NADPH-cytochrome P450 reductase (POR) (Shephard et al. 1992) and cytochrome-b5 and NADH-b5 reductase play important roles in cytochrome P450-mediated drug metabolism (Gan et al. 2009). POR can transfer electrons to many naturally occurring electron acceptors, including cytochrome P450 enzymes where it transfers the electrons from NADPH into the P450 catalytic cycle (Elmore & Porter 2002). The P450 catalytic cycle describes the transitional mechanistic steps by which P450s bind a substrate, act upon it and finally release the product, returning P450s to their initial state (see mini-review Guengerich 2007).