Cytochrome P₄₅₀ is the collective term for a family of haemoproteins found predominantly in the hepatic endoplasmic reticulum and which possess mixed function oxidase activity. Cytochrome P₄₅₀– dependent mono-oxygenases are a super gene family of enzymes which catalyse the oxidation of lipophilic chemicals through the insertion of one atom of molecular oxygen into the substrate. The enzyme systems include epoxide hydrolase, UDP-glucuronyltransferases, sulphotransferases and glutathione-S-transferases. They are largely responsible for the metabolism of a wide range of endogenous and exogenous compounds before excretion from the body in urine or bile or further metabolism.

1. Cytochrome P₄₅₀ is the collective term for a family of haemoproteins found predominantly in the hepatic endoplasmic reticulum and which possess mixed function oxidase activity. This super gene family of enzymes catalyses the oxidation of lipophilic chemicals through the insertion of one atom of molecular oxygen into the substrate. The enzyme systems include epoxide hydrolase, UDP-glucuronyltransferases, sulphotransferases, and glutathione-S-transferases.

2. P₄₅₀ oxidation may be sufficient for the elimination of a chemical, but sometimes the generation of a more hydrophilic metabolite, e.g. glucuronide or sulphate is required.

3. In the genes controlling the synthesis of the P₄₅₀ enzyme system, there is significant genetic polymorphism. These P₄₅₀ enzyme systems may be induced and inhibited in many ways.

4. The cytochrome P₄₅₀ 1A family are important in the biotransformation of pro- carcinogens and carcinogens.