Three functionally different cytochrome b redox centres, apparently of high metabolic activity, were detected in intact pigeon heart mitochondria; cytochrome b(1), b(m) and b(h), with maxima of absorption at 556.6 (State 5), 560.6, and 564.5 nm, respectively (alpha-bands, 77K). 2. Cytochrome (b(l) was reduced in the presence of either antimycin or HQNO (2-heptyl-4-hydroxyquinoline N-oxide). The absorption maximum was shifted by dithionite, cyanide, NNN‘N’-tetramethyl-p-phenylenediamine + ascorbate, HQNO and antimycin. The spectra obtained on simultaneous or successive addition of HQNO and antimycin favoured the assumption of a common binding site for the two inhibitors. 3. Cytochrome b(m) was reduced in the presence of HQNO, but not in the presence of antimycin. No shifts of absorption maximum was observed. 4. Cytochrome b(h) was reduced in the presence of antimycin. HQNO was unable to cause reduction of this cytochrome by endogenous substrates. The absorption maximum was shifted to lower wavelength by organic solvents. It was inseparable from that of cytochrome b(m) in the presence of 0.4% ethanol. 5. The pattern of reduction in the presence of HQNO or antimycin demonstrates the functional difference of the three redox centres and appears incompatible wih a linear respiratory chain.
