Complex II (succinate dehydrogenase/co-enzyme Q reductase) is one of the major sites of electron entry into the mitochondrial electron transport chain (ETC). Complex II catalyzes the oxidation of succinate to fumarate, and in the process reduces ubiquinone (Q) to ubiquinol (QH2). Ultimately, oxidation of succinate will lead to reduction of O2, the terminal step in mitochondrial respiration. Cayman's MitoCheck? Complex II Activity Assay allows for the activity of complex II to be determined without the need to isolate mitochondria or pre-incubate with antibodies. As complex II oxidizes succinate, electrons are passed to an analog of ubiquinone and then on to DCPIP, which, when oxidized, absorbs in the 600 nm range. The absorbance of DCPIP will decrease upon reduction. Thus, Complex II activity is measured as a decrease in absorbance at 600 nm over time.