A Nonheme High-Spin Ferrous Pool in Mitochondria Isolated from Fermenting Saccharomyces cerevisiae

Mossbauer spectroscopy was used to detect pools of Fe in mitochondria from fermenting yeast cells, including those consisting of nonheme high-spin (HS) Fe-II species, Fe-III nanoparticles, and mononuclear HS Fe-III species. At issue was whether these species were located within mitochondria or on their exterior. None could be removed by washing mitochondria extensively with ethylene glycol tetraacetic acid or bathophenanthroline sulfonate (BPS), Fe-II chelators that do not appear to penetrate mitochondrial membranes. However, when mitochondrial samples were sonicated, BPS coordinated the Fe-II species, forming a low-spin Fe-II complex. This treatment also diminished the levels of both Fe-III species, suggesting that all of these Fe species are encapsulated by mitochondrial membranes and are protected from dictation until membranes are disrupted. 1,10-Phenanthroline is chemically similar to BPS but is membrane soluble; it coordinated nonheme HS Fe-II in unsonicated mitochondria. Further, the HS Fe-III species and nanoparticles were not reduced by dithionite until the detergent deoxycholate was added to disrupt membranes. There was no correlation between the percentage of nonheme HS Fe-II species in mitochondrial samples and the level of contaminating proteins. These results collectively indicate that the observed Fe species are contained within mitochondria. Mossbauer spectra of whole cells were dominated by HS Fe-III features; the remainder displayed spectral features typical of isolated mitochondria, suggesting that the Fe in fermenting yeast cells can be coarsely divided into two categories: mitochondrial Fe and (mostly) HS Fe-III ions in one or more non-mitochondrial locations.