The Journal of Biological Chemistry

The electron acceptors in ubihydroquinone-cytochrome c reductase (Complex III) derived from beef heart mitochondria were investigated by low temperature electron paramagnetic resonance spectroscopy at 120 K and optical reflectance spectroscopy at 100 K. Three resonances were resolved in the region where the low field lines of low spin ferric heme compounds are usually found. These lines were assigned to cytochromes cl (gz = 3.33), bR (g, = 3.44), and bT (g, = 3.78) on the basis of correlation with optical spectra during reduction. A broad line typical of the center line (gy) of a low spin ferric heme is seen at g N 2. Other EPR-detectable electron acceptors in Complex III are the iron-sulfur center previously detected by EPR at 7’7” (J. S. RIESKE, D. H. MACLENNAX, AND R. COLEMAN (1964) Biothem. Biophys. Res. Commun. 15, 338); a species whose signal appears at g = 1.93 on reduction, presumably an iron-sulfur protein, and possibly due to contamination with succinate-ubiquinone reductase (Complex II); two acceptors of unknown structure and function, one with a signal centered at g = 2.01 (oxidized form) and the other with lines at g = 2.04 and 1.99. The signal of the latter is absent in the oxidized state, appears on partial reduction and disappears on complete reduction, indicating that three discrete oxidation states may be involved. The behavior of all of these species was studied during reductive anaerobic titration of Complex III with dithionite and of more integrated preparations, such as DPNH-cytochrome c reductase and particles derived from beef heart mitochondria by sonication, with DPNH or dithionite as reductant. An attempt is made to compare the experi-