Complex IV (cytochrome c oxidase) is the terminal electron acceptor in the mitochondrial electron transport chain. Complex IV functions by oxidizing cytochrome c and completing a four electron reduction of oxygen (O2) to form water. During this process, two H+ are translocated from the mitochondrial matrix to the intermembrane space, contributing the mitochondrial membrane potential required for ATP synthesis. Common inhibitors of complex IV include potassium cyanide, azide, O2 limitation, nitric oxide, and carbon monoxide. Cayman’s MitoCheck® Complex IV Activity Assay is designed to measure the direct oxidation of cytochrome c by complex IV, in an isolated bovine heart mitochondrial system that is supplied within the kit.