Pseudofeedback Inhibition of Purine Synthesis by 6-mercaptopurine Ribonucleotide and Other Purine Analogues.

Exogenous purines are known to suppress purine synthesis de novo in bacteria (2-4), HeLa and L cells (5), and ascites cells (6) growing in culture. High levels of purines, especially adenine, inhibit accumulation of late purine precursors in the media of bacteria with blocks in the pathway of purine synthesis (2, 3), or of formylglycinamide ribonucleotide in Ehrlich ascites cells grown in the presence of aaaserine (6). These studies have implicated an early step in purine biosynthesis as the site of a feedback regulatory mechanism. Studies from this laboratory have shown that this inhibition operates on the first specific reaction of purine biosynthesis in which ribosylpyrophosphate 5-phosphate and glutamine interact to form phosphoribosylamine, and that adenosine triphosphate is the most potent of the inhibitory purine ribonucleotides (7, 8). Several purine analogues have also been shown to affect purine synthesis de novo. Gots and Gollub (9) observed that 6-mercaptopurine and 6-thioguanine inhibited accumulation of 5amino-4-imidazolecarbosamide ribonucleoside by Escherichia coli B-96 grown on limiting concentrations of adenine. Sartorelli and Le Page (10) and Le Page and Jones (1 I) observed that purinethiols inhibited accumulation of formylglycinamide ribonucleotide in Ehrlich ascites cells or mouse neoplasms treated with azaserine. Simpson, Bennett, and Golden (12) described the inhibition of incorporation of formate-‘4C into purines in Ehrlich ascites cells under conditions in which the terminal steps of purine synthesis were not inhibited. All of these studies suggested that these purine analogues might also be inhibiting purine synthesis by acting on an early reaction of the sequence. This paper presents evidence that the ribonucleotide derivatives of 6-mercaptopurine, 6-thioguanine, %azaguanine, and 4-hydrosypyrazolo-(3,4-d)pyrimidine effect a pseudofeedback inhibition of glutamine ribosylpyrophosphate 5-phosphate amidot.ransferase, mimicking the inhibit.ory action of natural purine ribonucleotides on this enzyme.