Regulation of purine synthesis de novo in human fibroblasts by purine nucleotides and phosphoribosylpyrophosphate

Previous studies of purine nucleotide synthesis de novo have suggested that major regulation of the rate of the pathway is affected at either the phosphoribosylpyrophosphate (PP-Rib-P) synthetase reaction or the amidophosphoribosyltransferase (amido PRT) reaction, or both. We studied control of purine synthesis de novo in cultured normal, hypoxanthine-guanine phosphoribosyltransferase (HGPRT)-deficient, and PP-Rib-P synthetase-superactive human fibroblasts by measuring concentrations and rates of synthesis of PP-Rib-P and purine nucleotide end products, proposed effectors of regulation, during inhibition of the pathway. Incubation of cells for 90 min with 0.1 mM azaserine, a glutamine antagonist which specifically blocked the pathway at the level of conversion of formylglycinamide ribotide, resulted in a 5-16% decrease in purine nucleoside triphosphate concentrations but no consistent alteration in generation of PP-Rib-P. During this treatment, however, rates of the early steps of the pathway were increased slightly (9-15%) in normal and HGPRT-deficient strains, more markedly (32-60%) in cells with catalytically superactive PP-Rib-P synthetases, and not at all in fibroblasts with purine nucleotide feedback-resistant PP-Rib-P synthetases. In contrast, glutamine deprivation, which inhibited the pathway at the amido PRT reaction, resulted in time-dependent nucleoside triphosphate pool depletion (26-43% decrease at 24 h) accompanied by increased rates of PP-Rib-P generation and, upon readdition of glutamine, substantial increments in rates of purine synthesis de novo. Enhanced PP-Rib-P generation during glutamine deprivation was greatest in cells with regulatory defects in PP-Rib-P synthetase (2-fold), but purine synthesis in these cells was stimulated only 1.4-fold control rates by glutamine readdition. Stimulation of these processes in normal and HGPRT-deficient cells and in cells with PP-Rib-P synthetase catalytic defects was, respectively: 1.5 and 2.0-fold; 1.5 and 1.7-fold; and 1.6 and 4.1-fold. These studies support the following concepts. 1) Rates of purine synthesis de novo are regulated at both the PP-Rib-P synthetase and amido PRT reactions by end products, with the latter reaction more sensitive to small changes in purine nucleotide inhibitor concentrations. 2) PP-Rib-P exerts its role as a major regulator of purine synthetic rate by virtue of its interaction with nucleotide inhibitors to determine the activity of amido PRT. 3) Activation of amido PRT by PP-Rib-P is nearly maximal at base line in fibroblasts with regulatory defects in PP-Rib-P synthetase.