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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.
Insights
Manganese (Mn2+) accumulation in rat liver mitochondria shows a unique electron paramagnetic resonance (EPR) signal. This signal, narrowed by spin exchange, indicates manganese is bound to internal mitochondrial sites, not just phosphate precipitates.
Area of Science:
- Biochemistry
- Mitochondrial Physiology
- Spectroscopy
Background:
- Rat liver mitochondria actively accumulate divalent manganese ions (Mn2+).
- Electron paramagnetic resonance (EPR) spectroscopy is used to study manganese interactions within mitochondria.
Purpose of the Study:
- To investigate the nature of Mn2+ accumulation in rat liver mitochondria.
- To determine if manganese phosphate precipitates account for the observed EPR spectra.
Main Methods:
- Electron paramagnetic resonance (EPR) spectroscopy was employed to analyze Mn2+ in mitochondria.
- Computer-generated simulations were used to interpret EPR spectral data.
- Quantification of inorganic phosphate (Pi) and observable Mn2+ within mitochondrial samples.
Main Results:
- A distinct EPR spectral component (E) was observed, characterized by spin exchange narrowing.
- The line width of component E was significantly narrower than expected for manganese phosphate precipitates.
- Mitochondrial Pi levels were substantially lower than the observable Mn2+ in component E.
Conclusions:
- The EPR spectral component E in mitochondria is not due to manganese phosphate precipitates.
- Mitochondria possess numerous internal sites (approx. 100 nmol/mg protein) capable of complexing Mn2+.
- These findings suggest alternative intracellular binding sites for divalent cations within mitochondria.