Abstract
The redox state of mitochondria is determined by the levels of reducing and oxidizing species in the organelle, which reflects mitochondrial metabolic activity and overall fitness. Mitochondria are also the primary endogenous source of reactive oxygen species (ROS). This chapter describes methods to measure the mitochondrial superoxide levels and the redox state of the organelle in mammalian cells and yeast. We describe the use of dihydroethidium (DHE) and MitoSOX (a derivative of dihydroethidium bound to a lipophilic cation) to detect mitochondrial superoxide in yeast and mammalian cells, respectively. We also describe the use of genetically encoded fluorescent biosensors for quantitative analysis of mitochondrial NADPH levels (iNap) in mammalian cells and mitochondrial redox state (mito-roGFP) in yeast.