Feedback inhibition purines

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

Purine and pyrimidine biosynthesis parallel one another mole for mole, suggesting coordinated control of their biosynthesis. Several sites of cross-regulation characterize purine and pyrimidine nucleotide biosynthesis. The PRPP synthase reaction (reaction 1, Figure 34-2), which forms a precursor essential for both processes, is feedback-inhibited by both purine and pyrimidine nucleotides. 

Hepatic purine nucleotide biosynthesis is stringently regulated by the pool size of PRPP and by feedback inhibition of PRPP-glutamyl amidotransferase by AMP and GMP. 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

A comparison of EDso values for feedback inhibition and for growth inhibition in H.Ep.-2 cells in culture is shown in Table 2.3. It is readily apparent that for most of the purine analogues listed, the correlation between feedback inhibition and cytotoxicity is good. The few discrepancies may be due to the fact that these particular compounds are not feedback inhibitors, but metabolism (or lack of it) of the analogue in question may be important. In comparing these ED50 values, the difference in experimental conditions for cytotoxicity determination (long... 

Apparently the acceleration of de novo purine biosynthesis by orotic acid results from a release of feedback inhibition imposed by hepatic purine nucleotides. In a related study, it was found that orotic acid feeding can prevent hyperlipaemia, which normally follows the administration of Triton WR-1339, a surface active agent [152]. The influence of orotic acid on lipid metabolism can be readily shown by the fact that depression of serum lipoproteins and milk production were observed in lactating goats when an aqueous suspension of orotic acid was administered orally [164]. 

The biosynthesis of the purines is also regulated by feedback inhibition. ADP and GDP inhibit the formation of PRRPP from ri-bose-5 -phosphate. Similarly, step 2a is inhibited by AMP and GMP. 

Nucleotide monophosphates based on purine and pyrimidine-derived systems play critical roles in various biological processes such as signal processing, feedback inhibition, energy transduction, gene replication, and enzymatic regula-... 

Purine Nucleotide Biosynthesis Is Regulated by Feedback Inhibition... 

Synthesis of purines is under complex control.273 Some of the mechanisms found in bacteria are outlined in Fig. 25-16. Both feedback inhibition and activation are involved. Very important is the fact that GTP is needed in the synthesis of ATP, and that ATP is needed for synthesis of GTP. This kind of control ensures that an excess of either nucleotide will not be formed for long. In bacteria all of the final end product nucleotides inhibit the initial reaction of step a in Fig. 25-15. 

Many lines of evidence indicate that the first committed step in de novo purine nucleotide biosynthesis, production of 5-phosphoribosylamine by glutamine PRPP amidotransfer-ase, is rate-limiting for the entire sequence. Consequently, regulation of this enzyme is probably the most important factor in control of purine synthesis de novo (fig. 23.24). The enzyme is inhibited by purine-5 -nucleotides, but the most inhibitory nucleotides vary with the source of the enzyme. Inhibition constants (A, ) are usually in the range 10-3-10-5 M. The maximum effect of this end-product inhibition is produced by certain combinations of nucleotides (e.g., AMP and GMP) in optimum concentrations and ratios, indicating two kinds of inhibitor binding sites. This is an example of a concerted feedback inhibition. 

Regulation of purine biosynthesis. Red arrows show points of inhibition 0 or activation . In addition to the feedback inhibition, GTP stimulates ATP synthesis, and ATP stimulates GTP synthesis, thus helping to ensure a balance between the pools of the two nucleoside triphosphates. The full biosynthetic pathways are shown in figures 23.10 and 23.11. 

In eukaryotes, carbamoyl phosphate synthase is inhibited by pyrimidine nucleotides and stimulated by purine nucleotides it appears to be the most important site of feedback inhibition of pyrimidine nucleotide biosynthesis in mammalian tissues. It has been suggested that under some conditions, orotate phosphoribosyltransferase may be a regulatory site as well. 

