Complex III (CoQ cytochrome c oxidoreductase) is an essential protein for mitochondrial oxidative phosphorylation. Complex III functions as both a gatekeeper for mitochondrial respiration, and as a major source of reactive oxygen species III. It accepts electrons from complexes I and II in the form of QH2, the reduced form of the electron carrier ubiquinone. Once bound to complex III, QH2 undergoes a series of redox reactions, known as the Q-cycle. During the Q-cycle, electrons are passed from QH2 onto cytochrome c via the Reiske iron-sulfur protein and cytochrome c1 resulting in the translocation of 4H+ and the generation of 02•-. Cayman's MitoCheck® Complex II/III Activity Assay measures the reduction of excess cytochrome c (550 nm absorbance) as catalyzed by complex III. This assay is coupled to succinate co-enzyme Q oxidoreductase (complex II) for the generation of QH2. Due to the dependence on complex II activity, a counterscreen for complex II activity (Item No. 700940) should be performed in order to truly measure complex III activity.