Some Aspects of the Role of Folic Acid in Nucleic Acid Synthesis in a Neoplastic C3H Mouse Cell Strain

Summary

A neoplastic murine cell strain, NCTC 3681, in culture in chemically defined medium for several years, requires thymidine for continued growth. The thymidine requirement could be replaced by deoxycytidine or 5-methyldeoxycytidine or by increasing the folate content of the medium. Labeled thymidine or deoxycytidine was used for DNA-thymidine synthesis to a greater extent than cytidine, uridine, deoxy-uridine, or orotic acid. The relative extent of utilization of these precursors for DNA-deoxycytidine and DNA-thymidine synthesis is indicated by the specific activity ratios of DNA-deoxycytidine to DNA-thymidine for each precursor used: deoxycytidine, 1; cytidine, 5; uridine, 10. When the folate content of the medium was increased tenfold, there was a greater utilization of cytidine for both DNA pyrimidines and, to a lesser extent, an increase in the utilization of deoxycytidine, deoxyuridine, and uridine for DNA-thymidine. A cell strain was derived from strain NCTC 3681 by growing these cells for 13 transplant generations in medium containing the tenfold higher level of folate. There was a marked increase in the extent of utilization of cytidine, uridine, and deoxyuridine for DNA-thymidine by the derived cells and the specific activity ratios of DNA-deoxycytidine to DNA-thymidine decreased: deoxycytidine 0.3, cytidine 1, uridine 1. The utilization of orotic acid remained comparatively low.