Ribonucleotide reductase and DNA synthesis in Ehrlich ascites tumor cells.

The levels of ribonucleotide reductase, DNA polymerase, thymidine kinase, and labeled thymidine incorporation into DNA were measured in Ehrlich ascites tumor cells as a function of time after inoculation of the tumor in mice. The levels of ribonucleotide reductase and thymidine kinase activities decreased with tumor growth, while the level of DNA polymerase activity did not decrease. Thymidine incorporation into DNA in vivo decreased with the decrease in ribonucleotide reductase activity. The level of ribonucleotide reductase was 70-fold less than thymidine kinase activity. The addition of other exogenous deoxyribonucleosides to the tumor cells either in vitro or in vivo stimulated thymidine incorporation into DNA.