Abstract
The content of enzymes involved in uracil metabolism (pyrimidine nucleoside phosphorylase, pyrimidine nucleoside kinase and enzymes of uracil catabolism) in extracts from rat liver, mouse liver and the Ehrlich ascites tumour was studied. Much higher levels of enzymes involved in anabolic reactions were found in ascites tumour extracts than in liver, whereas the enzymes of uracil catabolism which were present in rat and mouse liver were absent from the tumour. Especially the relatively high concentration of pyrimidine nucleoside kinase in the tumour extracts probably explains the better utilization of free uracil for polynucleotide synthesis in this tissue as compared to rat liver. Pyrimidine nucleoside kinase needed a high concentration of uridine ( ca. 0.01 M) for optimal synthesis of nucleosides, while a considerably lower concentration of cytidine (less than 0.0055 M) was sufficient. This partly explains the much better utilization of cytidine for polynucleotide synthesis as compared to uridine.
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