Human heme oxygenase-1 (HO-1) carries out heme catabolism supported by electrons supplied from the NADPH through NADPH P450 reductase (POR, CPR) Previously we have shown that mutations in human POR cause a rare form of congenital adrenal hyperplasia In this study, we have evaluated the effects of mutations in POR on HO-1 activity We used purified preparations of wild type and mutant human POR and in vitro reconstitution with purified HO-1 to measure heme degradation in a coupled assay using biliverdin reductase. Here we show that mutations in POR found in patients may reduce HO-1 activity, potentially influencing heme catabolism in individuals carrying mutant POR alleles POR mutants Y181D, A457H, Y459H, V492E and R616X had total loss of HO-1 activity, while POR mutations A287P. C569Y and V608F lost 50-70% activity The POR variants P228L, R316W and G413S, A503V and G504R identified as polymorphs had close to WT activity Loss of HO-1 activity may result in increased oxidative neurotoxicity, anemia, growth retardation and iron deposition Further examination of patients affected with POR deficiency will be required to assess the metabolic effects of reduced HO-1 activity in affected individuals (C) 2010 Elsevier Inc All rights reserved