text: Cytochrome c has been used to detect peroxide production in biological systems. As superoxide is produced, the number of oxidised cytochrome c increases, and reduced cytochrome c decreases. However, superoxide is often produced with nitric oxide. In the presence of nitric oxide, the reduction of cytochrome c is inhibited. This leads to the oxidisation of cytochrome c to cytochrome c by peroxynitrous acid, an intermediate made through the reaction of nitric oxide and superoxide. Presence of peroxynitrite or HO and nitrogen dioxide NO in the mitochondria can be lethal since they nitrate tyrosine residues of cytochrome c, which leads to disruption of cytochrome c's function as an electron carrier in the electron transport chain.