Variability in human drug metabolizing cytochrome P450 CYP2C9, CYP2C19 and CYP3A5 activities caused by genetic variations in cytochrome P450 oxidoreductase.

A broad spectrum of human diseases are caused by mutations in the NADPH cytochrome P450 oxidoreductase (POR). Cytochrome P450 proteins perform several reactions, including the metabolism of steroids, drugs, and other xenobiotics. In 2004 the first human patients with defects in POR were reported, and over 250 variations in POR are known. Information about the effects of POR variants on drug metabolizing enzymes is limited and has not received much attention. By analyzing the POR sequences from genomics databases, we identified potentially disease-causing variations and characterized these by in vitro functional studies using recombinant proteins. Proteins were expressed in bacteria and purified for activity assays. Activities of cytochrome P450 enzymes were tested in vitro using liposomes prepared with lipids into which P450 and P450 reductase proteins were embedded. Here we are reporting the effect of POR variants on drug metabolizing enzymes CYP2C9, CYP2C19, and CYP3A5 which are responsible for the metabolism of many drugs. POR Variants A115V, T142A, A281T, P284L, A287P, and Y607C inhibited activities of all P450 proteins tested. Interestingly, the POR variant Q153R showed a reduction of 20-50% activities with CYP2C9 and CYP2C19 but had a 400% increased activity with CYP3A5. The A287P is most common POR mutation found in patients of European origin, and significantly inhibited drug metabolism activities which has important consequences for monitoring and treatment of patients. In vitro, functional assays using recombinant proteins provide a useful model for establishing the metabolic effect of genetic mutations. Our results indicate that detailed knowledge about POR variants is necessary for correct diagnosis and treatment options for persons with POR deficiency and the role of changes in drug metabolism and toxicology due to variations in POR needs to be addressed.