On the Mechanism of Feedback Inhibition of Purine Biosynthesis De Novo in Ehrlich Ascites Tumor Cells in Vitro.

Feedback inhibition of biosynthetic pathways in animal cells may be investigated at three levels of complexity. (a) Enzymological studies may evaluate the relative sensitivity of various enzymes of the pathway to inhibition by end products, the relative inhibitory potencies of different end products, and the mode of interaction of inhibitors and enzymes. In the absence of other information, however, such inhibition must be interpreted as only potentially a control mechanism. (b) Studies with intact cells in vitro may demonstrate that the “control mechanism” is indeed operative in the complex of interacting pathways within the cell, but will probably be less precise with respect to the details of the inhibition. Even these results should be qualified as demonstrating a potential control mechanism. (c) Finally, studies on cells in the environment of the whole animal, with physiological levels of inhibitory end products, are required to establish the above as a physiologically significant control mechanism. Few potential control mechanisms have been studied at all three levels. Inhibition of partially purified 5-phosphoribosyl-l-pyrophosphate amidotransferase from pigeon liver by purine ribonucleotides and ribonucleotides of purine analogues has been studied in some detail by Wyngaarden et al. and has been proposed by them as a potential mechanism of control of purine biosynthesis de vwuo (l-3). Feedback inhibition of this enzyme has also been shown with extracts of rat liver (3) and of Bacillus subtilis (4), although it, could not be demonstrated with a highly purified enzyme from pigeon liver (5). Other studies showed that purines added to media in which HeLa cells (6) or Ehrlich ascites carcinoma cells (7) were incubated caused inhibition of purine biosynthesis de novo. and many observations have demonstrated the “pseudofeedback” inhibitory effects of purine analogues on this pathway in various animal cells both in vitro and in viva (8-13). Although the results of studies with intact cells are consistent with the site of inhibition suggested by Wyngaarden, the identity of the two phenomena has not previously been demonstrated conclusively. This paper reports further studies of the forms of purines which are active in feedback inhibition of purine biosynthesis de novo, and studies of the site and mechanism of this inhibition in intact Ehrlich ascites tumor cells in vitro.