Member of a superfamily of heme-containing mono-oxygenases involved in xenobiotic metabolism, cholesterol biosynthesis, and steroidogenesis, in eukaryotic organisms found mainly in the endoplasmic reticulum and inner mitochondrial membrane of cells. ‘P-450’ refers to the observation that a solution of this enzyme exposed to carbon monoxide strongly absorbs light at a wavelength of 450nm compared with the unexposed solution, (a difference spectrum caused by a thiolate in the axial position of the heme opposite to the carbon monoxide ligand).