Abstract

Several iron-sulfur centers in the NADH-ubiquinone segment of the respiratory chain in pigeon heart mitochondria and in submitochondrial particles were analyzed by the combined application of cryogenic EPR (between 30 and 4.2 °K) and potentiometric titration. Center N-1 (iron-sulfur centers associated with NADH dehydrogenase are designated with the prefix “N”) resolves into two single electron titrations with E m 7.2 values of −380±20 mV and −240±20 mV (Centers N-1a and N-1b, respectively). Center N-1a exhibits an EPR spectrum of nearly axial symmetry with g // = 2.03, g ⊥ = 1.94, while that of Center N-1b shows more apparent rhombic symmetry with g z = 2.03, g y = 1.94 and g x = 1.91. Center N-2 also reveals EPR signals of axial symmetry at g // = 2.05 and g ⊥ = 1.93 and its principal signal overlaps with those of Centers N-1a and N-1b. Center N-2 can be easily resolved from N-1a and N-1b because of its high E m 7.2 value (−20±20 mV). Resolution of Centers N-3 and N-4 was achieved potentiometrically in submitochondrial particles. The component with E m 7.2 = − 240±20 mV is defined as Center N-3 ( g z = 2.10, ( g y = 1.93?), g x = 1.87); the −405±20 mV component as Center N-4 ( g z = 2.11, ( g y = 1.93?), g x = 1.88). At temperatures close to 4.2 °K, EPR signals at g = 2.11, 2.06, 2.03, 1.93, 1.90 and 1.88 titrate with E m 7.2 = −260±20 mV. The multiplicity of peaks suggests the presence of at least two different ironsulfur centers having similar E m 7.2 values (−260±20 mV); hence, tentatively assigned as N-5 and N-6. Consistent with the individual E m 7.2 values obtained, addition of succinate results in the partial reduction of Center N-2, but does not reduce any other centers in the NADH-ubiquinone segment of the respiratory chain. Centers N-2, N-1b, N-3, N-5 and N-6 become almost completely reduced in the presence of NADH, while Centers N-1a and N-4 are only slightly reduced in pigeon heart submitochondrial particles. In pigeon heart mitochondria, the E m 7.2 of Center N-4 lies much closer to that of Center N-3, so that resolution of the Center N-3 and N-4 spectra is not feasible in mitochondrial preparations. E m 7.2 values and EPR lineshapes for the other ironsulfur centers of the NADH-ubiquinone segment in the respiratory chain of intact mitochondria are similar to those obtained in submitochondrial particle preparations. Thus, it can be concluded that, in intact pigeon heart mitochondria, at least five iron-sulfur centers show E m 7.2 values around -250 mV; Center N-2 exhibits a high E m 7.2 (−20±20 mV), while Center N-1a shows a very low E m 7.2 (−380±20 mV).

Full Text
Paper version not known

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call