1: Histochem Cell Biol. 1995 Apr;103(4):287-92.

Localization of dihydroorotate oxidase in myocardium and kidney cortex of the
rat. An electron microscopic study using the cerium technique.

Angermuller S, Loffler M.

Department of Anatomy and Cell Biology II, University of Heidelberg, Germany.

Biochemical studies have demonstrated that dihydroorotate dehydrogenase
(DHOdehase; EC 1.3.3.1 or 1.3.99.11) is the sole enzyme of de novo pyrimidine
synthesis in mitochondria, whereas the rest of the pathway takes place in the
cytosol. The dehydrogenation of dihydroorotate to orotate is linked to the
respiratory chain via ubiquinone. In this study, we show for the first time the
ultrastructural localization of DHOdehase. Since the purified enzyme was found to
act both as dehydrogenase and as oxidase, the cerium capture technique for
detecting enzymatically generated hydrogen peroxide could be applied to pin-point
the in situ activity of DHOdehase oxidase in mitochondria of rat heart and kidney
cortex. Cerium perhydroxide as the final reaction product was detected
predominantly in the matrix with some focal condensation along the inner
membrane, but not in the intermembrane space. From this pattern of localization, 
it is concluded that the active site of the membrane-bound enzyme could face the 
mitochondrial matrix similar to succinate dehydrogenase. The reliability of the
applied method for the demonstration of DHOdehase oxidase was demonstrated by the
addition of Brequinar sodium to the incubation medium. This quinoline-carboxylic 
acid derivative is a potent inhibitor of DHOdehase and has proven
anti-proliferative activity. The present observations do not ascertain whether
the oxidase is permanently active as a constant portion of the enzyme in vivo,
similar to xanthine oxidase/dehydrogenase. However, DHOdehase should be
considered as a source of radical oxygen species under pathophysiological
conditions.

Publication Types: 
    Research Support, Non-U.S. Gov't

PMID: 7648404 [PubMed - indexed for MEDLINE]