Allison Cockrell - Mössbauer and EPR Study of Iron in Vacuoles Isolated from Saccharomyces cerevisiae

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      Allison L.   Cockrell,   Gregory P. Holmes-Hampton, Sean P. McCormick, Mrinmoy   Chakrabarti, and Paul A. Lindahl (2011) Biochemistry 50   (47), 10275 – 10283


        Vacuoles were isolated from fermenting yeast cells grown on   minimal medium supplemented with 40 μM (57)Fe. Absolute   concentrations of Fe, Cu, Zn, Mn, Ca, and P in isolated vacuoles   were determined by ICP-MS. Mössbauer spectra of isolated vacuoles   were dominated by two spectral features: a mononuclear   magnetically isolated high-spin (HS) Fe(III) species coordinated   primarily by hard/ionic (mostly or exclusively oxygen) ligands   and superparamagnetic Fe(III) oxyhydroxo nanoparticles. EPR   spectra of isolated vacuoles exhibited a g(ave) ~ 4.3 signal   typical of HS Fe(III) with E/D ~ 1/3. Chemical reduction of the   HS Fe(III) species was possible, affording a Mössbauer quadrupole   doublet with parameters consistent with O/N ligation. Vacuolar   spectral features were present in whole fermenting yeast cells;   however, quantitative comparisons indicated that Fe leaches out   of vacuoles during isolation. The in vivo vacuolar Fe   concentration was estimated to be ~1.2 mM while the Fe   concentration of isolated vacuoles was ~220 μM. Mössbauer   analysis of Fe(III) polyphosphate exhibited properties similar to   those of vacuolar Fe. At the vacuolar pH of 5, Fe(III)   polyphosphate was magnetically isolated, while at pH 7, it formed   nanoparticles. This pH-dependent conversion was reversible.   Fe(III) polyphosphate could also be reduced to the Fe(II) state,   affording similar Mössbauer parameters to that of reduced   vacuolar Fe. These results are insufficient to identify the exact   coordination environment of the Fe(III) species in vacuoles, but   they suggest a complex closely related to Fe(III) polyphosphate.   A model for Fe trafficking into/out of yeast vacuoles is   proposed.

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