title: Soluble cytochrome P-450 from housefly microsomes. Partial purification and characterization of two hemoprotein forms.
abstract: Housefly microsomes contain two spectrally different forms of cytochrome P-450 which we have termed P-450 and P-450I. Methods have been developed for the fractionation and chromatographic purification of these two hemoprotein forms. Microsomes are solubilized first with Triton X-100 in the presence of glycerol, dithiothreitol, ethylenediaminetetra-acetic acid, and phenobarbital. Cytochrome P-450 is recovered in a floating pellet after the addition of 25% ammonium sulfate followed by centrifugation, whereas cytochrome P-450I remains in the 25% ammonium sulfate supernatant fluid. Cytochrome P-450 is purified further by Sephadez G-200 and DEAE-Sephadex A-50 column chromatography, which also allows the isolation of cytochrome b5 and NADPH-dependent cytochrome P-450 reductase in good yields and with little cross-contamination. Cytochrome P-450 apparently is free of cytochromes b5 and P-420 as well as of reductase and is obtained in a final yield of approximately 16% with a 6.9-fold purification. Its maximum absorbance is at 45 mn in the CO-difference spectrum and its average extinction coefficient is 103 cm-1 nm-1. Cytochrome P-450I is purified by Sephadex G-25 column chromatography but still contains some cytochromes b5 and P-420 as well as reductase. Its maximum absorbance is at 448.5 nm in the CO-difference spectrum and its extinction coefficient is 83 to 86 cm-1 mM-1. Both cytochromes hydroxylate type I substrates such as aminopyrine. Sufficient amounts of reductase are present in the cytochrome P-450I preparation to sustain activity, but the reductase has to be added to cytochrome P-450 in a reconstituted system for activity. Cytochrome P-450 is fairly stable, whereas cytochrome P-450I can be isolated only when protected by a substrate (phenobarbital). Detergent-solubilized housefly cytochromes P-450 and P-450I seem to correspond to either aggregates or oligomeric proteins. Cytochrome P-450 appears to correspond to a tetramer, each subunit having a molecular weight of 45,000, whereas cytochrome P-450I may correspond to an aggregate of at least 10 subunits. The cytochrome P-450 aggregate is dissociated by 6 M urea, but cytochrome P-450I remains as such.
mesh_terms: D000588:Amines; D000818:Animals; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification / Q000378:metabolism; D004227:Dithionite; D006793:Houseflies / Q000201:enzymology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology*; D009570:Nitriles; D010084:Oxidation-Reduction; D010634:Phenobarbital; D011092:Polyethylene Glycols; D012685:Sepharose; D012995:Solubility; D013053:Spectrophotometry; D013056:Spectrophotometry, Ultraviolet; D013997:Time Factors
chemical_list: D000588:Amines; D009570:Nitriles; D004227:Dithionite; D011092:Polyethylene Glycols; D012685:Sepharose; D003577:Cytochrome P-450 Enzyme System; D010634:Phenobarbital