AbstractWe have investigated the de novo synthesis of intermediates of purine nucleotides in 3T6 fibroblasts and determined the manner by which the activity of this pathway is increased in resting cells by the addition of fresh serum. Within 30 minutes after stimulation, 3T6 cells began to synthesize increased amounts of purines by the de novo pathway as measured by increased amounts of formylglycinamide ribonucleotide, a representative intermediate of this pathway. Within 15 minutes after serum‐stimulation 3T6 cells exhibited a substantial increase in their capacity to synthesize ribose compounds, particularly in the form of 5‐phosphoribosylpyrophosphate (PRPP). The availability of PRPP appeared to be limiting for the synthesis of purine nucleotides in resting fibroblasts, but not necessarily in serum‐stimulated cells.The amount of the enzyme PRPP synthetase as measured in vitro remained constant for at least the first four hours. Therefore, a study was made of various compounds known to activate PRPP synthetase in vitro. No evidence was found that suggested involvement of concentrations of cyclic nucleotides or phosphate. Experiments with methylene blue, an artificial electron acceptor that stimulates the production of ribose 5‐phosphate by the hexose monophosphate shunt, indicated that one of the immediate consequences of the addition of serum is increased cycling of the pyridine nucleotide coenzymes, NADP+and NADPH, and that the rapid increase in formation of ribose compounds and, consequently, purine nucleotides was caused as a result of modulation by this coenzyme. The relative ration of ATP:ADP:AMP as well as their concentrations remain constant in resting and serum‐stimulated cells under normal assay conditions. However, there was a substantial decrease in ATP concentrations with a corresponding increase in AMP concentration with methylene blue in the assay buffer. The production of AMP from ATP was 5‐fold greater in the serum‐stimulated than in the resting fibroblasts. The increased production of AMP is thus serum‐dependent and may reflect a basic enzymatic function of proliferative as compared t
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