Effects of 5-fluorouracil on various melanoma cell metabolisms. Its use for assessing the cold thymidine pool

5-Fluorouracil (5FU) increases the rate of incorporation of tritiated thymidine (3HTdR) and iododeoxyuridine (125 IUdR) into the cell and its DNA. 5FU has differential effect on metabolism of DNA through its specific action on thymidylate synthetase. Effects of 5FU on the incorporation of other metabolic precursors were studied, as well as the sensitivity of different cells to 5FU. This multiparametric approach enables us to predict the effect of 5FU on a cell type depending on its cold thymidine (TdR) pool size and the permeation of the cell to the drug. Furthermore, 5FU was used as a tool to recognize the difference between the effects of TdR on the 3HTdR and 125IUdR incorporation. Three parameters are superimposed: isotope dilution, inhibition of metabolic activity of the pyrimidine synthesis and metabolic competition between TdR and UdR. The measurement of the TdR pool size of the cell can be obtained using a specific equation, which is a function of the slope of the inhibition curve of the labelled precursors (3HTdR and 125IUdR) incorporation.