APA Dictionary of Psychology

a group of proteins located in the smooth endoplasmic reticulum of liver and other cells that, in combination with other oxidative enzymes, is responsible for the metabolism of various chemicals, including many psychotropic drugs. Approximately 50 cytochrome P450 enzymes (so named because their reduced forms show a spectroscopic absorption peak at 450 nm) are currently identified as being active in humans, of which cytochromes belonging to the CYP2D6 subclass, CYP2C variants, and CYP3A4/5 subclass predominate. Cytochromes are mainly active in Phase I drug metabolism; by donating an atom of oxygen, they tend to make parent drugs more water soluble and therefore more easily excreted. Because numerous drugs are metabolized via the same cytochrome, these enzymes are important in drug interactions. Numerous functional genetic polymorphisms of cytochrome P450 can now be genotyped clinically.