Any of a large group of enzymes that perform oxidation-reduction reactions in which one atom of the oxygen molecule is incorporated into the chemical donor substrate and the other oxygen atom is combined with hydrogen ions to form water. The different classes of monooxygenases are defined according to the type of hydrogen-providing cosubstrate (or coenzyme, e.g. FAD, NADPH) required in the mixed-function oxidation. Such enzymes are commonly involved in the detoxification of harmful substances by tissues, being located in the vertebrate liver and in the hepatopancreas of invertebrates. For example, herbivorous insects employ monooxygenases to neutralize the natural plant toxins (e.g. phenolics, terpenoids, and quinones) they ingest. Such enzymes are vital components of numerous metabolic pathways; cytochrome oxidase is an example.