Molecular Pharmacology, Vol 11, 319-325, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

Inhibition of the Biosynthesis of 5'-Phosphoribosyl-N-formylglycinamide in Sarcoma 180 Cells by Homofolate

¹ Department of Experimental Therapeutics, Grace Cancer Drug Center, Roswell Park Memorial Institute, New York State Department of Health, Buffalo, New York 14203

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.