Biophysical Investigation of the Ironome of Human Jurkat Cells and Mitochondria

The speciation of iron in intact human Jurkat leukemic cells and their isolated mitochondria was assessed using biophysical methods. Large-scale cultures were grown in medium enriched with ⁵⁷Fe citrate. Mitochondria were isolated anaerobically to prevent oxidation of iron centers. 5 K Mössbauer spectra of cells were dominated by a sextet due to ferritin. They also exhibited an intense central quadrupole doublet due to S = 0 [Fe₄S₄]²⁺ clusters and low-spin (LS) Fe^II heme centers. Spectra of isolated mitochondria were largely devoid of ferritin but contained the central doublet and features arising from what appear to be Fe^III oxyhydroxide (phosphate) nanoparticles. Spectra from both cells and mitochondria contained a low-intensity doublet from non-heme high-spin (NHHS) Fe^II species. A portion of these species may constitute the “labile iron pool” (LIP) proposed in cellular Fe trafficking. Such species might engage in Fenton chemistry to generate reactive oxygen species. Electron paramagnetic resonance spectra of cells and mitochondria exhibited signals from reduced Fe/S clusters, and HS Fe^III heme and non-heme species. The basal heme redox state of mitochondria within cells was reduced; this redox poise was unaltered during the anaerobic isolation of the organelle. Contributions from heme a, b, and c centers were quantified using electronic absorption spectroscopy. Metal concentrations in cells and mitochondria were measured using inductively coupled plasma mass spectrometry. Results were collectively assessed to estimate the concentrations of various Fe-containing species in mitochondria and whole cells  the first “ironome” profile of a human cell.