title: On the role of inorganic phosphate in divalent-cation sequestration by mitochondria.
abstract: Rat liver mitochondria which have actively accumulated Mn2+ ions exhibit an electron para-magnetic resonance (EPR) spectrum dominated by a component E in which the usual hyperfine sextet has been collapsed to a single line through the process of spin exchange narrowing. When uptake occurs in the absence of added Pi, the line width of E shows considerable variation. By comparing experimentally observed spectra with computer-generated simulations it is demonstrated however that, under these conditions, E can be as narrow as approximately equal to 190 Oe, considerably narrower than the spectrum from a manganese phosphate precipitate. Furthermore parallel quantification on a single mitochondrial sample shows the level of Pi to be several times lower than the amount of Mn2+ observable in E. It is concluded therefore that the spectral component E, obtained under these conditions, cannot be accounted for in terms of manganese phosphate precipitate. These findings further indicate that a substantial number of sites (approximately equal to 100 nmol/mg protein), other than Pi, must exist inside the mitochondria which are capable of complexing divalent cations in regions of high local concentration.
mesh_terms: D000818:Animals; D002413:Cations, Divalent / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D008345:Manganese / Q000378:metabolism*; D008930:Mitochondria, Liver / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D051381:Rats
chemical_list: D002413:Cations, Divalent; D010710:Phosphates; D008345:Manganese