This study investigates how loss of the ER transporter SLC33A1 affects glutathione redox balance and protein cysteine reactivity in the endoplasmic reticulum. Oxidized cysteine reactivity profiling and whole-cell proteomics were performed in Slc33a1 knockout KPK cells and cDNA-complemented controls. Quantitative TMT-based proteomics identified increased oxidation of protein disulfide isomerases and other ER-resident proteins, indicating disruption of ER redox homeostasis and proteostasis. Additional experiments examined unfolded protein response activation and genetic interactions with ER quality-control pathways. The dataset includes cysteine reactivity profiling and unenriched proteomics data acquired by LC–MS/MS and processed using standard proteomics pipelines.