Abstract

The role of allopurinol in the pharmacologic regulation of purine biosynthesis was studied in cell-free enzyme preparations and in intact cells in vitro. Studies were carried out in two cell lines, in Ehrlich ascites tumor cells and in human lymphoma cells (Burkitt). The effect of this compound on the rate of the early steps of purine biosynthesis as measured by the incorporation of isotopic glycine into a N-formylglycinamide ribonucleotide was compared with the effect of allopurinol ribonucleotide, oxipurinol and purine ribonucleotides on phosphoribosyl-1-amine synthesis in cell-free enzyme preparations of these two cell lines. Burkitt lymphoma cells are deficient in xanthine oxidase, while Ehrlich ascites tumor cells are not. A pharmacogenetic difference in the regulation of purine biosynthesis unrelated to xanthine oxidase activity was noted. Allopurinol inhibited de novo purine biosynthesis in intact human lymphoma cells but failed to inhibit purine biosynthesis in intact Ehrlich ascites tumor cells. The response of intact cells in vitro to allopurinol seemed to be related to the sensitivity of cell-free enzyme preparations to inhibition of phosphoribosyl-1-amine synthesis by allopurinol ribonucleotide. Enzyme preparations of human Burkitt lymphoma cells were significantly more sensitive to inhibition by allopurinol ribonucleotide and purine ribonucleotides than were preparations of Ehrlich ascites tumor cells. Oxipurinol did not inhibit the early steps of purine biosynthesis of either cell line, nor was it a potent inhibitor of phosphoribosyl-1-amine synthesis in cell-free extracts. Ammonia was found to be a substrate for purine biosynthesis in intact Burkitt lymphoma cells and in Ehrlich ascites tumor cells in vitro and in cell-free preparations of these cell lines. Ammonia-dependent purine biosynthesis, like glutamine-dependent purine biosynthesis was more sensitive to inhibition in human cells than in Ehrlich ascites tumor cells.