Redox-optimized ROS balance and the relationship between mitochondrial respiration and ROS.

The Redox-Optimized ROS Balance [R-ORB] hypothesis postulates that the redox environment [RE] is the main intermediary between mitochondrial respiration and reactive oxygen species [ROS]. According to R-ORB, ROS emission levels will attain a minimum vs. RE when respiratory rate (VO2) reaches a maximum following ADP stimulation, a tenet that we test herein in isolated heart mitochondria under forward electron transport [FET]. ROS emission increased two-fold as a function of changes in the RE (~400 to ~900mV·mM) in state 4 respiration elicited by increasing glutamate/malate (G/M). In G/M energized mitochondria, ROS emission decreases two-fold for RE ~500 to ~300mV·mM in state 3 respiration at increasing ADP. Stressed mitochondria released higher ROS, that was only weakly dependent on RE under state 3. As a function of VO2, the ROS dependence on RE was strong between ~550 and ~350mV·mM, when VO2 is maximal, primarily due to changes in glutathione redox potential. A similar dependence was observed with stressed mitochondria, but over a significantly more oxidized RE and ~3-fold higher ROS emission overall, as compared with non-stressed controls. We conclude that under non-stressful conditions mitochondrial ROS efflux decreases when the RE becomes less reduced within a range in which VO2 is maximal. These results agree with the R-ORB postulate that mitochondria minimize ROS emission as they maximize VO2 and ATP synthesis. This relationship is altered quantitatively, but not qualitatively, by oxidative stress although stressed mitochondria exhibit diminished energetic performance and increased ROS release.