1. 1. 5-Phosphoribosyl 1-pyrophosphate (PRPP) was determined in several murine and human cancer cell lines grown in monolayer, and harvested by trypsinization. 2. 2. For all cell lines a large variation in the PRPP concentration (5-1300 pmol/106 cells) was found. 3. 3. A 1-hr incubation in Dulbecco's medium reduced the variation in PRPP concentration. After this incubation the highest concentration was found in the murine B16 melanoma cell line (about 200 pmol/106 cells). 4. 4. The human melanoma cell lines IGR3 and M5 and the human colon carcinoma cell line WiDr contained about 100 pmol/106 cells. 5. 5. After this preincubation of 1-hr these cell suspensions were used to study the effect of several antimetabolites on PRPP concentration. A 2-hr incubation with 1mM N-(phosphonacetyl)-l-aspartate (PALA) increased the PRPP concentration only in M5 cells, whereas methotrexate caused an increase in all cell lines. 6. 6. When 5-fluorouracil (5FU) was added, no significant decrease was found in any cell line. Addition of 5FU after a 2-hr preincubation with PALA resulted in a lower concentration in B16, M5 and WiDr cells. 7. 7. The prodrug, 5-fluoro-5′ deoxyuridine altered the PRPP concentration only WiDr cells when it was added after PALA. 8. 8. The activity of the 5FU metabolizing enzyme orotate phosphoribosyl transferase was comparable in B16, M5 and WiDr cells, but much lower in IGR3 cells.
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