Complete inhibition of dihydro-orotate oxidation and superoxide production by 1,1,1-trifluoro-3-thenoylacetone in rat liver mitochondria.

1,1,1-Trifluoro-3-thenoylacetone was shown to cause complete inhibition of dihydroorotate oxidation in rat liver mitochondria as measured by orotate formation and the rate of dihydro-orotate-dependent reduction of 2,6-dichlorophenol-indophenol or cytochrome c. The inhibition by trifluorothenoylacetone was dose-dependent, and a concentration of 1 mM completely inhibited dihydro-orotate dehydrogenase activity. 1,10-Phenanthroline, another iron-chelating agent, also caused total inhibition of the liver enzyme. Whereas the iron chelators inhibited 100% of dihydro-orotate dehydrogenase activity in liver mitochondria, they inhibited only a maximum of 72% in the case of the brain enzyme. The inhibition by trifluorothenoylacetone was not prevented by addition of phenazine methosulphate or ubiquinone. Dihydro-orotate dehydrogenase-mediated generation of superoxide was abolished when the enzyme was fully inhibited by trifluorothenoylacetone or when the electron-transport system was blocked by antimycin A. These results suggest that the iron component(s) of dihydro-orotate dehydrogenase is of strategic importance for catalytic activity and transfer of reducing equivalents from the primary enzyme to the electron-transport chain. Furthermore, the study indicates that production of superoxide radicals during dihydro-orotate dehydrogenase-catalysed oxidation of dihydro-orotate may be at the cytochrome b-c1 segment of the electron-transport chain (as a consequence of autooxidation of ubisemiquinone) rather than at a site on the primary enzyme.