Quantitation and characterization of cytochrome c oxidase in complex systems

Quantitation of cytochrome c oxidase in complex systems such as tissue homogenates is often hampered by the presence of other hemoproteins. Cyanide can bind to reduced cytochrome c oxidase from diverse sources with a dissociation constant in the range of 0.1-0.5 mM and induces a characteristic optical change. This contrasts with the very weak binding of cyanide to reduced forms of many other hemoproteins, including hemoglobin and myoglobin. Hence, difference spectra of cyanide binding to reduced samples can provide an improved method to resolve and quantitate cytochrome c oxidase. In addition, the cyanide compound of cytochrome c oxidase is photolabile. This property can be exploited to further enhance the sensitivity of detection and analysis of cytochrome c oxidase.