The rates of these two complementary reactions can control the amount of either AMP or GMP present in the cell. Each of these reactions is feedback-inhibited by its nucleotide product. Thus, if more adenosine nucleotides exist than guanosine nucleotides, the synthesis of AMP slows down until the purine nucleotides balance. 

The synthesis of purine nucleotides is controlled by feedback inhibition at several sites (Figure 25.16). 

The committed step in purine nucleotide biosynthesis is the conversion of PRPP into phosphoribosylamine by glutamine phosphoribosyl amidotransferase. This important enzyme is feedback-inhibited by many purine ribonucleotides. It is noteworthy that AMP and GMP, the final products of the pathway, are synergistic in inhibiting the amidotransferase. 

Figure 25.16. Control of Purine Biosynthesis. Feedback inhibition controls both the overall rate of purine biosynthesis and the balance between AMP and GMP production.

Pyrimidine biosynthesis in E. coli is regulated by the feedback inhibition of aspartate transcarbamoylase, the enzyme that catalyzes the committed step. CTP inhibits and ATP stimulates this enzyme. The feedback inhibition of glutamine-PRPP amidotransferase by purine nucleotides is important in regulating their biosynthesis. 

The utilization of ammonia resulting from the combination of carbamyl phosphate with aspartic acid, the initial reaction for the synthesis of the pyrimidine nucleotides, continues only as long as there is a requirement for them (Fig. 3). Regulation of this biosynthetic pathway is probably by way of feedback inhibition of aspartate transcarbamylase. The rat liver enzyme is inhibited by uridine, cytidine or thymidine or such derivatives as CMP, UTP, or TMP, all intermediates or products of this pathway (B8). This is not the only enzyme of the pathway which may be subject to feedback regulation. Dihydroorotase from rat liver is also inhibited by some pyrimidines and purines (B9). 

The Synthesis of Purine Nucleotides Is Controlled by Feedback Inhibition at Several Sites... 

The answer is c. (Ivlurray, pp 375— /O I. Scrivt i, pp 2513—2570. Sack, pp 121—138. Wilson, pp 287—320.1 Several control sites exist in the path of purine synthesis where feedback inhibition occurs, AMP, GMP, or IMP may inhibit the first step of the pathway, which is the synthesis ol 5-phosphoribosyl-l-pyrophosphate (PRPP). PRPP synthetase is specifically inhibited. All three nucleotides can inhibit glutamine PRPP aminotranslerase, which catalyzes the second step of the. pathway. AMP blocks the conversion ol IMP to adenylosuccinate. GMP inhibits the lormation ol xanthylate Irom IMP Thus, blockage rather than enhancement ol IMP metabolism to AMP and GMP effectively inhibits purine biosynthesis. 

Feedback inhibition is indicated by red arrows. The stimulation of AMP synthesis by GTP and GMP synthesis by ATP ensures a balanced synthesis of both families of purine nucleotides. 

Thioguanine has multiple metabolic effects. Its tumor inhibitory properties may be due to one or more of its effects on feedback inhibition of de novo purine synthesis inhibition of purine nucleotide interconversions or incorporation into DNA and RNA. The net conseqnence of its actions is a sequential blockade of the synthesis and utilization of the purine nucleotides. 

PRPP is an important intermediate in the de novo synthesis of purines pathway (Figure 22.4). Defects in PRPP synthetase may render it insensitive to feedback inhibition by purine nucleotides. Thus, purine nucleotides are overproduced, leading to excessive uric acid synthesis and gout (Figure 22.9). 

The first step in the pathway, formation of carbamoyl aspartate from aspartate and carbamoyl phosphate, is the primary regulatory point in the pathway. The enzyme, aspartate transcarbamoylase (ATCase) (see here), is activated by ATP and inhibited by CTP, which is the end product of the pathway. Another point of regulation is CTP synthetase, which is feedback inhibited by CTP and activated by GTP. In bacteria, synthesis of ATCase subunits is inhibited by high levels of UTP. The inverted regulatory effects of purine and pyrimidines in the pathway are yet another way cells maintain a proper balance of nucleotides. 

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