Dihydroorotate-Ubiquinone Reductase Complex of Escherichia coli B

A dihydroorotate-oxidizing complex of high molecular weight was purified from respiratory particles of Escherichia coli, strain B. This material was found to be an active dihydroorotate-ubiquinone-30 reductase. The complex lacked cytochrome reductase and oxidase activities but catalyzed the reduction of various artificial electron acceptors, including 2,6-dichloroindophenol, simple 1,4-quinones, and ferricyanide. Molecular oxygen was also an oxidant, although a less efficient one than the artificial electron acceptors. Detergent-dispersed ubiquinone-30 was the most effective oxidizing agent that was tested. This compound was also rapidly reoxidized in a coupled reaction with 2,6-dichloroindophenol in the presence of the complex. The complex contained, in addition to the dihydroorotate-oxidizing enzymes, an active peroxidase, catalase, succinate dehydrogenase, and DPNH oxidase. In crude soluble fractions and in particulate systems of aerobic microorganisms, the oxidation of dihydroorotate is cytochrome-linked. This linkage may occur via non-heme iron proteins and ubiquinone. The oxygen-mediated oxidation of dihydroorotate leads to the quantitative conversion to orotate. Linkage of the reaction with the respiratory chain may represent an important site of respiratory control over pyrimidine biosynthesis.