Mitochondrial function under hypoxic conditions: The steady states of cytochrome a+ a3 and their relation to mitochondrial energy states

Abstract

The oxidation-reduction states of mitochondrial cytochromes were studied under low O 2 concentrations. 1. 1. Relative oxidation states of cytochromes caused by increasing concentrations of O 2 apparently followed the sequence of their half reduction potentials only under the uncoupled conditions. 2. 2. In the presence of antimycin A, the O 2-induced reduction of cytochrome b T was seen in the difference spectrum. 3. 3. The O 2 dependency of the relative reduction state of cytochrome a+ a 3 with respect to that of cytochrome c altered significantly depending upon the presence or absence of ATP. The most significant change in the O 2 dependency was that due to cytochrome a 3. 4. 4. When compared at a given low O 2 concentration below 0.5 μM, the reduction states of cytochrome a+ a 3, 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 O 2 concentration required for 50% oxidation of cytochrome c (P50 c ) depended upon the respiratory rate, the O 2 concentration required for 50% oxidation of cytochrome a 3 (P50 a3 ) required information on the energy state of mitochondria. Under conditions where the redox states of cytochrome c and a+ a 3 are measured continuously and as a function of the O 2 concentration it may be possible to evaluate the energetic state of the mitochondria.