Cytochrome P450 monooxygenases catalyze the oxidation and metabolism of a large number of xenobiotics and endogenous compounds. CYP450 enzymes evolved as the primary defense against xenobiotics and in this process are also responsible for the bioactivation of drugs and toxicants to more reactive intermediates. Cytochrome P450 enzymes are primarily found in liver cells but are also located in cells throughout the body.