PURINE SYNTHETIC CAPACITIES OF DE NOVO AND SALVAGE PATHWAYS IN RAT HEPATOMA 3924A CELLS: 139

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The purpose was to elucidate whether the failure of anti-tumor action of purine de novo synthetic inhibitors might be due to the high capacity of salvage synthesis in neoplastic cells. Hepatoma 3924A cells in monolayer culture were used to compare enzymic activities and metabolic fluxes of de novo and salvage pathways. The specific activity of the rate-limiting enzyme for IMP de novo synthesis, amidophosphoribosyltransferase, was 21, whereas the activities of the salvage enzymes, adenine, hypoxanthine and guanine phosphoribosyltransferase, were 38, 34 and 99 μmol/hr/g cells, respectively. Moreover, the Km value for the shared substrate, PRPP, of the salvage enzymes was orders of magnitude lower than that of amidophosphoribosyltransferase. In the hepatoma cells the initial rate kinetics of [14C] formate, [14C] adenine, [14C] hypoxanthine and [14C] guanine incorporation into acid-soluble nucleotides and acid-insoluble nucleic acids followed Michaelis-Menten kinetics. The apparent Vmax of de novo synthesis was 213, whereas those of salvage pathways from adenine, hypoxanthine and guanine were 2,200, 530 and 590 nmol/hr/g cells, respectively. The higher capacity of purine salvage than that of the de novo pathway indicates the important role purine salvage synthesis might play in circumventing the action of inhibitors of purine de novo synthesis in cancer chemotherapy. (Supported by NCI grants CA-13526 and 05034).