Cytochrome P-450 is a name for a family of heme proteins that perform hydroxylation reactions, as well as epoxidation, peroxygenation, desulfuration, dealkylation, deamination, and dehalogenation reactions. Most vertebrate genomes contain more than 40 different structural genes for cytochrome P-450. The proteins resemble mitochondrial cytochrome oxidase in being able to bind both O2 and carbon monoxide. Cytochrome P-450 proteins are usually found in the endoplasmic reticulum of eukaryotic cells.

Cytochrome P-450 hydroxylate many compounds. These include the hydroxylations of steroid hormone synthesis and the hydroxylation of thousands of xenobiotics (foreign compounds), including drugs such as phenobarbital and environmental carcinogens such as benzpyrene, a constituent of the smoke from tobacco and backyard grills. Hydroxylation of foreign substances usually increases their solubility and is a step in their detoxification, or metabolism and excretion. In some cases, however, some of these reactions activate potentially carcinogenic substances to more reactive species. Aflatoxin B, for example, is converted to a more reactive species either by hydroxylation or epoxidation.

A key to the reactivity of cytochrome P-450 is its ability to split O2, with one oxygen atom binding to the cytochrome's heme iron. This bond forms a perferryl ion, which can be represented as FeO³⁺. This highly reactive group can abstract a hydrogen atom, even from an unreactive substrate such as a hydrocarbon. In such a hydroxylation, reducing equivalents are typically transferred to the cytochrome from NADPH. A general mechanism is shown in Figure 15.25. In this mechanism, substrate binding is followed by O2 binding. Transfer of two electrons reduces one oxygen atom, such that splitting of the oxygen molecule generates water plus the perferryl ion, which then hydroxylates the substrate.