Pyrimidines and purine (deoxy)nucleotides are the building blocks of DNA and RNA. Nucleoside diphosphate sugars, e.g. UDP-glucose, are the reactive intermediates in the synthesis of nearly all glycosidic bonds between sugars. In mammals the requirement for pyrimidines is met by UMP de novo synthesis and, to a greater or lesser extent, by salvage of free nucleosides. The exceptional compartmentation of the de novo synthesis with respect to mitochondrially-bound dihydroorotate dehydrogenase ('DHOdehase' or 'DHODH', EC 126.96.36.199) is one focus of the present work. DHODH activity was determined by the dihydroorotate-dependent oxygen consumption or by the UV absorption of the product orotate with mitochondria isolated from rodent and porcine tissues. For comparison, the cytochrome c and choline-dependent oxygen consumption of mitochondria from different tissues was measured. The highest specific activity of the rat DHODH was found in liver (2.3 x 10(-3) mumol/min x mg protein) > kidney > heart. The application of known enzyme inhibitors Brequinar Sodium and Leflunomide for DHODH and sodium cyanide for cytochrome c oxidase verified the specificity of the activity tests used. The relation of DHODH activity versus that of cytochrome c oxidase revealed the lowest ratios in heart mitochondria and the highest in liver mitochondria. Since disorders in the mitochondrial energy metabolism could entail severe impairment of pyrimidine biosynthesis via respiratory-chain coupled DHODH, it is suggested to include improvement of pyrimidine nucleotide status in therapy protocols.