Cytochrome Oxidase (CO) is considered to be an inherent enzyme of the mitochondrial membrane, and is highly active in cells containing a large number of mitochondria (such as myocardium, renal tubular epithelium, as well as gastric lining cells and hepatocytes), and the activity of this enzyme is often used as an indicator of oxidative metabolism within the cell, and as one of the hallmark enzymes of mitochondria.

Cytochrome oxidase staining solution (p-phenylenediamine method) uses N-phenyl-p-phenylenediamine as the substrate, in the presence of oxygen, by the action of cytochrome oxidase can be generated with naphthol colored indophenol blue, i.e., the Nadi reaction, this enzyme is sensitive to the fixative, so it is necessary to use a fresh section.