NADPH-cytochrome P-450 reductase has been purified to electrophoretic homogeneity from rabbit liver microsomes by a procedure that may be used in conjunction with the isolation of the major forms of cytochrome P-450. The purified reductase is active in a reconstituted hydroxylation system containingp-45OLM, or P-~~OLM.,. The enzyme contains one molecule each of FMN and FAD per polypeptide chain having an apparent minimal molecular weight of 74,000. Immunological techniques provided evidence for only a single form of the reductase; lower molecular weight forms occasionally seen are believed to be due to degradation by contaminating microsomal or bacterial proteases. Upon anaerobic photochemical reduction, the rabbit liver reductase undergoes spectral changes highly similar to those previously described by Vermilion and Coon for the rat liver enzyme; the fully reduced rabbit liver enzyme is converted to the three-electron-reduced form by the addition of NADP and then to the stable one-electron-reduced form by exposure to oxygen. The CD spectra of the fully oxidized enzyme, one-electron-reduced form (air-stable semiquinone), three-electron-reduced form, and fully reduced form are presented. The results obtained provide evidence that the FMN and FAD are in highly different environments in the enzyme, as also indicated by the different redox potentials and oxygen reactivities of the flavins.

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