The Biogenesis of Pentosenucleic Acid Pyrimidines in the Ehrlich Ascites Tumor Free

The Ehrlich ascites tumor can synthesize PNA pyrimidines from the same precursors as have been earlier found in rat liver to be pyrimidine precursors. The specific incorporation of N¹⁵-labeled ammonia, l-aspartate, l-ureidosuccinic acid, and orotic acid was demonstrated.

It was also found that uracil-N¹⁵ was incorporated into pyrimidines at about the same rate as orotic acid-N¹⁵. This applied also generally for mouse tissues in vivo. N¹⁵-labeled β-alanine, β-ureidopropionic acid, and dihydrouracil were not specifically incorporated into the pyrimidines of the ascites tumor.

Evidence was obtained from dilution experiments that orotic acid and dihydroorotic acid were intermediates in pyrimidine synthesis from smaller molecules. Corresponding evidence could not be obtained for l-ureidosuccinic acid, though permeability effects made a definite interpretation of the results impossible. Dilution experiments with labeled ammonia and nonlabeled uracil indicated that uracil was not situated on the main synthetic pathway of PNA pyrimidine synthesis.