Mitochondrial control of cellular redox states is a fundamental component of cell signaling in the coordination of core energy metabolism and homeostasis during normoxia and
hypoxia. We investigated the relationship between
cytochrome redox states and mitochondrial oxygen consumption at steady-state levels of
hypoxia in mitochondria isolated from beef and mouse heart (BHImt, MHImt), comparing two species with different cardiac dynamics and local
oxygen demands. A low-noise, rapid spectrophotometric system using visible light for the measurement of
cytochrome redox states was combined with high-resolution respirometry. Monophasic hyperbolic relationships were observed between oxygen consumption, JO2, and
oxygen partial pressure, Po2, within the range <1.1 kPa (8.3 mmHg; 13 μM). P50j (Po2 at 0.5·Jmax) was 0.015 ± 0.0004 and 0.021 ± 0.003 kPa (0.11 and 0.16 mmHg) for BHImt and MHImt, respectively. Maximum oxygen consumption, Jmax, was measured at saturating
ADP levels (OXPHOS capacity) with Complex I-linked substrate supply. Redox states of
cytochromes aa3 and c were biphasic hyperbolic functions of Po2. The relationship between
cytochrome oxidation state and oxygen consumption revealed a separation of distinct phases from mild to severe and deep
hypoxia. When
cytochrome c oxidation increased from fully reduced to 45% oxidized at 0.1 Jmax, Po2 was as low as 0.002 kPa (0.02 μM), and trace amounts of
oxygen are sufficient to partially oxidize the
cytochromes. At higher Po2 under severe
hypoxia, respiration increases steeply, whereas redox changes are small. Under mild
hypoxia, the steep slope of oxidation of
cytochrome c when flux remains more stable represents a cushioning mechanism that helps to maintain respiration high at the onset of
hypoxia.