We have developed procedures for sensitive measurement of specific radioactivities of pyrimidine nucleosides excreted from cells in culture. The changes in the observed values reflect dilution of the added isotope through de novo biosynthesis of nonradioactive pyrimidine nucleosides or by shifting and equilibration of other nucleotide pools into the free uridine pool. It is thus possible to monitor uridine biosynthesis occurring in intact cells without destroying or disrupting the cell population. On comparing a series of normal and transformed lines, we have observed several growth-dependent patterns of tivity and levels of uridine excretion and the temporal appearance of these changes.
Hamster embyro fibroblasts slows pyrimidine biosynthesis at mid-growth while the hamster cell line V79 continues t o dilute the pyrimidine pool at about 7% of the rate observed during exponential growth at confluence. Both cells exhibit Urd excretion beginning at one-half maximal growth. midine biosynthesis with a prior increase in uridine excretion. Two chemically transformed lines IARC-28 and IARC-19 derived from IARC-20 show different patterns. IARC-I 9 begins uridine excretion in early log growth and the specific activity continues t o decrease at about 2% of the rate observed during exponential growth at confluence. The IARC-28 cells also begin excretion in early log growth but pyrimidine biosynthesis stops at about midlog. This method may prove t o be an additional aid in recognizing and differentiating transformed cells in culture that d o not exhibit the transformed phenotype.
Passageable normal rat liver cells (IARC-20) also show a cessation of pyri-Key words: pyrimidine biosynthesis, V79, IARC19, IARCZO, IARC28, biomarker, pleiotropy, confluence, rat liver epithelial cells
We have recently discovered uridine and cytidine t o be normal excretion products of cells in culture and the excretion process is regulated so that the maximal rate of excretion occurs as the cells enter G, /Go [ 11 . The excretion of uracil and deoxynucleosides is also known t o occur in fibroblast cultures, especially with mutants blocked in a utilization step . It would appear that the processes leading to the normal excretion of these compounds is related t o the balance of salvage and de novo synthetic pathways.