Oxidized Redox State of Glutathione in the Endoplasmic Reticulum

TLDR

The ER’s oxidative environment may be maintained by preferential transport of GSSG over GSH into the lumen, a mechanism not previously understood. The authors measured ER redox using a glycosylated peptide that diffuses into the lumen, undergoes reversible thiol‑disulfide exchanges, and is transported into microsomes, revealing glutathione’s role as the major redox buffer. Glycosylated peptides were disulfide‑linked to glutathione, showing glutathione as the main redox buffer, and the secretory pathway’s GSH/GSSG ratio (1:1–3:1) was far more oxidative than the cytosol’s (30:1–100:1).

Abstract

The redox state of the endoplasmic reticulum (ER) was measured with the peptide N -Acetyl-Asn-Tyr-Thr-Cys-NH 2 . The peptide diffused across cellular membranes; some became glycosylated and thus trapped within the secretory pathway, and its cysteine residue underwent reversible thiol-disulfide exchanges with the surrounding redox buffer. Glycosylated peptides from cells were disulfide-linked to glutathione, indicating that glutathione is the major redox buffer in the secretory pathway. The redox state of the secretory pathway was more oxidative than that of the cytosol; the ratio of reduced glutathione to the disulfide form (GSH/GSSG) within the secretory pathway ranged from 1:1 to 3:1, whereas the overall cellular GSH/GSSG ratio ranged from 30:1 to 100:1. Cytosolic glutathione was also transported into the lumen of microsomes in a cell-free system. Although how the ER maintains an oxidative environment is not known, these results suggest that the demonstrated preferential transport of GSSG compared to GSH into the ER lumen may contribute to this redox compartmentation.