Ref: Goodman and Gilmans "Pharmacological basis of therapeutics" 12th edExplanation:The cytochrome P450 superfamily (CYP) is a large and diverse group of enzymes that catalyze the oxidation of organic substances.The cytochrome P450 (CYP) mixed function monooxygenases are located on the smooth endoplasmic reticulum of cells throughout the body, but the highest concentrations are found in the liver (hepatocytes) and small intestine.These enzymes are responsible for the oxidative (Phase I) metabolism of a wide number of compounds, including many medications.They biotransform lipophilic drugs to more polar compounds that can be excreted by the kidneys.The metabolites are usually less active than the parent compound, although some drugs undergo biotransformation to pharmacologic active agents.The substrates of CYP enzymes include metabolic intermediates such as lipids and steroidal hormones, as well as xenobiotic substances such as drugs and other toxic chemicals.CYPs are the major enzymes involved in drug metabolism and bioactivation, accounting for about 75% of the total number of different metabolic reactions.Based on the nature of the electron transfer proteinsCYPs can be classified into several groups:Microsomal P450 systems in which electrons are transferred from NADPH via cytochrome P450 reductase (variously CPR, POR. or CYPOR). Cytochrome b5 (cyb5) can also contribute reducing power to this system after being reduced by cytochrome b5 reductase (CYB5R).Mitochondrial P450 systems, that employ adrenodoxin reductase and adrenodoxin to transfer electrons from NADPH to P450.Bacterial P450 systems, that employ a ferredoxin reductase and a ferredoxin to transfer electrons to P450.CYB5R/cyb5/P450 systems in which both electrons required by the CYP come from cytochrome b5.FMN/Fd/P450 systems originally found in Rhodococcus sp. in which a FMN-domain- containing reductase is fused to the CYP.P450 only systems, which do not require external reducing power. Notable ones include CYP5 (thromboxane synthase), CYP8 (prostacyclin synthase), and CYP74A (allene oxide synthase).