While oxygen is essential for oxidative phosphorylation, O 2 can form reactive species (ROS) when interacting with electrons of mitochondrial electron transport system. ROS is dependent on O 2 pressure (PO 2 ) and has traditionally been assessed in O 2 saturated media, PO 2 at which mitochondria do not typically function in vivo. Mitochondrial ROS can be significantly elevated by the respiratory complex II substrate succinate,...