To clarify the contributions of amidophosphoribosyltransferase (ATase) and its feedback regulation to the rates of purine de novo synthesis, DNA synthesis, protein synthesis, and cell growth, mutated human ATase (mhATase) resistant to feedback inhibition by purine ribonucleotides was engineered by site-directed mutagenesis and expressed in CHO ade (-)A cells (an ATase-deficient cell line of Chinese hamster ovary fibroblasts) and in transgenic mice (mhATase-Tg mice). In Chinese hamster ovary transfectants with mhATase, the following parameters were examined: ATase activity and its subunit structure, the metabolic rates of de novo and salvage pathways, DNA and protein synthesis rates, and the rate of cell growth. In mhATase-Tg mice, ATase activity in the liver and spleen, the metabolic rate of the de novo pathway in the liver, serum uric acid concentration, urinary excretion of purine derivatives, and T lymphocyte proliferation by phytohemagglutinin were examined. We concluded the following. 1) ATase and its feedback inhibition regulate not only the rate of purine de novo synthesis but also DNA and protein synthesis rates and the rate of cell growth in cultured fibroblasts. 2) Suppression of the de novo pathway by the salvage pathway is mainly due to the feedback inhibition of ATase by purine ribonucleotides produced via the salvage pathway, whereas the suppression of the salvage pathway by the de novo pathway is due to consumption of 5-phosphoribosyl 1-pyrophosphate by the de novo pathway. 3) The feedback inhibition of ATase is more important for the regulation of the de novo pathway than that of 5-phosphoribosyl 1-pyrophosphate synthetase. 4) ATase superactivity leads to hyperuricemia and an increased bromodeoxyuridine incorporation in T lymphocytes stimulated by phytohemagglutinin.