ATP-induced oxidation of the a3+-2CO compound in pigeon heart mitochondria

Simultaneous oxidation-reduction potential versus absorbance titrations on intact mitochondria have allowed us to measure the individual spectral and thermodynamic properties of cytochromes a and a₃ in the presence or absence of added ligands. In coupled pigeon heart mitochondria, saturated with CO, the reduced a₃CO compound can be fully oxidized by ferricyanide under conditions of high phosphate potential, i.e., on addition of ATP. These ATP-induced effects are reversed by the presence of oligomycin or uncouplers with complete restoration of the reduced a₃CO compound. This observation has permitted us to examine the spectral and thermodynamic properties of the a₃CO compound in some detail. The data indicate that, in the presence of ATP, the midpoint potential of the a₃CO compound varies as a function of CO concentration in a thermodynamically predictable fashion. The ATP-induced changes on the a₃CO compound are directly analogous to the changes reported earlier for cytochrome a₃ in the absence of added ligands.