NADPH-cytochrome P-450 reductase. Physical properties and redox behavior in the absence of the FAD moiety.

NADPH-cytochrome P-450 reductase contains one molecule each of FMN and FAD. The FAD moiety has been selectively removed, producing the FMN reductase. The FMN reductase is stable and enzymatic activity is reconstituted with either FAD or FMN. FMN remains tightly bound, but can both dissociate from the FMN site and bind to the vacant FAD site. The amount of FMN bound in the FAD site is minimal under specific experimental conditions. There are at least two conformational subpopulations of the FMN reductase; NADP dissociates readily from one but extremely slowly from the other. Rapid dissociation of NADP is regained upon reconstitution with FAD. The one-electron redox state of the FMN reductase is thermodynamically stabilized, though to a lesser degree than in the holoreductase. When two-electron reduced FMN reductase is exposed to oxygen, a stable species with an absorbance peak at 580 nm forms rapidly and quantitatively. This species has been identified by electron paramagnetic resonance spectroscopy as the neutral radical of FMN and is indistinguishable from the air-stable radical of the holoreductase. The redox behavior of the FMN reductase is in agreement with properties proposed previously for the FMN site.