Labile Iron Pool of Isolated Escherichia coli Cytosol Likely Includes Fe-ATP and Fe-Citrate but not Fe-Glutathione or Aqueous Fe

The existence of labile iron pools (LFePs) in biological systems has been recognized for decades, but their chemical composition remains uncertain. Here, the LFeP in cytosol from Escherichia coli was investigated. Mo''ssbauer spectra of whole vs lysed cells indicated significant degradation of iron-sulfur clusters (ISCs), even using an unusually gentle lysis procedure; this demonstrated the fragility of ISCs. Moreover, the released iron contributed to the non-heme high-spin Fe(II) species in the cell, which likely included the LFeP. Cytosol batches isolated from cells grown with different levels of iron supplementation were passed through a 3 kDa cutoff membrane, and resulting flow through-solutions (FTSs) were subjected to SEC-ICP-MS. Mo''ssbauer spectroscopy was used to evaluate the oxidation states of standards. FTSs exhibited iron-detected peaks likely due to different forms of Fe-citrate and Fe-nucleotide triphosphate complexes. Fe-Glutathione (GSH) complexes were not detected using physiological concentrations of GSH mixed with either Fe(II) or Fe(III); Fe(II)-GSH was concluded not to be a significant component of the LFeP in E. coli under physiological conditions. Aqueous iron was also not present in significant concentrations in isolated cytosol and is unlikely a major component of the pool. Fe appeared to bind ATP more tightly than citrate, but ATP also hydrolyzed on the timescale of tens of hours. Isolated cytosol contained excess ligands that coordinated the added Fe(II) and Fe(III). The LFeP in healthy metabolically active cells is undoubtedly dominated by the Fe(II) state, but the LFeP is redox-active such that a fraction might be present as stable and soluble Fe(III) complexes especially under oxidatively stressed cellular conditions.