Electron Paramagnetic Resonance-detectable Electron Acceptors in Beef Heart Mitochondria

The EPR-detectable (24.2 K) electron acceptors of the DPNH-ubiquinone reductase segment of the electron transfer system of beef heart mitochondria were studied in preparations of varying complexity. Four distinct iron-sulfur centers associated with this segment of the electron transfer system were identified. They occur in approximately equal concentrations, and this concentration approximates that of the bound flavin of DPNH dehydrogenase. Quantitative reductive titrations with DPNH and dithionite showed that the order of the oxidation-reduction potentials of the ironsulfur centers is: Center 3 2 2 >> 4 > 1, indicating that there are two centers of high and two of low potential. Complex I accepts approximately 20 electron neq per mg of protein from DPNH. In rapid reaction studies using the freeze-quench method the rates of reduction of the ironsulfur centers by DPNH could not be resolved. With acetylpyridine DPNH, however, the rates of appearance of the signals were in this sequence: Center 2 > 3 + 4 > 1.