Abstract
Prior incubation of mouse Sarcoma 180 cells in vitro for 24 hr with 2-40 µM homofolate in either the presence or absence of 50 µM hypoxanthine inhibited the subsequent incorporation of [2-14C]glycine into 5'-phosphoribosyl-N-formylglycinamide by 20-90%. Since hypoxanthine protects these cells against the growth-inhibitory effects of homofolate, this inhibition of 5'-phosphoribosyl-N-formylglycinamide synthesis was clearly a specific effect and not a consequence of growth arrest. Folinic acid (0.1-10 µM), when added after 24 hr of incubation with 10 µM homofolate, reversed only slightly the inhibition of 5'-phosphoribosyl-N-formylglycinamide synthesis. When present simultaneously with homofolate, folinic acid provided a competitive type of protection with respect to growth and 5'-phosphoribosyl-N-formylglycinamide synthesis. In view of the correlation between inhibition of growth and of 5'-phosphoribosyl-N-formylglycinamide synthesis, it appears that the latter is the site of action of homofolate in Sarcoma 180 cells. In the parent Sarcoma 180 cells and in a subline, which contains 300 times more dihydrofolate reductase than the parent cells, a 30-min prior incubation with homofolate was insufficient to inhibit 5'-phosphoribosyl-N-formylglycinamide synthesis significantly. The necessity for the prolonged initial incubations suggests that homofolate must undergo a time-dependent cellular conversion in order to inhibit 5'-phosphoribosyl-N-formylglycinamide synthesis.
- Copyright ©, 1975, by Academic Press, Inc.
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