Metabolic resistance to tight-binding inhibitors of enzymes involved in the de novo pyrimidine pathway. Simulation of time-dependent effects.
When a tight-binding inhibitor interacts with a target enzyme of a metabolic pathway, the end products of the pathway are depleted and the substrate(s) for the inhibited reaction accumulate. The accumulating substrate(s) can reach a concentration which is sufficiently high that the inhibition is effectively reversed, with restoration of the original flux through the inhibited reaction. We have recently developed a theoretical model for this phenomenon which we have called 'metabolic resistance' [R. I. Christopherson and R. G. Duggleby (1983) Eur. J. Biochem. 134, 331-335]. In the present communication, we have successfully used the technique of numerical integration to simulate the effects of inhibition of several enzymes in the pathway of pyrimidine biosynthesis. Utilizing appropriate dissociation constants and enzyme concentrations for the inhibited systems, these simulations are consistent with published experimental data for the interactions of N-phosphonacetyl-L-aspartate (P AcAsp) with mammalian aspartate transcarbamoylase in vitro [R. I. Christopherson and M. E. Jones (1980) J. Biol. Chem. 255, 11 381 -11 395] and of 5-fluorodeoxyuridine 5'-phosphate (FdUMP) with thymidylate synthetase in vivo [R. G. Moran, C. P. Spears, and C. Heidelberger (1979) Proc. Natl Acad. Sci. USA 76, 1456- 1460]. In addition we present a simulation of the expected effect in vivo of phosphoribofuranosyl barbituric acid (BMP), a tight-binding inhibitor of OMP decarboxylase [H. L. Levine, R. S. Brody, and F. H. Westheimer (1980) Biochemistry 19, 4993- 4999]. This simulation shows that on addition of BMP, there is a sequential depletion of all pyrimidine intermediates between UMP and dCDP and a concomitant accumulation of OMP. Eventually, OMP reaches a new steady-state concentration and the concentrations of the depleted intermediates rise to their original levels. We have simulated the depletion of dCDP at various concentrations of BMP; since dCDP is comitted to DNA synthesis we can integrate the dCDP concentrations over time to calculate the amount of DNA synthesis and thereby predict the delay in cell division which would be elicited by BMP. This form of analysis may help to explain in quantitative terms why inhibitors of nucleic acid biosynthesis have a selective toxicity for rapidly growing tumour cells.
D002262:Carboxy-Lyases / Q000037:antagonists & inhibitors*; D004247:DNA / Q000378:metabolism; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D009711:Nucleotides / Q000494:pharmacology; D009964:Orotidine-5'-Phosphate Decarboxylase / Q000037:antagonists & inhibitors*; D011743:Pyrimidines / Q000378:metabolism*; D013997:Time Factors; D014500:Uracil Nucleotides / Q000494:pharmacology*; D014542:Uridine Monophosphate / Q000031:analogs & derivatives / Q000494:pharmacology*
D009711:Nucleotides; D011743:Pyrimidines; D014500:Uracil Nucleotides; C028372:1-(5'-phospho-beta-D-ribofuranosyl)barbituric acid; D004247:DNA; D014542:Uridine Monophosphate; D002262:Carboxy-Lyases; D009964:Orotidine-5'-Phosphate Decarboxylase