Thymidylate synthetase of in vitro chemically and virally transformed rat cells.

Thymidylate synthetase (EC 2.1.1) from in vitro 3-methylcholanthreneor Rauscher leukemia virus-transformed rat embryo cells was studied. The enzyme from control or transformed rat cells exhibited a Km for 2'-deoxyuridylic acid of 4.5 +/- 0.2 muM, but the transformants had a higher level of enzyme activity than did control cells. Titration of the enzyme with tritiated 5-fluoro2'-deoxyuridylic acid indicated that the increased enzyme activity in the transformants was due to a greater level of cellular enzyme. While the level of enzyme activity in control cells was dependent on both the availability of nutrients in the medium and cell density, the level of enzyme activity in the transformants appeared to depend only on the availability of nutrients.