The enzyme systems associated with the synthesis and degradation of pyrimidine nucleotides in rat liver after the repeated administration of phenobarbital were studied. The incorporation of [2-14C]orotic acid into uridine components of the free nucleotide pool remains unchanged, whereas incorporation into cytidine components is decreased. The cytidine triphosphate synthetase, UTPase, and CTPase activities of cytosol are increased. The activities of 5'-nucleotidases of the cytosol and microsomal fraction as well as UTPase and CTPase of the microsomal fraction are decreased. The activities of nucleoside, nucleoside monophosphate, and diphosphate kinases of uridine and cytidine components and the activity of cytosol uridine phosphorylase are not affected. Deamination of cytidine in the presence of the whole homogenate, the microsomal fraction, and the cytosol is not altered. Finally, the ATPase activity of the cytosol is not affected, while that of the microsomal fraction is decreased.
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