Human novel reductase 1 (NR1) is an NADPH dependent diflavin oxidoreductase related to cytochrome P450reduc-tase (CPR). The FAD/NADPH- and FMN-binding domains of NR1 have been expressed and purified and their redox properties studied by stopped-flow and steady-state kinetic methods, and by potentiometry. The midpoint reduction potentials of the oxidized/semiquinone ()315 ± 5 mV) and semiquinone/dihydroquinone ()365 ± 15 mV) couples of the FAD/NADPH domain are similar to those for the FAD/NADPHdomain of humanCPR, but the rate of hydride transfer from NADPH to the FAD/NADPH domain of NR1 is 200-fold slower
