Alternative Pathways of Thymine and Uracil Metabolism in the Liver and Hepatoma23

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Abstract
The catalyzing enzymes are found in the supernatant fraction after high-speed centrifugation of the rat liver and Dunning LC 18 hepatoma homogenates. In the normal liver the ratio between the activities for the deoxyriboside-transferring enzyme and the riboside-transferring enzyme is about 3.5, and in the Dunning hepatoma about 0.06. Cytosine riboside and deoxyriboside do not participate in the exchange reactions, while thymine and uracil participate equally well. Exchange reaction (1) is sensitive to arsenate ions, in contrast to reaction (2). The comparative inhibitory effect on the exchange reactions exerted by the three naturally occurring pyrimidines and by 5-fluorouracil has been studied. Rat kidney, brain, and heart do not show any measurable amounts of the deoxyribose-transferring enzyme, nor does tissue from the Novikoff rat hepatoma.