Primary cholesterol oxygenation reactions mediated by different cytochrome P-450 species or occurring nonenzymatically in the presence of reactive oxygen species (ROS) (43). The oxygenation of cholesterol into 25-hydroxycholesterol is catalyzed by cholesterol 25-hydroxylase, a non-heme iron protein (7). The quantitatively most important oxysterols present in human circulation are underlined.