Cells were grown on 100-mm tissue culture dishes and analyzed for metals by inductively coupled plasma mass spectrometry (ICP-MS) as we described previously (Choi et al. 2020 (link); Choi et al. 2019 (link); Choi et al. 2018 (link)). Briefly, the cell samples were digested with 2 mL/g total wet weight nitric acid (BDH ARISTAR®ULTRA) for 24 h and then digested with 1 mL/g total wet weight hydrogen peroxide (BDH Aristar® ULTRA) for 24 h at room temperature. The samples were stored at 4°C until metals were quantified. Ultrapure water was used for final sample dilution. For mitochondrial iron levels, mitochondria were isolated through differential centrifugation as we previously described (Choi et al. 2018 (link)), and then mitochondrial iron levels were measured via ICP-MS.