Significance of Furine Salvage in Circumventing the Action of Antimetabolites in Rat Hepatoma Cells1

The flux activities of de novo and salvage purine synthesis were compared in rat hepatoma 3924A cells in various growth phases. The initial rate assays of [I4C]adenine, ("C]hypoxanthine, and |"C]guanine incorporation yielded Michaelis-Menten kinetics with A'msof 5, 7, and 7 #tM,respectively. After replating plateau phase cells in lag and log phases the activity of purine de novo pathway increased 4.5to 8-fold with a preferential rise in guanylate synthesis, whereas purine salvage activities increased only 1.6to 2.1-fold. However, for the syntheses of IMP, AMP, and GMP, the activities of purine salvage pathways were 2to 7-fold, 5to 28-fold, and 2to 32-fold higher than those of the de novo purine pathway. Treatment of cells with acivicin, an inhibitor of the activity of amidophosphoribosyltransferase, phosphoribosylformylglycinamidine synthase, and GMP synthase, inhibited the flux activities of de novo purine, adenylate, and guanylate syntheses to 37, 73, and 3% of the controls and decreased the concentration of GTP to 42%; the concentra tion of ATP did not change and that of 5-phosphoribosyl 1-pyrophosphate increased 3.1-fold. Under these conditions the activities of salvage syn thesis from hypoxanthine and guanine were enhanced 2.5-fold. Treatment of hepatoma cells with IMP dehydrogenase inhibitors, tiazofurin, ribavirin, and 4-carbamoylimidazolium 5-olate, to block de novo guanylate synthesis accelerated the flux activity of guanine salvage pathway. The higher capacity of purine salvage pathway than that of the de novo one and the further rise of the activity in response to the drugs targeted against the de novo pathway highlight the important role salvage synthe sis might play in circumventing the impact of antimetabolites of de novo purine synthesis in cancer chemotherapy.