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Mammalian Dihydroorotate – Ubiquinone Reductase Complex

Richard W. Miller, Carolyn T. Kerr, Jack R. Curry

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An L-dihydroorotate–ubiquinone reductase system was obtained from beef liver mitochondria in the form of a high molecular weight complex. Whole mitochondrial preparations of mouse liver and electron-transport particles of beef heart exhibited some cytochrome-linked oxidation of the pyrimidine precursor. Solubilization of the complex from beef liver by deoxycholate and subsequent purification largely removed cytochrome components and yielded a complex which was incapable of reducing oxygen. Non-heme iron, flavin, ubiquinone, and lipids were present in the complex. The exact role of each of these components in the linkage of dihydroorotate oxidation with parts of the respiratory chain is not known.A convenient and sensitive spectrophotometric assay for dihydroorotate oxidation based on the coupled reduction of 2,6-dichlorophenolindophenol by ubiquinone and possibly, to a much lesser extent, by other redox components of the complex was developed. This assay procedure was applicable to the whole particles as well as submitochondrial fractions. With the purified complex, ubiquinone-30 greatly stimulated the dye reaction. Since the dye reacts rapidly with detergent-dispersed reduced ubiquinone, it is likely that the cofactor represents an intermediate electron carrier in the reduction of the dye. Ubiquinone and other 1,4-quinones were catalytically reduced by dihydroorotate in the presence of the purified complex.