Mitochondrial function under hypoxic conditions: the steady states of cytochrome alpha+alpha3 and their relation to mitochondrial energy states.

The oxidation-reduction states of mitochondrial cytochromes were studied under low O2 concentrations. 1. 1. Relative oxidation states of cytochromes caused by increasing concentrations of O2 apparently followed the sequence of their half reduction potentials only under the uncoupled conditions. 2. 2. In the presence of antimycin A, the O2-induced reduction of cytochrome bT was seen in the difference spectrum. 3. 3. The O2 dependency of the relative reduction state of cytochrome a+a3 with respect to that of cytochrome c altered significantly depending upon the presence or absence of ATP. The most significant change in the O2 dependency was that due to cytochrome a3. 4. 4. When compared at a given low O2 concentration below 0.5 μM, the reduction states of cytochrome a+a3, as well as that of cytochrome c, were higher in the presence of ADP or uncoupler than in the presence of ATP. 5. 5. Whereas the O2 concentration required for 50% oxidation of cytochrome c (P50c) depended upon the respiratory rate, the O2 concentration required for 50% oxidation of cytochrome a3 (P50a3) required information on the energy state of mitochondria. Under conditions where the redox states of cytochrome c and a+a3 are measured continuously and as a function of the O2 concentration it may be possible to evaluate the energetic state of the mitochondria.