (GSSG) concentration is much more tightly regulated than expected and provides a mechanistic explanation for the discrepancy with conventional measurements. GSSG that is not immediately reduced in the cytosol is rapidly transported into the vacuole by the ABC-C transporter Ycf1. The amount of whole-cell GSSG is entirely dependent on Ycf1 and uninformative about the cytosolic glutathione pool. Applying these insights, we identify Trx2 and Grx2 as efficient backup systems to glutathione reductase for cytosolic GSSG reduction.

Glutathione, the most abundant small-molecule thiol in eukaryotic cells, is synthesized de novo solely in the cytosol and must subsequently be transported to other cellular compartments. The mechanisms of glutathione transport into and out of organelles remain largely unclear. We show that budding yeast Opt2, a close homolog of the plasma membrane glutathione transporter Opt1, localizes to peroxisomes. We demonstrate that deletion of OPT2 leads to major defects in maintaining peroxisomal, mitochondrial, and cytosolic glutathione redox homeostasis. Furthermore, ∆opt2 strains display synthetic lethality with deletions of genes central to iron homeostasis that require mitochondrial glutathione redox homeostasis. Our results shed new light on the importance of peroxisomes in cellular glutathione homeostasis.