NADPH is a cofactor required to support reactions by drug metabolizing enzymes such as cytochrome P450 (CYP, including EROD activity), and flavin-containing monooxygenases (FMO). NADPH is an obligatory cofactor, and initiates the monooxygenase reaction as a source of electrons necessary to reduce CYP-substrate complex in its oxidation-reduction cycle. NADPH is used in Microsomes, S9 and Recombinant CYPs (Bactosomes) to support the long-term metabolism. Also, NADPH- regenerating systems are commonly used.