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Pyrimidine Metabolism in Human Leukocytes

III. The Utilization of Thymine for DNA-Thymine Synthesis by Leukemic Leukocytes

Laboratory of Physiology and Medicine Branch, National Cancer Institute, NIH, Bethesda, Maryland

There was only slight incorporation of thymine-³H into DNA of intact human leukemic leukocytes in vitro in the absence of an exogenous source of deoxyribose. Thymine-³H incorporation was promoted by both purine and pyrimidine deoxynucleosides. The net conversion of thymine-³H to thymidine (TdR)-³H was greatest in the presence of TdR or deoxyuridine. Addition of deoxycytidine resulted in less net conversion, and the purine deoxynucleosides produced little net TdR-³H. Addition of pyrimidine deoxynucleosides but not purine deoxynucleosides resulted in decreased thymine-³H catabolism.

The results indicate that human leukocytes have the enzymatic capability to convert thymine to TdR. This enzymatic capability is not expressed in the absence of added deoxynucleosides because of rapid catabolism of TdR and thymine and a limited supply of an available deoxyribose source.

¹ To whom requests for reprints should be addressed at the Laboratory of Physiology, National Cancer Institute, NIH, Bethesda, Md.

² Present address: II and IV (Harvard) Medical Service, Boston City Hospital, Boston, Mass.

Received 1/28/66. Accepted 5/24/66.