Dependence of the metabolic fate of IMP on the rate of total IMP synthesis Studies in cultured fibroblasts from normal subjects and from purine-overproducing mutant patients

In normal cultured human skin fibroblasts, increasing P_i concentration in the incubation medium resulted in increased IMP synthesis by both de novo and salvage pathways, indicating that cellular content of substrate phosphoribosylpyrophosphate is limiting for IMP synthesis by these pathways. The metabolic fate of IMP labeled from precursor [¹⁴C]formate was studied. The relative utilization of IMP for synthesis of adenine and guanine nucleotides and the excretion into the incubation medium of hypoxanthine, the degradation product of IMP, exhibited dependence on the rate of total (i.e., both de novo and salvage) IMP synthesis. Increasing the rate of total IMP synthesis, by increasing P_i concentration in the incubation medium, resulted in an increase in the proportional labeling of cellular adenine nucleotides and of the excreted hypoxanthine but in a decrease in the proportional labeling of cellular guanine nucleotides. The metabolic fate of IMP was studied in cultured fibroblasts obtained from two purine-overproducing mutant patients, one with physiologically superactive phosphoribosylpyrophosphate synthetase, the other with virtually complete deficiency of hypoxanthine-guanine phosphoribosyltransferase (the transferase) activity. In the former cells, with excessive de novo and salvage IMP synthesis, the metabolic fate of the newly formed IMP, at physiological 1.4 mM P_i concentration, was indeed compatible with excessive total IMP synthesis in that the proportional labeling of the various purine fractions resembled that in normal cells incubated at high P_i concentration. In the transferase-deficient cells, without detectable salvage IMP synthesis from hypoxanthine but with a many-fold accelerated de novo IMP synthesis, the fate of IMP at physiological 1.4 mM P_i was similar to that in normal cells at the same P_i concentration, indicating normal total IMP production.