Feedback inhibition of purine biosynthesis in ascites tumor cells by purine analogues.

Feedback inhibition of purine biosynthesis de novo in bacteria has been studied in some detail by Gots and his colleagues (l-4). Other observations have suggested feedback inhibition, but these are complicated by dilution of intermediary pools by the added inhibitor (5-10). This reaction has been studied less extensively in mammalian cells. Sartorelli and LePage (11) and LePage and Jones (12) have conclusively demonstrated feedback inhibition by various 6-thiopurines in several azaserine-treated ascites tumors. McFall and Magasanik (13) studied this reaction in cultured L cells and reported both feedback inhibition and enzyme repression, although the extent of inhibition did not exceed 50%. It had long been assumed by analogy with other feedback systems that the inhibition was exerted at the first step in synthesis de novo, 5-phosphoribosylpyrophosphate amidotransferase. LePage and his co-workers (11, 12) showed that inhibition was before the fourth reaction, and Wyngaarden, Silberman, and Sadler later showed that inhibition preceded the third step (14). More recently Wyngaarden and Ashton (15, 16) showed that partially purified pigeon liver PP-ribose-P amidotransferase was strongly inhibited by several purine ribonucleotides. The present study was undertaken to establish the conditions necessary for the expression and measurement of feedback inhibition of purine biosynthesis de novo in Ehrlich ascites tumor cells in vitro and to determine some characteristics of this reaction. The extent of cu-N-formylglycinamide ribonucleotide accumulation in azaserine-treated ascites cells was used as a measure of purine synthesis de novo. The relative inhibitory activities of several natural purines have been determined as an approach to the identification of the actual inhibitor(s), and the potentiative action of combinations of purines on this process is demonstrated.