Levels of Thymidylate Synthetase during Normal Culture Growth of L1210 Cells1

The binding of 5-fluoro-2'-deoxyuridylate generated from 5fluoro-2'-deoxyuridine in intact cells was used to measure changes in the level of thymidylate synthetase during the course of population growth of murine leukemia L1210 cells. By the use of elutriation techniques and flow cytometric analysis, the amount and activity of thymidylate synthetase associated with the var ious phases of the cell cycle were determined for the L1210 cells during unperturbed in vitro culture growth. Fluctuations of thy midylate synthetase levels were associated with the cell cycle; there was a positive correlation (P < 0.001) between the per centage of the total cell population in S phase and the concen tration of thymidylate synthetase, although there was an increase in the level of this enzyme in association with an increase in G2M cells, this did not achieve statistical significance. A negative correlation between d cells and the concentration of thymidylate synthetase was also observed. The maximum amount of thymi dylate synthetase was nearly 900 fmol/106 cells and occurred in cell populations during logarithmic growth when the percentage of the population in S phase and G2-M phase was greater than 50 and 20%, respectively. In late culture growth (plateau) when only 25% of the cell population was in S phase and nearly 75% of the population was in d phase, the level of enzyme was reduced to 200 fmol/106 cells.