Summary
The pathway of the utilization of orotic acid and cytidine for DNA synthesis in vivo was examined in regenerating liver. Availability of radio-active precursors for the synthesis of DNA was demonstrated many hours after the injection of orotic acid-6-C14 into normal and partially hepatectomized rats.
The labeled pools present in the acid-soluble fraction were examined during the period 10–28 hours postoperatively. Orotic acid-6-C14 was rapidly metabolized, and the uracil and cytosine ribonucleotides and -sides accounted for almost all of the radioactivity. Labeled pyrimidine deoxyribonucleotides were not detected. Deoxycytidine was isolated, and it could be shown that this fraction became labeled at a time coincident with the incorporation of C14 into the DNA. The specific activity of deoxycytidine was, however, insufficiently high to implicate deoxycytidine as a direct intermediate in the synthesis of pyrimidine deoxyribonucleotides.
The extent of labeling of the acid-soluble uracil and cytosine ribonucleotides at short intervals following the administration of orotic acid-6-C14 or cytidine-C14 was compared with the relative labeling of the corresponding DNA and RNA pyrimidine nucleotides. The data are consistent with the hypothesis of a direct conversion of cytidine (phosphate) to deoxycytidine (phosphate), and of uridine (phosphate) to deoxyuridine (phosphate) and thymidine (phosphate). The degree of interconversion of deoxycytidine (phosphate) to thymidine (phosphate) could not be evaluated.
This work was supported by a grant (No. C-646) from the National Cancer Institute, National Institutes of Health, United States Public Health Service. A preliminary report of this work was given previously (4).